Secondary Oxide Phosphines to Promote Tandem AcylâAlkyl

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Secondary Oxide Phosphines to Promote Tandem Acyl−Alkyl Coupling/Hydrogen Transfer to Afford (Hydroxyalkyl)rhodium Complexes. Theoretical and Experimental Studies Virginia San Nacianceno,† Lourdes Ibarlucea,† Claudio Mendicute-Fierro,† Antonio Rodríguez-Diéguez,‡ José M. Seco,† Antonio J. Mota,*,‡ and María A. Garralda*,† †

Facultad de Química de San Sebastián, Universidad del País Vasco (UPV-EHU), Apdo. 1072, 20080 San Sebastián, Spain Facultad de Ciencias, Universidad de Granada, Avda. Fuentenueva s/n, 18002 Granada, Spain

‡

S Supporting Information *

ABSTRACT: Acyl(σ-norbornenyl)rhodium(III) dimer [Rh(μ-Cl)(C9H6NCO)(C7H9)L]2 (1) (C7H9 = σ-norbornenyl; L = 4-picoline, isoquinoline) reacts with diphenylphosphine oxide (SPO) to undergo a one-pot reaction involving (i) cleavage of the chloride bridges and coordination of the phosphine, (ii) C−C bond coupling between acyl and norbornenyl in a 18e species, and (iii) ligand-assisted outer-sphere O(P)-to-O(C) hydrogen transfer, to afford mononuclear 16e species [RhCl{(C9H6NC(O)C7H9)(Ph2PO)H}(L)] (2) containing a quinolinyl-(norbornenylhydroxyalkyl) fragment hydrogen-bonded to a κ1-Pphosphinite ligand. Pentacoordinated 2, which adopt a distorted trigonal bipyramidal structure, are kinetic reaction products that transform into the thermodynamic favored isomers 3. Structures 3 contain an unusual weak η1-C anagostic interaction involving the rhodium atom and one carbon atom of the olefinic C−H bond of the norbornenyl substituent in the chelating quinolinyl-hydroxyalkyl moiety. Their structure can be described as pseudoctahedral, through a 5 + 1 coordination, with the anagostic interaction in a trans disposition with respect to the phosphorus atom of the phosphinite ligand. Complexes were characterized in solution by NMR spectroscopy and electrospray ionization mass spectrometry. Complex [RhCl{(C9H6NC(O)C7H9)(Ph2PO)H}(4-picoline)] (3a) was also identified by X-ray diffraction. Density functional theory calculations confirm the proposed structures by a plausible set of mechanisms that accounts for the 1 (monomer) → 2 → 3 transformation. Lowest-energy pathways involve reductive elimination of quinolinylnorbornenylketone, still coordinated in the rhodium(I) species thus formed, followed by O-to-O hydrogen transfer from κ1-P-SPO to the sp3 hybridized carbonyl group (formal alkoxide) avoiding the otherwise expected classical release of ketone. Theoretical 13C NMR studies also confirm the experimental spectral data for the considered structures.

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reductive elimination of the ketone.4 The use of tethered aldehydes has allowed the isolation of stable intermediates, such as hydrido-acylrhodium(III) complexes,4c,5 and detection or isolation of a few alkenyl- or alkyl-acylrhodium(III) compounds or η2-ketone bound products.6 Previously we reported on the reaction of [Rh(nbd)Cl]2 (nbd = norbornadiene, C7H8) with quinoline-8-carbaldehyde (C9H6NCHO) and pyridines (L) that gave dinuclear alkyl-acylrhodium(III) complexes [Rh(μCl)(C9H6NCO)(nbyl)L]2 (1) (nbyl = σ-norbornenyl-C7H9), which, upon addition of triphenylphosphine, afforded the corresponding hydroacylation quinolinylnorbornenylketone product C9H6NC(O)(nbyl) (I in Scheme 1).7 More recently we isolated the 16e [RhCl(C9H6NCO)(nbyl)(P(p-F-C6H4)3)] complex, an active species in the catalytic hydroacylation of norbornadiene with quinoline-8-carbaldehyde, which requires

INTRODUCTION Transition-metal acyl complexes are required intermediates in many stoichiometric and catalytic reactions. Relevant examples are the hydroformylation of olefins, where the reductive elimination of aldehyde from acyl-hydrido intermediates is assumed to be the final step, and the hydroacylation reactions that involve acyl formation and subsequent new C−C bond formation.1 Also, the active site of the H2 activating enzyme [Fe]-hydrogenase has been shown to contain a C,N-chelating 2-hydroxy-6-acylmethylenepyridine.2 The hydroacylation is an economic method to prepare ketones due to the availability of the starting materials, and phosphine−rhodium complexes have been reported as efficient catalysts in the hydroacylation of unsaturated substrates with aldehydes.3 The details of the mechanism for a particular reaction depend on the substrate, catalyst, and reaction conditions and usually involve steps such as C−H oxidative addition of the aldehyde, coordination of the substrate, insertion through an M−H bond, and, finally, © XXXX American Chemical Society

Received: February 6, 2018

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DOI: 10.1021/acs.inorgchem.8b00320 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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Scheme 1. Reaction of 1 with PPh3 to Get the Final, Expected, Hydroacylation Product

Article

RESULTS AND DISCUSSION

The reaction of the acyl(σ-norbornenyl)rhodium(III) dimers [Rh(μ-Cl)(C9H6NCO)(nbyl)L]2 (L = 4-picoline, 4pic, 1a; isoquinoline, iquin, 1b) with diphenylphosphine oxide leads to the cleavage of the chloride bridges to afford complexes 2a,b, whose formation formally also involves C−C coupling and O(P)-to-O(C) hydrogen transfer (Scheme 2). These complexes were characterized in solution by electrospray ionization mass spectrometry (ESI-MS) and NMR spectroscopy. ESI-MS spectra show a strong signal corresponding to the [M−1]+ peak (Figures SI-1 and SI-2 in Supporting Information), which fits with structures on considering mononuclear species. The 31P{1H} NMR spectra of 2a,b contain a unique signal: a doublet at 80 ppm (J(Rh,P) = 153 Hz) that could be attributed to a rhodium-coordinated diphenylphosphinous acid moiety, though ca. 20 ppm at higher field than that reported for this ligand in hexacoordinated rhodium12b or ruthenium9c complexes. Features of 1H and 13C{1H} NMR spectra indicate the occurrence of further transformations (Figures SI-3−SI-6 in Supporting Information). Thus, 13C{1H} NMR spectra of complexes 2a,b certainly contain the complete set of signals due to the norbornenyl fragment with an uncoordinated olefin as shown by the chemical shifts of C4 and C5 (see Table 2) at 137.0 and 131.1 ppm, respectively, the same values presented in the starting materials. At variance, the signal due to C1 of norbornenyl appears in 2a,b at lower field (50 ppm) than in complexes 1a,b (39 ppm) and, more significantly, as singlet instead of as the multiplet observed for 1a,b. Other spectral features are similar to those reported for norbornenyl ketones7,8 or for a ruthenium complex containing an acylnorbornenyl ligand.20 Also, the resonance in the low-field region due to the acyl group in 1a,b (230 ppm) is no longer observed; instead, a doublet at 93.9 ppm (J(Rh,C) = 26 Hz) appears. These spectral changes suggest the transformation of the rhodium-bonded sp2 acyl carbon atom in 1 into a rhodium-bonded sp3 hydroxyalkyl carbon atom. The spectroscopic features agree well with the occurrence of, to the best of our knowledge, an unprecedented reaction involving an acyl insertion into the rhodium norbornenyl bond accompanied by a (possibly concomitant) hydrogen transfer reaction from the oxygen atom of the phosphinous acid to the oxygen atom of the RhCO moiety. Kinetic products thus consist of coordinatively unsaturated chloridehydroxyalkyl-κ1-P-

an additional phosphine to undergo the reductive elimination of the C9H6NC(O)(nbyl) ketone in the last step.8 We thought of interest to perform the reaction of 1 with diphenylphosphine oxide, PPh2OH, which could promote the C−C bond formation step and, via hydrogen bonding, also allow the isolation and characterization of ketone-coordinated rhodium species. Secondary phosphine oxides (SPOs) exist as two tautomeric isomers in equilibrium, the pentavalent form R1R2P(O)H and the trivalent phosphinous acid form R1R2P−OH. Metal atoms usually prefer a P-coordination through the trivalent form.9 Coordinated SPOs may lead to the formation of intra- or intermolecular hydrogen bonds, related to their electronic and steric effects.10 Strong intramolecular hydrogen bonds have been observed in complexes containing the phosphinous acid−phosphinito moiety R1R2PO−H··· OPR1R2,11 and also in complexes containing phosphinous acid and acyl fragments.12 SPOs have been used as ligands for the synthesis of versatile catalysts for a variety of reactions, such as hydrophosphinylation,13 hydroformylation,12a,14 cross-coupling,15 nitrile hydration,16 asymmetric hydrogenation,17 or cycloaddition,18 or as useful reagents for nanoparticle stabilization.19 We report now on the reaction of saturated acyl(σnorbornenyl)rhodium(III) complexes with diphenylphosphine oxide, leading to unexpected pentacoordinated (norbornenylhydroxyalkyl)rhodium(III) complexes (see below in Scheme 2). This reaction could represent a useful step in a transition-metal catalyzed tandem hydroacylation/ligand-assisted hydrogenation of olefins to afford alcohols.

Scheme 2. Reaction of 1 to Get the Final, Thermodynamic Product, 3 (yield: 63−70%)a

a

The kinetic product 2 is isolable (yield: 61−73%) and could be characterized. Reaction conditions: (i) PPh2OH in C6H6. (ii) Refluxing C6H6 (2 h). The possible reaction mechanism (supported by DFT calculations) requires the generation of the transient structure A. B


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Inorganic Chemistry

tends to stabilize the trigonal bipyramid.27 Repeated attempts to obtain single crystals suitable for X-ray diffraction that could ascertain the geometry of these compounds proved unsuccessful, and this is related to complexes 2a,b, being kinetic reaction products, transforming readily into the thermodynamically stable isomers 3a,b; see Scheme 2. Thus, we based our geometry discussion for 2 on DFT calculations (structure E in the Theoretical DFT Study section, see below). In solution, complexes 2a,b slowly transform at room temperature (48 h), or faster in refluxing benzene (2 h), into the thermodynamically favored species 3a,b shown in Scheme 2. The 31P{1H} NMR spectra of 2a in CDCl3 showed the appearance of a new doublet at higher field, 71.3 ppm (J(Rh, P) = 147 Hz), due to the increase of 3a, whereas the intensity of the corresponding signal due to 2a decreased. The 1H NMR olefinic chemical shifts in the 5−6 ppm range disappeared (Figure SI-9 in Supporting Information), and we also account for important changes in the 13C NMR spectra; see below (Table 2). Although, compounds 2a,b could be transformed, respectively, into 3a,b as already mentioned, structures 3 could be directly obtained by reaction of 1 with PPh2OH in refluxing benzene preventing by this the appearance of the kinetic products 2a,b. These complexes were characterized by IR (Figures SI-10 and SI-11 in Supporting Information) and NMR spectroscopies and by ESI-MS. The ESI-MS spectra show a strong signal corresponding to the [M−1]+ peak (Figure SI-12 in Supporting Information) and suggest complex 3 to be also mononuclear species. In the NMR spectra (Figures SI-13−SI-16 in Supporting Information), the carbon atom of the hydroxyalkyl moiety appears at slightly higher field than in 2 as doublet of doublet, 91.6 ppm (J(Rh, C) = 29; J(P, C) = 2 Hz), and the hydroxyl proton is clearly observed at room temperature at 9.54 ppm and disappears upon addition of a few drops of CD3OD. The transformation of 2 into 3 involves significant changes in the chemical shifts of the olefin: taking the 4-picoline complexes as an example, (i) the olefinic protons undergo a shift toward lower field from 5.90 and 5.25 ppm in 2a to 7.08 and 6.95 ppm in 3a, respectively, and (ii) the corresponding carbon atom shifts from 137.0 and 131.1 ppm (2a) to 154.7 and 127.5 ppm (3a), respectively. The isomerization of 2a,b, containing a noncoordinated norbornenyl group in the chelating hydroxyalkylquinoline moiety, into 3a,b is related to the formation of slightly thermodynamically more stable hexacoordinated species (see below). Unsaturated species containing ligands with olefin substituents can reach saturation by coordination of their πelectrons,28 leading to significant upfield chemical shift of the olefinic 1H NMR signals.29 Weaker interactions of the agostic or anagostic type, more frequent when involving sp3 C−H bonds than when sp2 C−H bonds are involved, have also been observed.30 Agostic interactions involve proton shifts toward higher field and M−H coupling with metals active in NMR, while H-anagostic interactions involve proton shifts toward lower fields and C-anagostic interactions, involving η1-C bonding only, are characterized by carbon shifts toward higher field.31 The NMR spectra of 3a,b include displacement of one olefinic proton chemical shift toward lower field, and that of the corresponding carbon toward higher field, with respect to the parent 2a,b. These data suggest, therefore, the existence of anagostic interactions between the Rh(III) center and the olefinic part of the norbornenyl moiety in 3a,b; see Figure 1.

phosphinite rhodium(III) derivatives, in which a strong intramolecular hydrogen bond [d(O···H) = 1.855 Å] is established between the oxygen atom of the phosphinite moiety and the hydrogen atom of the already formed Rh−C− OH group. The resonance observed at 10.17 ppm in the 1H NMR spectrum of 2a obtained at −60 °C, which disappears upon addition of a few drops of CD3OD due to exchange with deuterium and unobserved at 25 °C, suggests the presence of a hydroxylic proton being involved in hydrogen bonding.12a,13,21 The IR spectra of 2a,b show a band at 1100 cm−1 due to ν(P O) (Figures SI-7 and SI-8 in Supporting Information). We thus observe that the addition of a SPO to acylalkyl rhodium(III) complexes promotes a new C−C bond formation, but, instead of leading to the corresponding reductive elimination reaction of the ketone (I in Scheme 1), as observed with triarylphosphines,7 an additional O(P)-to-O(C) hydrogen transfer gives a stable hydroxyalkyl moiety. The stereochemistry presented in Scheme 2 for compound 1 is based on its corresponding crystal structure,7 whereas those of compounds 2a,b are based on density functional theory (DFT) calculations (see below) made from the crystal structure of 3 (see below), since we think that 2a,b and 3a,b should be structurally closely related, as both of them are monomeric structures, and 3a,b is an evolution product from 2a,b. The formation of 16e complexes 2a,b can be rationalized by a P-coordination of SPO, after the chloride-bridge splitting of 1, yielding an undetected and necessary intermediate A (see Scheme 2), as the precursor of the C−C bond formation. The requirement of the phosphine addition suggests that the dimer breaking process is triggered by the coordinative saturation reached for intermediate A upon the addition of the richelectron P-derivative. We reported an activation entropy of −100.9 ± 38.1 J mol−1 K−1 in the hydroacylation of norbornadiene with quinoline-8-carbaldehyde to afford C9H6NC(O)(nbyl) ketone catalyzed by triarylphosphine rhodium complexes.8 Intermediate A could contain a PO− H···Cl−Rh or a PO−H···OC−Rh hydrogen bond. The hydroxyalkyl formation, via a mechanism involving acyl insertion and ligand-assisted outer-sphere hydrogen transfer that could be concerted, appears feasible. Ligand-assisted outersphere mechanisms that can be concerted are involved in dehydrogenation of isopropanol to afford acetone22 or in protonation of acyl complexes to afford hydroxyalkyl intermediates leading to alcohols.23 An alternative mechanism, probably less likely, could involve reductive elimination of potentially chelating ketone followed by oxidative addition of the P−H bond in SPO and insertion of the carbonyl into the Rh−H bond. Insertion of an aldehyde into an M−H bond may afford hydroxyalkyl complexes,24 and an insertion of an η2ketone into a Rh−H bond to afford an α-hydroxyalkyl species has been recently reported.25 The oxidative addition of the P− H bond of SPO to afford M−H bonds is known.9b,13,21b,26 Complexes 1 were intermediates in stoichiometric hydroacylation of norbornadiene with quinoline-8-carbaldehyde promoted by triphenylphosphine,7 therefore complexes 2 could well represent models for intermediates in tandem hydroacylation/ligand-assisted hydrogenation of olefins to afford alcohols. As for the geometry of 2a,b, pentacoordinated complexes may adopt a trigonal bipyramidal or a square-planar pyramidal structure. Both types of geometries have been reported for unsaturated 16e rhodium(III) complexes,5a,7,8 and it is generally accepted that a π-donor ligand such as chloride C


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Inorganic Chemistry

square pyramid with the apical position occupied by the P1 atom and the equatorial positions occupied by the nitrogen atoms N1 and N2 of the quinoline and picoline ligands mutually trans and by the carbon atom of the hydroxyalkyl fragment and the chlorine atom also mutually trans. Alternatively, if considering a Rh···H−C interaction filling the potentially free apical coordination site trans to the P1 atom, the geometry may be described as pseudoctahedral. The H17 olefinic proton could be clearly located as a Fourier peak in a difference map. The Rh1−H17 distance of 2.5603(7) Å and the Rh1−H17−C17 angle of 87° in 3a are outside the accepted ranges for η2-C−H agostic interactions: 1.8−2.3 Å and 90− 140°, respectively,32 and the Rh1−C17 distance of 2.686(7) Å agrees with a weak η1-C interaction.31,33 The C16−C17 bond distance, 1.329(9) Å, corresponds to that of a nonbonded olefin. The crystal structure confirms the sp3 hybridization of the coordinated carbon atom C1 in the hydroxyalkylnorbornenyl fragment with C(1)−O(1) and C(1)−C(11), distances of 1.405(6) and 1.537(9) Å, respectively, and with a C2−C1−O1 angle of 109.9(4)°. The short P1−O2 distance of 1.513(4) Å supports double bond PO character,9c,10d,12b,21,25b and the rather short Rh1−P1 distance, 2.243(2) Å, is close to that observed for complexes with diarylphosphinous acid ligands with electron-withdrawing substituents.20b Finally, a strong34 hydrogen bond P−O···H−O−C (O1−O2 distance of 2.596(4) Å and O1−H2−O1 angle of 148(5)°) is observed between the hydroxyalkyl (O1−H1 = 0.834 Å) and the phosphoryl ligand (O2···H1 = 1.855 Å). A few examples of olefins in bimetallic complexes forming a π-bond to a metallic center and an agostic interaction of the sp2 C−H bond with the other metallic center have been reported.35

Figure 1. Molecular structure of 3a. Displacement ellipsoids are drawn at 50% probability level. Hydrogen atoms, except H1, H16, and H17, behaving to groups participating in significant intramolecular interactions, were ommitted. Selected bond lengths (Å) and angles (deg): Rh(1)−C(1), 2.094(6); Rh(1)−N(2), 2.092(3); Rh(1)−N(1), 2.063(4); Rh(1)−Cl(1), 2.517(2); Rh(1)−P(1), 2.243(2); C(1)− O(1), 1.405(6); C(1)−C(11), 1.537(9); P(1)−O(2), 1.513(4); O(1)−O(2), 2.596(4); Rh(1)−H(17), 2.5603(7); Rh(1)−C(17), 2.686(7); C(16)−C(17), 1.329(9); Cl(1)−Rh(1)−C(1), 167.0(2); N(1)−Rh(1)−N(2), 179.6(2); C2−C1−O1, 109.9(4); O(1)−H(1)− O(2), 148(5); Rh(1)−H(17)−C(17), 87.8(4).

Complex 3a was also characterized by single-crystal X-ray diffraction (Figure 1) that confirms the spectroscopic findings. The geometry about the Rh(III) center can be described as a

Figure 2. Calculated structure for 3a: 3T (left), the corresponding rotational conformer 3Trot (right), and the corresponding rotational transition state (negative frequency at −41.8 cm−1). D


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Inorganic Chemistry

Table 1. Calculated τ Values (in degrees), Relative Energies, Enthalpies, and Gibbs Free Energies (in kcal·mol−1) for the Four Considered Isomers and the Corresponding Transition States Associated to the Cnbyl−Cacyl Coupling Reaction, along with Their Respective Unique Imaginary Frequenciesa (in cm−1)

a

conformers

τ

A1 (syn/gauche) A2 (syn/anti) A3 (anti/gauche) A4 (anti/anti) A1‑TS (syn/gauche) A2‑TS (syn/anti) A3‑TS (anti/gauche) A4‑TS (anti/anti)

+59.26 −173.37 +62.34 −168.31 +99.78 −163.19 +46.54 −163.03

ΔE +5.0 +4.0 +0.5 0.0 +22.7 +21.2 +22.7 +20.5

ΔH

(+27.8) (+25.3) (+23.3) (+20.5)

+5.4 +4.2 +0.6 0.0 +21.8 +20.3 +21.7 +19.4

(+27.2) (+24.5) (+22.4) (+19.4)

ΔG +6.7 +4.4 +1.1 0.0 +22.8 +21.9 +23.8 +20.7

frequency

(+29.4) (+26.3) (+24.8) (+20.7)

b b b b −239.8 −304.3 −241.0 −323.7

Energies with respect to its parent conformer. In parentheses, values with respect to the most stable conformer (A4). bNonapplicable.

We first pay attention to the anagostic C−H···Rh interaction to evaluate its strength. For this purpose, we calculated the complex in benzene (PCM) from its geometry as appeared in the crystal structure, giving the computed 3T structure. We then calculated the rotational conformer 3Trot (by turning the norbornenyl group), for which the anagostic interaction is not operative, finding that the enthalpy rose by only 1.8 kcal·mol−1. This value should be very close to the Rh···CH interaction energy, since the 3Trot conformer does not present any important additional steric hindrance (see Figure 2), confirming that such interaction is actually weak. For comparison, in the investigation of several titanium and tungsten compounds showing in this case agostic interactions, the strength of the interaction was estimated to be smaller than or equal to 10 kcal·mol−1, in the range of typical hydrogen bonds.47 The activation enthalpy for the nbyl rotation event was calculated as 6.2 kcal·mol−1. From the above considerations, this relatively high value should mainly correspond to strain energy. Weak interactions are characterized, in terms of the properties of the Laplacian of the electron density ∇2ρ(r), as closed-shell (∇2ρ(r) > 0, at the bond critical points (BCPs)) interactions.48 Furthermore, the values of the Lagrangian kinetic energy, G(r), at the BCPs could be correlated with the strength of the interaction, which in the case of hydrogen bonds could be calculated by the equation EHB = 0.429 × G(r).49 Yet, the intramolecular PO···HO hydrogen bond at the BCP in structure 3T got a ∇2ρ(r) value of +0.050 448 au, in the range of hydrogen bonding,47,50 and a G(r) value of 0.043 397 au, which results in a strength of ∼11.7 kcal·mol−1, thus meaning a significantly strong interaction that could greatly influence the reaction coordinate. For comparison, for the anagostic interaction we calculated a ∇2ρ(r) value of +0.024 041 au accompanied by a G(r) value of 0.018 706 au. So, we would expect in this case an interaction of ∼2−3 kcal· mol−1, close to the previously calculated value of 1.8 kcal·mol−1, by simple rotation (see Figure 2 and related text in Supporting Information). Figure SI-18 (Supporting Information) shows all the through-space interatomic BCPs calculated for this structure. Coming back to the mechanism for the transformation of 1 into 3, we considered, taking into account the observed products, that compound 3a was achieved as shown in Scheme 2. Unfortunately, we have only spectroscopic data for the isolated compound 2a, and its exact structure could not be confirmed by X-ray diffraction. Focusing on this puzzling behavior we tried to establish a solid mechanism from structure A, got by cleavage of 1. For this purpose, we modeled four

Nevertheless, it is unusual for an anagostic interaction to compete with the formation of an olefinic bond. Indeed, some palladium complexes with diphenylphosphine ligands can show an agostic C−H arene interaction rather than an η2-CC bond due to geometric constraints.36 In unsaturated complexes with π-coordinated norbornene, γ-agostic interactions involving the bridge proton have been observed.37 In the present case, we believe that inherent geometric restrictions imposed by the particular geometry of the norbornenyl group, though allowing the olefin to approach the metal, avoid a better approach of the double bond preventing the expected π-coordination. Those and additional weak interactions operating in 3a, namely, a CH(L)···OH interaction, several C−H(ar)···Cl interactions, and π−π interactions between the phenyl rings in the phosphorus moiety and the corresponding nearest quinoline and L ligands, are shown in Figure SI-17 (Supporting Information). Theoretical DFT Study. To give response to some important questions that arose from the experimental work, we performed DFT calculations using the Gaussian09 suite.38 Structures were optimized using the widely used B3LYP functional39 with the standard Pople’s 6-31G(d,p) basis set40 for all atoms except Rh, for which the LANL2DZ basis set (using the corresponding Effective Core Potential)41 was used instead. All minima and transition states were fully optimized and assessed by frequency calculations. Enthalpy and Gibbs free energy values were obtained by taking into account zero-point energies, thermal motion, and entropy contribution at standard conditions (temperature of 298.15 K, pressure of 1 atm). A Polarizable Continuum Model (PCM)42 was taken into account, except if otherwise indicated, to reproduce the solvent (benzene) in which reactions were performed. The geometrical parameters obtained with this basis set are in good agreement with those found in the crystal structure. Focusing on the more significant Rh−X distances, we achieved mean differences of ∼0.05 Å. For instance, in the 3a DFT-optimized structure in vacuum, 3Tv, the computed Rh−P, Rh−C(OH), and Rh− N(quinoline) bond lengths were 2.315, 2.135, and 2.106 Å, to be respectively compared to 2.242, 2.094, and 2.063 Å in the 3a crystal structure. The Rh···CH anagostic interaction was faithfully reproduced, as the computed Rh−C(H) and Rh− H(C) distances were 2.736 and 2.625 Å, to be respectively compared to 2.690 and 2.560 Å in the 3a complex. To evaluate the strength of some key hydrogen bonds, we also performed AIM calculations43 by means of the Multiwfn software.44 We first then need to reoptimize it using the polarized, diffuse, triple-ζ 6-311++G(d,p) basis set45 for all the atoms but rhodium, for which the all-electron DGDZVP basis set46 was used instead, keeping the PCM solvent model (benzene). E


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Inorganic Chemistry

Figure 3. Selected geometric parameters for the lowest-energy calculated structures in the A → B conversion.

different isomers, namely, A1−A4 (Figure SI-19 in Supporting Information), consisting in the syn or anti disposition of the P− OH/CO groups and the change in the orientation of the norbornenyl (nbyl) moiety through the H−Cnbyl−Rh−C(O) torsion angle τ, leading to gauche and anti conformations. Accordingly, we got syn/gauche (A1), syn/anti (A2), anti/ gauche (A3), and anti/anti (A4) isomers. From each isomer, a transition state (TS), respectively, from A1‑TS to A4‑TS (see Figure SI-20 in Supporting Information) could be established for the Cnbyl−Cacyl coupling reaction to not be biased for the most stable conformer but determine the best pathway through the lowest-energy transition state, given that conformer energy differences are low enough to be reached at room temperature. Table 1 shows the torsional τ angle values and the calculated energies for the four considered isomers along with the corresponding transition states associated with the rhodiummediated C−C coupling process. Thus, the lowest-energy pathway probably begins with the anti/anti A4 isomer, whose corresponding TS is located ∼3 kcal·mol−1 (enthalpic value) below the next isomer (anti/ gauche) and almost 8 kcal·mol−1 with respect to the highestenergy one (syn/gauche); see Table 1. In the A4 conformer, Rh has a C2N2PCl coordination environment conforming to an octahedral arrangement. Continuous shape measurements, performed by the program SHAPE,51 gave a distorted octahedral geometry (see Figure 3), with a continuous shape measure (CSM) of 1.460. This is not a very low value (lower the value, close to zero, closer to a perfect geometry), but, just for comparison, the next value is 14.623 corresponding to a trigonal prism (D3h). With respect to its corresponding A4‑TS transition state, the Rh−C(O) distance augmented from 2.133, in A4, to 2.360 Å. Interestingly, the Rh−Cl distance also enlarged significantly from 2.871 in A4 to 3.122 Å in the

corresponding TS, in which the new C−C forming bond distance went to 2.071 Å. From the lowest-energy transition state, A4‑TS, a natural intermediate, B, could be obtained upon the nbyl transfer event (Figure 3). For this intermediate, an agostic Rh···H(C)(nbyl) interaction is clearly stablished, with a Rh···H distance of 1.966 Å (2.633 Å in 3T). As a consequence, the Rh···Cl distance becomes even larger: 3.486 Å, still far from the sum of the van der Waals radii for Cl and Rh (4.26 Å).52 Intermediate B is located 6.2 kcal·mol−1 above its parent A4 structure (enthalpic value). Structure B could be understood as the resulting reductive elimination ketone product and, therefore, a formal Rh(I) intermediate. This is supported by the considerable elongation of the Cl−Rh bond, whereas the dihapto hydrido−carbido coordination mode supplies the necessary electron density to have a complete coordinating bond. On the one hand, the C O distance changes from 1.225 Å in transient A4 structure to 1.251 Å in intermediate B, and the Rh−C(O) distance respectively moves from 1.972 to 2.371 Å. On the other hand, the Rh−O(C) distance in B is 3.110 Å with a Rh−C−O angle of 114.7°, avoiding a dihapto CO coordination; see inset in Figure 3. Structure B is remarkable in several aspects. Its formation illustrates a C−C coupling leading to reductive elimination of ketone occurring in a six-coordinated rhodium(III) complex. This is unusual, as the most commonly accepted mechanism requires a five-coordinate species undergoing the reductive elimination.3a,5b,6a Also, chelate-assistance has allowed the detection or isolation of intermediates containing the reductive coupling product coordinated in an η1-O- or η2-(C O)-fashion.4c,5b,6 At variance with this, in B chelation of the hydroacylation product involves an up to our knowledge unprecedented interaction with the carbonyl carbon atom and the alkyl (norbornenyl) group. Since B is located above in F


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Inorganic Chemistry Scheme 3. Enthalpy Profile of the Considered Pathways for the A4 → 3T Conversiona

All values (in kcal·mol−1) are referred to A4. Black profile corresponds to the lowest-energy pathway (pathway (i)). Red profile (pathway (ii)) is the mechanism on considering first the chlorine isomerization.52 Blue one is the profile of the ketone release product. a

Figure 4. Potential energy variation for the nonsymmetric rotation of the Ph2POH moiety (Ph groups were removed). A geometry optimization was done at every step.

energy (ΔH = +6.2 kcal·mol−1) with respect to A4, the thermodynamic product would be A4. Interestingly, B structure can follow a lowest-energy sequence (largely below the A4‑TS energy, vide infra in Scheme 3, pathway (i), in which Rh(I) reoxidizes to Rh(III) by reducing the carbonyl group to an alcohol through a (P)O−H···O(C) proton transfer that finally leads to the final product 3T. This event needs a previous Ph 2 POH group rotation to attain a POH/CO syn disposition. The Lowest-Energy B → 3T Pathway (pathway (i)). Although many attempts to catch any TS for the rotation of the Ph2POH moiety were made, none succeeded. We could, however, calculate the energy of the system for the rotation process freezing the corresponding (O)C−Rh−P−O dihedral angle in 10° step for a complete 360° turn, optimizing the system at each point; see Figure 4. This optimization is necessary, since the direct calculation by a series of single points through simple rotation leads to high strain energy structures with the possibility of atom overlapping. The maximum energy of the clockwise (CW) and counterclockwise (CCW) curves obtained allowed us to approach the energy barrier for the rotation and the most probable direction taken: + 7.7 kcal· mol−1 at 50° and +2.4 kcal·mol−1 at 100° (260° in the whole

turn), respectively, for the CW and CCW directions (Figure 4). The unsymmetrical energy profile then points to a strongly CCW preferred direction, having a very low energy barrier. Note that this rotation event lowers the enthalpy (from B) by 10.2 kcal·mol−1, and so, this is a quite favorable, exothermic step. Interestingly, during the rotation, the chlorine atom isomerizes52 adopting a trans disposition with respect to the Ph2POH moiety. Therefore, the final product is structure D (Figure 5 and Scheme 3), which undergoes a P−OH···O(C) proton transfer following a very low-energy pathway to get E; see Figure 5. Yet, the corresponding transition state, DTS, is very close in energy to D: just 0.3 kcal·mol−1. In fact, the energy difference is so small that the enthalpy variation is negative by 1.2 kcal·mol−1. This is a typical behavior in very flat potential energy surfaces, and it could be argued that the hydrogen transfer process takes place, in practice, with no energy barrier. In E, the +3 oxidation state for Rh is recovered, and the carbon atom of the carbonyl moiety is completely sp3 hybridized, formally having a Rh−C−O− coordination. A new chlorine reisomerization through the ETS transition state (see Figure SI23 and related text in Supporting Information) allows to achieve structure 3Trot, from which the obtaining of final 3T G


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Inorganic Chemistry

Figure 5. Selected geometric parameters for the lowest-energy calculated structures in the B → E conversion.

Table 2. Computed and Experimental 13C Chemical Shifts (δ, in ppm) of the Optimized Structures B, C, D, E (equivalent to 3), 3Trot, and 3T (equivalent to 3a) for the Norbornenyl (blue) Moiety

δ (ppm)

B

C

D

E

2

3Trot

3T

3

1′ 1 2 3 4 5 6 7

164.02 41.82 32.63 41.23 138.70 137.46 44.48 47.61

212.21 56.92 32.76 40.58 136.10 139.69 44.42 47.81

101.10 49.35 28.84 40.87 138.13 134.81 47.86 49.42

101.10 49.34 28.84 40.87 138.13 134.81 47.86 49.42

93.9 49.9 30.4 42.6 137.0 131.1 46.9 50.9

98.40 49.83 25.77 42.18 135.67 136.52 46.34 49.94

98.86 58.62 31.68 40.45 156.16 132.28 40.83 53.28

91.6 59.3 30.2 42.6 154.7 127.5 43.3 53.1

H


Article

Inorganic Chemistry

to the olefinic double bond coordination, due to steric constraints. DFT calculations provide a suitable mechanism for this reaction that confirms the reductive elimination of ketone from an 18e Rh(III) species, leading to Rh(I) intermediates that, upon rotation of the Ph2POH moiety or chlorine isomerization, allow an O-to-O hydrogen transfer from SPO to the sp3 hybridized ketone (formal alkoxide), which thus forms the hydroxyalkyl−rhodium(III) species and also explains release of free ketone being avoided. Theoretical 13C NMR calculations on different structures provide a solid support for the mechanism through the experimental isolated intermediate 2. The hydroxyalkyl complexes can model intermediates in rhodium-catalyzed tandem hydroacylation/ligand-assisted hydrogenation of olefins to afford alcohols and could be relevant concerning an enantioselective version if using chiral SPOs. In this respect, more studies should be done to evaluate the scope of the title reaction using SPOs as promotor.

structure is straightforward (Figure 2). This TS (imaginary frequency at −43.3 cm−1) is very flat compared with the first Cl isomerization (+5.5 kcal·mol−1), the energy difference being +0.14 kcal·mol−1 from E, whereas the enthalpy variation was, once more, negative by 0.32 kcal·mol−1. For structural aspects of the D to 3Trot transformation, see the related text in the Supporting Information. An enthalpy profile of the whole mechanism is depicted in Scheme 3 (black profile). Alternatively, although the presented calculations show the lowest-energy pathway found for the B → 3T transformation, another possible pathway was also envisaged considering first the chlorine isomerization process and then the rotation of the Ph2POH moiety (pathway (ii) in Scheme 3, red profile). This pathway also explains the formation of the final product instead of the otherwise expected release of ketone. By considering first the chlorine isomerization process, displacement of the Rh··· H(C) agostic interaction by chlorine occurs, through a transition state BTS (imaginary frequency at −53.9 cm−1), located 5.5 kcal·mol−1 above B (see Figure SI-21 and related text). C structure is enthalpically more stable than B by just 0.14 kcal·mol−1. Thus, both structures could be considered as isoenergetic in practice. Release of free ketone from C to afford P, see Figure SI-24 in Supporting Information, is an endothermic process and follows a high-energy TS that even would allow first the backward process to the starting A4 structure (Scheme 3 (blue profile)). Theoretical calculations also helped us to ascertain the structure of compound 2 based on the determination of reliable 13 C NMR theoretical spectra. Some of us recently published an article in which the use of the LC-ωPBE/6-31G(d,p) functional/basis set couple demonstrated to be high coincident for both 1H and 13C NMR spectra.53 Since 13C NMR spectrum has a wide window for chemical shifts it could be ascribed to a fingerprint for any organic compound. Thus, we focused our analysis on it by calculating the corresponding 13C NMR chemical shifts for structures B, C, D, E, and 3Trot (see Figures 2, 3, 5, and SI-21) in chloroform (the same solvent in which experimental 13C NMR spectra were recorded), to be compared to those of the experimental, unknown compound 2. In particular, we paid attention to the norbornenyl (blue) moiety; see Table 2. The corresponding 4-picolinyl (green) and quinolinyl (red) signals, less affected by the chemistry involved in the chemical changes, are presented in the Supporting Information; see Table SI-1. Data in Table 2 show close values for the 13C NMR chemical shifts of the experimental structure 3 and that of the calculated 3T. With these reliable data, we can argue that the unknown structure of compound 2 corresponds in fact to that of structure E, see Figure 5 and the inset in Table 2, which is positioned just 2.5 kcal·mol−1 (enthalpy value) above the final 3T product.

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EXPERIMENTAL SECTION

General Procedures. The preparation of the metal complexes was performed at room temperature under nitrogen by standard Schlenk techniques. The complexes [Rh(μ-Cl)(C9H6NCO)(Nbyl)L]2 (L = 4methylpyridine, 1a; or isoquinoline, 1b) were prepared as previously reported.7 All other reagents were purchased from commercial sources and used without further purification. Microanalysis was performed with a Leco CHNS-932 microanalyzer. IR spectra were recorded with a Nicolet FTIR 510 spectrophotometer in the range of 4000−400 cm−1 using KBr pellets. 1H and 13C{1H} (TMS internal standard) and 31 1 P{ H} (H3PO4 external standard) NMR were recorded with Bruker Avance DPX 300 or Bruker Avance 500 spectrometers. ESI-MS spectra were recorded on a Bruker MicroTOF-Q Instrument. In all ESI-MS spectra there was a good fit to both the principal molecular ion and the overall isotopic distribution.

Preparation of [RhCl{(C9H6NC(O)C7H9−Nbyl)(Ph2PO)H}(L)] (2) (L = 4-pic, 2a; iquin, 2b). To a C6H6 suspension of 1 (0.06 mmol) PPh2(O)H (0.12 mmol) was added. After it was stirred for 2 h at room temperature a yellow solution was obtained. Addition of hexane afforded pale yellow solids that were filtered off, washed with hexane, and vacuum-dried. Data for 2a: Yield: 60 mg, 73%. IR (KBr, cm−1): 1099 (m), ν(PO). ESI-MS: calcd. For C35H33ClN2O2PRh: 682.0; observed: 681.1 [M−H]+. Anal. Calcd for C35H33ClN2O2PRh: C 61.55, H 4.87, N 4.10; found: C 61.88, H 5.03, N 4.03%. 1H NMR (CDCl3, 298 K): δ 9.64 (m, 1H, HCN, quin); 9.13 (m, 2H, HC N, pic); 8.08−6.60 (m, 17H, aromatic); 5.90 (br, 1H, H4 Nbyl); 5.25 (br, 1H, H5 Nbyl); 3.13 (m, 1H, H1 Nbyl); 2.55 (s, 1H, H3 Nbyl) 2.48 (s, 1H, H6 Nbyl); 2.22 (s, 3H, CH3); 1.89 (d, 1H, Jgem = 13.0 Hz, H2, Nbyl); 1.10 (m, 3H, 1H2, 2H7 Nbyl) ppm. 13C{1H} NMR (CDCl3, 298 K): δ 154.8 (CN, quin); 153.5 (CN, pic); 152.9; 149.2; 148.6; 137.0 (C4 Nbyl); 136.6; 131.3; 131.1 (C5 Nbyl); 129.5; 128.5; 128.4; 127.9; 127.7; 127.6; 126.8; 125.6; 124.8; 121.8; 93.9 (d, JRh,CO = 26 Hz, C−O); 50.9 (C7 Nbyl); 49.9 (C1 Nbyl); 46.9 (C6 Nbyl); 42.6 (C3 Nbyl); 30.4 (C2 Nbyl); 20.9 (CH3) ppm. 31P{1H} NMR (CDCl3, 298 K): δ 80.6 (d, JRh, P = 153 Hz) ppm. Data for 2b: Yield: 53 mg, 61%. IR (KBr, cm−1): 1100 (m), ν(P = O). ESI-MS: calcd. For C38H33ClN2O2PRh: 718.0; observed: 717.1 [M−H]+. Anal. Calcd for C38H33ClN2O2PRh: C 63.48, H 4.63, N 3.90; found: C 63.58, H 3.94, N 3.86%. 1H NMR (CDCl3, 298 K): δ 10.01 (s, 1H, HCN, quin); 9.69 and 9.16 (m, 1H, HCN, quin); 8.12−6.62 (m, 20 H, aromatic); 5.91 (br, 1H, H4 Nbyl); 5.31 (br, 1H, H5 Nbyl); 3.21 (m, 1H, H1 Nbyl); 2.52 (s, 2H, 1H3, 1H6 Nbyl); 1.88 (m, 1H, H2 Nbyl); 1.15 (d, 1H, Jgem = 9.3 Hz, H2, Nbyl); 1.10 (m, 2H, H7 Nbyl) ppm. 13C{1H} NMR (CDCl3, 298 K): δ 158.4 (CN, quin); 154.6 (CN, quin); 152.9; 148.6; 145.3; 137.2 (C4 Nbyl); 136.7;

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CONCLUSIONS Addition of an SPO to coordinatively saturated acyl-alkylrhodium(III) complexes leads to a rare transformation promoting C−C bond formation and an additional ligandassisted outer-sphere O-to-O hydrogen transfer that inhibits the release of free ketone. The carbon−carbon coupling occurs from saturated 18e species to afford a pentacoordinated 16e hydroxyalkyl species as kinetic product. The thermodynamic product, coordinatively saturated, is attained by transformation of a distorted trigonal bipyramid into a distorted square-planar pyramid that allows an unusual C-anagostic interaction involving the carbon atom of an olefinic double bond, preferred I


Inorganic Chemistry

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135.0; 132.3; 131.1 (C5 Nbyl); 130.8; 129.4; 128.6; 128.5; 127.9; 127.7; 127.4; 126.8; 126.0; 125.7; 121.9; 120.7; 93.9 (d, JRh,CO = 26 Hz, C−O); 50.9 (C7 Nbyl); 49.9 (C1 Nbyl); 46.9 (C6 Nbyl); 42.6 (C3 Nbyl); 30.4 (C2 Nbyl) ppm. 31P{1H} NMR (CDCl3, 298 K): δ 80.1 (d, JRh, P = 153 Hz) ppm. Preparation of [RhCl{(C9H6NC(O)C7H9−Nbyl)(Ph2PO)H}(L)] (3) (L = 4-pic, 3a; iquin, 3b). Method a. To a C6H6 suspension of 1 (0.06 mmol) PPh2(O)H (0.12 mmol) was added. The suspension was heated at reflux for 2 h, whereupon a solution was formed. Addition of hexane afforded pale yellow solids that were filtered off, washed with hexane, and vacuum-dried. Method b: A C6H6 solution of 2 was heated at reflux for 2 h. Addition of hexane afforded pale yellow solids that were filtered off, washed with hexane, and vacuum-dried. Data for 3a: Yield: 57 mg, 70%. IR (KBr, cm−1): 1123 (s), ν(PO). ESI-MS: calcd. For C35H33ClN2O2PRh: 682; observed: 681 [M−H]+. Anal. Calcd for C35H33ClN2O2PRh: C 61.55, H 4.87, N 4.10; found: C 61.20, H 4.45, N 4.08%. 1H NMR (CDCl3, 298 K): δ 9.88 (m, 1H, HCN, quin); 9.54 (s, 1H, O−H···O); 9.44 (br, 2H, HCN, pic); 8.75−6.75 (m, 17H, aromatic); 7.08 (m, 1H, H4 Nbyl); 6.95 (m, 1H, H5 Nbyl); 2.63 (s, 1H, H3 Nbyl); 2.25 (m, 1H, H1 Nbyl); 2.18 (s, 3H, CH3); 1.83 (d, 1H, Jgem = 12.6 Hz, H2 Nbyl); 1.60 (s, 1H, H6 Nbyl); 1.51 (d, 1H, Jgem = 8.2 Hz, H7 Nbyl); 1.35 (m, 1H, H7 Nbyl); 0.86 (d, 1H, H2 Nbyl) ppm. 13C{1H} NMR (CDCl3, 298 K): δ 156.9; 155.6 (CN, quin); 155.5 (CN, pic); 155.3 (CN, pic); 154.7 (C4 Nbyl); 154.0; 149.3; 147.8; 136.5; 131.3; 130.7; 129.3; 129.0; 128.8; 128.5; 127.6; 127.5 (C5 Nbyl); 127.1; 126.9; 125.3; 124.7; 122.3; 93.9 (d, JRh,CO = 26 Hz, C−O); 50.9 (C7 Nbyl); 49.9 (C1 Nbyl); 46.9 (C6 Nbyl); 42.6 (C3 Nbyl); 30.4 (C2 Nbyl); 20.9 (CH3) ppm. 31P{1H} NMR (CDCl3, 298 K): δ 71.3 (d, JRh, P = 149 Hz) ppm. Data for 3b: Yield: 54 mg, 63%. IR (KBr, cm−1): 1124 (m), ν(PO). Anal. Calcd for C38H33ClN2O2PRh: C 63.48, H 4.63, N 3.90; found: C 63.27, H 4.27, N 3.95%. 1H NMR (CDCl3, 298 K): δ 10.19 (br, 1H, HCN, quin); 9.94 (s, 1H, HCN, quin); 9.75 (s, 1H, O−H···O); 9.63 (br, 1H, HCN, quin); 8.34−6.61 (m, 20 H, aromatic); 7.21 (m, 1H, H4 Nbyl); 7.01 (m, 1H, H5 Nbyl); 2.58 (s, 1H, H3 Nbyl); 2.28 (m, 1H, H1 Nbyl); 1.78 (m, 1H, H2 Nbyl); 1.64 (s, 1H, H6 Nbyl); 1.52 (m, 1H, H7 Nbyl); 1.34 (m, 1H, H7 Nbyl); 0.94 (d, 1H, Jgem = 9.3 Hz, H2 Nbyl) ppm. 13C{1H} NMR (CDCl3, 298 K): δ 160.2 (C N, quin); 159.0; 156.8; 155.6 (CN, quin); 154.5 (C4 Nbyl); 147.7; 136.4; 135.0; 132.3; 131.3; 130.7; 129.2 (C5 Nbyl); 128.9; 128.5; 127.7; 127.5; 126.9; 126.8; 125.7; 125.4; 122.3; 120.4; 91.6 (d, JRh,CO = 29 Hz, C−O); 59.9 (C1 Nbyl); 53.2 (C7 Nbyl); 43.2 (C6 Nbyl); 42.6 (C3 Nbyl); 30.1 (C2 Nbyl) ppm. 31P{1H} NMR (CDCl3, 298 K): δ 70.7 (d, JRh, P = 146 Hz) ppm. X-ray Structure Determination of 3a. Yellow prismatic single crystals of 3a suitable for X-ray diffraction experiments were successfully grown by slow vapor diffusion of pentane onto benzene solutions. Data collections were performed at 100 K on a Bruker Smart CCD diffractometer, with graphite-monochromated Mo Kα radiation (λ = 0.710 69 Å), operating at 50 kV and 30 mA. The data were collected over a hemisphere of the reciprocal space by combination of the three exposure sets. Each frame exposure time was of 10 s, covering 0.3° in ω. Lorentz polarization and empirical absorption corrections were applied.54 The first 100 frames were recollected at the end of the data collection to monitor crystal decay. No appreciable drop in the intensities of standard reflections was observed. The cell parameters were determined and refined by a least-squares fit of all reflections. A summary of the fundamental crystal data and refinement data is given in Table SI-2 in the Supporting Information. The structure was solved by direct methods and conventional Fourier techniques. The refinement was done by full-matrix least squares on F2 (SHELXS-2014).55 Anisotropic parameters were used in the last cycles of refinement for all non-hydrogen atoms. Hydrogen atoms bonded to O1 and C17 atoms were located in a Fourier synthesis, included and refined as riding on bonded atom. The remaining hydrogen atoms were included in calculated positions and refined as riding on their respective carbon atoms. Final R(Rw) values were 0.0604(0.1218). The supplementary crystallographic data were passed to the Cambridge Crystallographic Data Centre (CCDC deposition numbers 1531315).

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00320. ESI-MS, IR, and NMR spectra, calculated structures of transition states, computed and experimental chemical shifts, and Cartesian coordinates for the calculated structures described in the manuscript (PDF) Accession Codes

CCDC 1531315 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (M.A.G.) *E-mail: [email protected]. (A.J.M.) ORCID

Antonio Rodríguez-Diéguez: 0000-0003-3198-5378 María A. Garralda: 0000-0001-6058-0412 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Partial financial support by Ministerio de Economiá y Competitividad MINECO/FEDER(CTQ2015-65268-C2-1-P), Univ. del Paiś Vasco (UPV/EHU), and Diputación Foral de Gipuzkoa is gratefully acknowledged. We also thank the Centro de Supercomputación (ALHAMBRA-CSIRC) of the Univ. of Granada for computational resources.

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REFERENCES

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