Stabilization of an Electron-Unsaturated Pd(I)âPd(I) Unit by Double

Article pubs.acs.org/Organometallics

Stabilization of an Electron-Unsaturated Pd(I)−Pd(I) Unit by Double Hemichelation Christophe Werlé,† Lydia Karmazin,† Corinne Bailly,† Louis Ricard,‡ and Jean-Pierre Djukic*,† †

Institut de Chimie de Strasbourg, UMR 7177 CNRS, Université de Strasbourg, 4 rue Blaise Pascal, F-67070 Strasbourg Cedex, France ‡ Département de Chimie, Ecole Polytechnique CNRS, Route de Saclay, F-91128 Palaiseau Cedex, France S Supporting Information *

ABSTRACT: The competition between conventional coordination and hemichelation of a Pd(II) center of three different palladacycles was probed by reacting the tricarbonyl(η6-3-phenylprop-1-enyl)chromium anion with μ-chloro-bridged palladacycles. Structural X-ray diffraction analysis indicates that the main products of the reaction are conventional (η3-allyl)Pd(II) complexes. The latter display significant dynamic behavior in solution, as suggested by 1H NMR spectroscopy. According to DFT calculations this dynamic behavior can be related to conformational equilibria and to possible chemical exchanges leading to Pd(II) hemichelates. The (η3-allyl)Pd(II) complexes convert readily to homoleptic bischelates of Pd(II) and to a bis-hemichelate of a Pd(I)−Pd(I) unit via a reductive disproportionation reaction. The latter Pd(I)−Pd(I) complex bears structural features very similar to those of electron-saturated bis(μ-allyl)-bridged Pd(I) complexes in the literature: the Pd−Pd interaction is only weakly covalent and is dominated by noncovalent attractive interactions, as revealed by NCI analyses. The incipient covalent interaction of the Pd(I) centers with CO ligands of the Cr(CO)3 moieties is too weak to significantly hinder the motion of the metalcarbonyl rotor in solution, which leaves each Pd center formally unsaturated with a 14-valence-electron count. DFT investigations sustained by QTAIM, NCI, ELF, and ETS-NOCV analyses suggest the predominance of noncovalent attractive forces in the stabilization of the bis(μ-allyl)-bridged Pd(I)−Pd(I) complex.

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INTRODUCTION The quest for a comprehensive account of the forces that contribute to chemical bonding in transition-metal complexes containing metal−metal interactions1 is a challenging domain of research that underwent renewed interest2 with the recent development of theoretical methods3 capable of accounting for nonlocal attractive interactions such as the London force4 in large complexes (up to 120 atoms) at moderate computational cost.5 Such new tools pave the way for a more comprehensive description of the forces that are responsible for molecular cohesion1b,2b,c,6 and potentially allow the rational elaboration of new paradigms for coordination chemistry that can now include noncovalent attractive forces as a significant source of molecular cohesion and stability. The term hemichelation7 is one of these new concepts that was recently proposed to define a nonclassical chelation mode of d-block transition metals by a heteroditopic ligand with which both covalent and noncovalent interactions between the ligand and a metal are central to the stabilization of the associated transition-metal complex.8 The concept of hemichelation emerged from earlier cases of electron-unsaturated Mn(I),9 Re(I),8c and Pd(II)8b,10 complexes reported in the literature, which were found to be exceptionally stable and persistent in solution. From these observations it was concluded that the tricarbonyl(η6-benzyl)chromium unit has strong hemichelating capabilities;10 it is able to establish a conventional covalent coordinative bond with the © 2015 American Chemical Society

metal through the benzylic carbon position and a bonding interaction of dominating noncovalent character with the same metal through the swiftly rotating Cr(CO)3 moiety. This latter property of the arene-bound Cr(CO)3 moiety is not new; it was already underlined several decades ago by Lewis11 and Magomedov,12 who outlined the “Lewis-donor properties” of (η6-arene)tricarbonylchromium complexes.13 Our recent efforts concentrated on validating the concept of hemichelation with a variety of Pd(II)-, Pt(II)-, and Rh(I)-based organometallic fragments, using as heteroditopic ligand the conjugated base, i.e. the anion, of a tricarbonyl(η6-indene)chromium derivative7,8b or of anti-bis(tricarbonylchromium)(η6:η6-fluorene).8a In all cases, and particularly with an indene-based ligand, the metal moiety displayed a marked propensity to form hemichelates in which the Cr(CO)3 moiety was formally occupying the fourth site of coordination of the hemichelated d10 or d8 metal. Due to the relative frailty of hemichelation the isolated hemichelates displayed dynamic behavior in solution. Theory suggested that the thermodynamically grounded drive toward stable electron-unsaturated complexes, rather than toward conventional coordination complexes when the option existed, was mostly dominated by attractive Cr(CO)3···M electrostatic interactions.14 Received: April 26, 2015 Published: June 9, 2015 3055

DOI: 10.1021/acs.organomet.5b00349 Organometallics 2015, 34, 3055−3064

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Organometallics

Scheme 2. Subsequent Reactions of 1 with n-BuLi and Palladacycles 2a−c To Afford Conventional (η3Allyl)palladium(II) Complexes

This quite unexpected conclusion was apparently in conflict with the main paradigm of coordination chemistry: that is, the so-called 18-electron rule. Indeed, from classical considerations, if the steric volume of the Cr(CO)3 moiety imposes a stereoelectronic control of reactions at the arene ligand, reaction of the Cr(CO)3-bound indenate ion with an electrophilic metal center should yield above all the less hindered species and lead to conventional coordination bonds with the five-membered ring, mostly of the (η3-indene)metal type. In the vast majority of cases covered in our previous investigations,7,8 this was not the case, the main reason being that in such tricarbonyl(η6-benzyl)chromium anions the charge density is largely delocalized from the benzylic position toward the Cr center and the terminal carbonyl’s O atoms. This delocalization causes a major Coulombic unbalance between the two faces of the Cr-bound arene ligand to which the incoming electrophilic transition metal center is sensitive. To gain more information on the importance of this charge density delocalization in the formation of hemichelates, we were interested in investigating the 3-phenylprop-1-ene-based ligand 115 in order to gauge the preference of a Pd(II) center for the only two possible modes of coordination in such a flexible system: i.e., η3 bonding to the allyl or hemichelation of the Pd(II) center (Scheme 1). In this article we report on our Scheme 1. Bonding Mode Preference for Pd(II) with the Flexible Benzylic Anion Derived from 1

attempts to isolate hemichelates from the conjugated base of complex 1 and disclose the unexpected formation of a bishemichelate of the Pd(I)−Pd(I) unit, the unprecedented structure of which was firmly established and the electronic structure of which was investigated theoretically.

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RESULTS AND DISCUSSION Complex 1 was synthesized by the conventional thermolysis of Cr(CO) 6 in the presence of 3-phenylprop-1-ene and deprotonated by n-BuLi in tetrahydrofuran (THF) at ca. −40 °C; this yielded a slightly brown-yellow solution of the corresponding anion. The latter was treated in distinct experiments with THF solutions of three different μ-chlorobridged palladacycles: i.e., 2a−c (Scheme 2). The reactions yielded the η3-allyl complexes 3a−c in yields spanning 40−57%. These complexes, when isolated by conventional methods, displayed moderate air-stability. The infrared spectra of the isolated species revealed, from inspection of the symmetric CO stretching bands, that none of the three new complexes 3a−c were hemichelates. Indeed, it was reported earlier that the IR spectrum of Pd(II) hemichelates of (η6-indenyl)Cr(CO)3 ligands displays an A band at a reciprocal wavelength ca. 10 cm−1 lower than that for the parent (η6-indene)Cr(CO)3 complex.7,8b For compounds 3a−c, the overall shift of COstretching bands was less than 5 cm−1 with respect to 1. The structures of 3a−c were determined by structural X-ray diffraction analysis on crystals grown by slow diffusion of nheptane in saturated solutions in CH2Cl2. ORTEP-type diagrams are displayed in Figures 1 and 2. For the sake of

Figure 1. ORTEP-type diagrams of the structures of 3a (a) and 3b (b). Thermal ellipsoids are drawn at the 30% probability level with idealized positions for hydrogen atoms. Selected interatomic distances (Å) and angles (deg) for 3a: Cr1−C13 2.27(3), C13−C12 1.50(4), C12−C11 1.440(13), C11−C10 1.312(14), C12−Pd1 2.248(8), C11−Pd1 2.108(11), C10−Pd1 2.101(4), N1−Pd1 2.143(3), C1− Pd1 2.022(4); C12−Pd1−C10 67.1(3), C1−Pd1−N1 82.09(15). Selected interatomic distances (Å) and angles (deg) for 3b: Cr1−C10 2.254(2), C10−C16 1.478(3), C16−C17 1.373(4), C17−C18 1.419(4), Pd1−C16 2.276(2), Pd1−C17 2.146(2), Pd1−C18 2.096(2), Pd1−C1 2.021(2), Pd1−N1 2.149(2); C1−Pd1−N1 82.07(9).

conciseness, acquisition and refinement parameters are given in the Supporting Information. For reasons related to steric repulsion, the (η3-allyl)palladium moiety is oriented in such a manner that the palladacycle remains in the farthest possible position away from the Cr(CO)3 rotor. The most remarkable feature of the structures of 3a−c is the orientation of the palladacyclic fragment with respect to the (η6-arene)Cr(CO)3 moiety. The Pd-bound heteroelement (i.e. the nitrogen atom) is systematically cis with the benzylic carbon position. As a 3056


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result of the theory is consistent with the observed temperature dependence of the 1H NMR spectra of 3a−c, which all display rather marked broadenings of the aromatic proton signals symptomatic of dynamic behavior and indicate the coexistence of two to three species in solution depending on the analyte. For example, the temperature dependence of the spectrum of 3a is characterized not only by band broadenings in the 6−8 ppm region but also by the coexistence of two main sets of signals particularly visible in the allylic ligand region comprised between 4 and 5 ppm at 298 K; the minor components vanish almost completely to traces in the 1H NMR spectrum at low temperatures starting from 243 K. Assigning the dynamic behavior of 3a−c to a single and unique structural change was deemed illusory in the light of further exploratory DFT calculations. Indeed, DFT investigations suggested another energetically plausible structural change in solution. It consists of a haptotropy of the palladacycle and subsequent hemichelation of the Pd center through transition state TS-3a (Scheme 3) that requires a Gibbs barrier of activation ΔG⧧ of ca. 10 kcal/mol at 298.15 K to produce an intermediate η1 species unbound to the Cr(CO)3 moiety: i.e., I-3a (Scheme 3). The conversion of I-3a into 3a″ is a barrierless process; the rather modest energetic requirements for conversion of 3a into 3a″ suggests that the dynamic nature of 3a−c in solution might be a hardly traceable combination of conformational and chemical exchanges. This led us therefore to conclude that unambiguous assignment of structures to the minor components in the spectrum of 3a was simply impossible at this stage. Reductive Disproportionation of 3a,b into Bis-Hemichelate 4. One interesting feature of complexes 3a,b, and of 3c to a lesser extent, is their instability in solution. The first two complexes kept in a stirred solution for several hours slowly decompose into products of a reductive disproportionation, mostly homoleptic chelates of Pd(II) and Pd(I). The spontaneous conversion of 3a,b into complexes 4 and 5a,b (Scheme 4) is not a clean process, though. A rather important

Figure 2. Ortep type diagram of the structure of 3c. Thermal ellipsoids are drawn at the 30% probability level with idealized positions for hydrogen atoms. Selected interatomic distances (Å): Cr1−C18 2.254(5), C18−C24 1.465(7), C24−C25 1.380(7), C25−C26 1.393(7), C24−Pd1 2.257(5), C25−Pd1 2.135(5), C26−Pd1 2.105(5), N1−Pd1 2.082(4), C1−Pd1 2.025(5).

consequence of the trans influence operated by the carbanionic part of the palladacycle, the Pd to benzylic carbon distance, e.g. C12−Pd1 in 3a (Figure 1a), is systematically longer than the Pd to terminal allyl carbon distance, e.g. C10−Pd1 (Figure 1a), by roughly 0.10−0.15 Å. This slight dissymmetry of the η3 bonding does not significantly affect the allylic carbon to carbon distances that vary only by ±0.03 Å around a mean value of 1.39 Å in 3a−c. The preferential orientation of the palladacycle observed in the structures of 3a−c can hardly be related to a dominant electronic effect of the electron-withdrawing Cr(CO)3 moiety, acting as an electron density sink to compensate for the trans effect donation of the carbanionic part of the palladacycle. Density functional theory calculations, which were carried out on model structures of 3a and its rotamer 3a′, wherein the palladacycle is rotated by 180° around an axis perpendicular to the mean plane of the allyl ligand (Scheme 3; cf. the Supporting Information) showed that the two relaxed gas phase singlet ground state structures are isoenergetic within 2 kcal/mol. This

Scheme 4. Conversion of 3a,b into Bis-Hemichelate 4 and Homoleptic Pd(II) Complexes 5a,b and Subsequent Decomposition of 4 into Compound 6

Scheme 3. Conformational and Chemical Exchanges Proposed for 3a According to DFT Investigations

amount of dark green solids precipitates out of the solution during the reaction, suggesting that the oxidation of Cr(0) centers is central to this disproportionation process that leads to the neutral tetranuclear complex 4, in which the two palladium centers supposedly bear formal oxidation state +1 within a coordinatively unsaturated environment. 3057


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allyl)-bridged Pd(I)−Pd(I) complexes reported in the recent literature: i.e., an interpalladium distance of ca. 2.7 Å.16,17 The Pd(I)−Pd(I) unit intersects the interfragment C1−C3 axis. It is worth noting that the positional disorder observed for 4 in the course of structure resolution did not affect the accuracy of atomic positions (see Table 1 for a selection of interatomic distances with esds) of the structure displayed in Figure 3. The Pd−Pd distance is only shorter than the Cr−Pd distance by 0.04 Å and lies in the empirical range expected for typical van der Waals contact interactions, thus suggesting that the Pd−Pd interaction bears a rather modest covalent character. Similarly to previously reported cases of hemichelates of Pd and Rh,7,8 the bis-hemichelate depicted here entails an interaction of a Cr-bound CO ligand with the vicinal Pd center, which is expressed by the Cr2−C11−O2 angle of ca. 170°, with atom C11 pointing toward Pd1 (Figure 3). The nature of this bending is not purely related to steric repulsion but is rather the result of an evanescent C11−Pd1 bonding interaction (vide infra). Quite consistent with previous results gathered for Pd(II)7,8b and Rh(I)8a hemichelates, the powder total reflectance FT-IR spectrum of 4 indicates an averaged shift of the CO ligand stretching bands of about −22 cm−1 with respect to reference compound 1. This shift is relatively strong and denotes a major electron density change at the Cr(CO)3 moiety. In this complex the asymmetric vibrational modes of the latter are not degenerate; they appear as two strong and sharp signals at 1874 and 1835 cm−1. The symmetric A mode gives a strong band at 1928 cm−1. In solution, the 13C NMR signature of the carbonyl carbons of the Cr(CO)3 moiety is a single signal at 233.6 ppm, shielded by 1.5 ppm with respect to the signal of the same ligands in reference compound 1: this spectral feature reveals the flexibility of the structure of 4 in solution in comparison to other hemichelates, for which the 13C NMR spectra at room temperature were generally characterized by two to three broad 13 C singlets for the Cr(CO)3 moiety.7,8b The 13C NMR spectrum clearly indicates that the incipient semibridging CO interaction18 with the Pd centers visible in Figure 3 is unimportant to the dynamics of the Cr(CO)3 moiety in solution. It is worth noting that the frozen CDCl3 solution of complex 4 analyzed by X-band EPR spectroscopy proved to be silent, which suggests that spin coupling is effective between the two Pd(I) centers. To the best of our knowledge the structures of homoleptic bischelates 5a,b determined by X-ray diffraction analysis are unprecedented (Figure 4). The synthesis of complex 5a was first reported by Kazahara and Izumi,19 and its cis was stereochemistry proposed by Longoni et al.20 To the best of our knowledge, the synthesis of compound 5b has not yet been reported in the literature. In contrast with analogous Pt(II) complexes,21 for which both cis and trans isomers can be isolated,20,22 in the case of Pd(II) complexes the cis isomer is purportedly strongly thermodynamically favored.23 The geometric features of 5b are akin to those of 5a. For example, the presence of the fluoro substituent does not greatly influence the aromatic carbon−palladium bond, which has the same length within experimental error. As in most of the structures of similar Pd bis-chelates reported in the literature, the chelating ligands are not coplanar but rather are tilted by ca. 42° to minimize steric repulsion in the immediate coordination sphere of the metal.24 Complex 4 is nonetheless a moderately stable species that decomposes in solution over long periods of time to afford a

Complex 4 was isolated in modest yields spanning 20−30% and purified by crystallization from the reaction mixture after filtration. X-ray diffraction analysis delivered the quasi-C2symmetric structure depicted in Figure 3. Compounds 5a,b

Figure 3. Ortep type diagram of the structure of 4. Thermal ellipsoids are drawn at the 30% probability level with idealized positions for hydrogen atoms. A disordered solvation molecule of benzene was omitted for the sake of clarity. Positions for atoms C1−3 and Pd1 suffered from 50% occupational structural disorder; only one set of positions is shown here. Selected interatomic angles (deg): Cr2− C11−O2 177.06(10), C11−Cr2−C12 95.15(12). See Table 1 for selected interatomic distances.

displayed a much lower stability than 4 and were isolated in minute but sufficient amounts to allow crystallization as well as X-ray diffraction analyses (Figure 4). The structure of 4, though rather uncommon and unique by the noticeable absence of donor ligands at two equatorial coordination sites at the two Pd center, shares the geometrical features of known cases of bis(μ-

Figure 4. ORTEP-type diagrams of the structures of 5a (a) and 5b (b). Thermal ellipsoids are drawn at the 30% probability level with idealized positions for hydrogen atoms. Selected interatomic distances (Å) and angles (deg) for 5a: Pd1−C1 2.001(3), Pd1−C10 1.998(3), Pd1−N1 2.210(3), Pd1−N2 2.216(3), C1−Pd1−C10 96.25(13); N1−Pd1−N2 101.58(10). Selected interatomic distances (Å) and angles (deg) for 5b: Pd1−C1 1.999(4), Pd1−C10 1.991(4), Pd1−N1 2.203(3), Pd1−N2 2.210(3); C1−Pd1−C10 95.92(15), N1−Pd1−N2 102.42(12). 3058


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Organometallics Table 1. Selected Geometrical and Electronic Features Determined for Models of Compounds 4 and 7 qPda

qCra

qC11a

0.44

−0.88

0.62

qPa

dcompb (Å) PBE0-Ddsc/PBE-D3(BJ)

dexpc (Å)

ρe (au)

−1/4∇2ρf (au)

0.046 0.066 0.088 0.088

−0.026 −0.037 −0.044 −0.046

0.281

0.174

0.051 0.102 0.087 0.082

−0.026 −0.040 −0.041 −0.042

Compound 4 Cr−Pd Pd−Pd C11−Pd C3−Pd C1−Pd C2−Pd C4−Pd C4−Cr C4−C3

2.727/2.747 2.690/2.758 2.293/2.297 2.166/2.210 2.167/2.198 2.353/2.301 2.603/2.599 2.244/2.277 1.436/1.444 Compound 7 0.36

2.7863(7) 2.7489(14) 2.526(5) 2.163(4) 2.175(3) 2.271(5) 2.512(5) 2.292(5) 1.430(7)

0.81

Pd−Pd Pd−P Pd−C11 Pd−C17

2.637/− 2.270/− 2.167/− 2.186/−

2.673d 2.284d 2.167d 2.219d

a

Natural population analysis partial charges. bInteratomic distances in computed structures. cExperimental interatomic distances. dData taken from CSDB structure BIPCIM. eElectron density at BCP (3,−1). fLaplacian of the density at BCP.

refcode BIPCIM; Figure 6) was taken arbitrarily as a representative reference case and therefore treated at the same theoretical level as 4.

complex mixture of palladium black, organic, and organometallic products, among which one component was isolated and identified thanks to X-ray diffraction structural analyses as complex 6, which is formally the product of the reductive coupling of the two phenylallyl moieties (Figure 5).

Figure 6. Formula of compound 7.

The hybrid functional PBE0-Ddsc29 was chosen (within the ZORA30 combined with ad hoc all electron singly polarized triple-ζ Slater-type basis sets, i.e. TZP) for NCI, QTAIM, and ELF analyses on geometry relaxed structures. All other computations were carried out with the conventional (ZORA)-PBE-D3(BJ) functional associated with all-electron TZP or quadruply polarized quadruple-ζ basis sets for all elements (QZ4P). Geometric parameters for a geometry of 4 optimized at the (ZORA)-PBE-D3(BJ)/all-electron TZP are given for information in Table 1. The optimized structures were generally consistent with experimental data. In the case of 4, the computed Cr−Pd and Pd−Pd distances were found to be shorter than those in the experimental structure by ca. 0.06 Å (Table 1). In the computed structure of 7 the Pd−Pd distance is only 0.04 Å shorter than that in the experimental structure deposited with the CSDB. Computed structures match the experimental trend of a Pd−Pd distance longer in 4 than in 7, which is correlated consistently with a slightly lower Wiberg bond index in the former complex (WBI = 0.16 in 4 in comparison to 0.18 in 7). Analysis of NCI isosurfaces produced from singlet ground state gas phase structures of 4 and 7 gave sound information on the contribution of attractive NCIs to the metal−metal interaction. Not surprisingly,7,8 in compound 4 the Pd−Cr interaction is noncovalent and attractive, whereas the C11−Pd interaction contains a covalent component that is visible from the ring of the attractive NCI isosurface that surrounds the corresponding

Figure 5. Ortep type diagram of the structure of 6. Thermal ellipsoids are drawn at the 30% probability level with idealized positions for hydrogen atoms. Selected interatomic distances (Å) and angles (deg): C1−C1 1.518(18), C2−C1 1.506(12), C2−C3 1.316(13), C3−C4 1.471(12), C4−Cr1 2.237(8); C10−Cr1−C11 87.7(5), C11−Cr1− C12 90.0(5).

Bonding in Complex 4. Chemical bonding in 4 was addressed using the methodology already called upon in previous reports.7,8 Fragment-wise analysis of orbital interactions between singlet state fragments was used within the frame of the extended transition state−natural orbitals for chemical valence (ETS-NOCV)25 method to assess the nature of the orbital interactions between the Cr(CO)3 fragments and the rest of the molecule. Bader’s quantum theory of atoms in molecules (QTAIM)26 topological analysis of electron density provided some insight into the relative strength of coordination bonds and intermetallic interactions, which was coupled to Yang’s materialization of noncovalent interactions (NCI),27 which produces an intuitive picture of noncovalent interactions between metallic centers in terms of attractive and repulsive interactions. Of course, a reference structure was sought in order to properly gauge the strength of the Pd−Pd interaction in 4. The reported structure of the bis(cyclopentenyl) bispalladium(I) complex 728 (Cambridge Structural Database 3059


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Organometallics “bond” (Figure 7) or “covalent through”. More striking is the apparent absence of such a through in the Pd−Pd segment,

Figure 7. ADFview2013 plot of noncovalent interaction (NCI) regions materialized by reduced density gradient isosurfaces (cutoff value s = 0.02 au, ρ = 0.05 au) colored according to the sign of the signed density λ2ρ (red and blue colors are associated with negatively and positively signed terms) for a gas-phase relaxed singlet ground state model of 4. Calculations were performed with the gas phase singlet ground state geometry optimized at the ZORA-PBE0-Ddsc/allelectron TZP level.

Figure 9. Partial molecular orbital (MO) interaction diagram between identical 3-[tricarbonyl(η6-phenyl)chromium]allyl palladium fragments of complex 4 idealized at C2 symmetry. Selected molecular orbitals 161 and 160 are related to Pd−Pd interactions. MO energies are expressed in hartrees. The red fragment disruption line is drawn over the formula of 4 (upper left). The highest occupied MO does not show any constructive (in-phase) orbital interaction between Pd atoms.

which therefore suggests a much lower covalent component for this interpalladium(I) interaction. In turn, such a covalent through is perceptible in the NCI analysis of 7 (Figure 8); it suggests that the Pd−Pd bond between the two formal Pd(I) centers has a stronger covalent component significantly assisted by attractive NCI.

(HOMO); those MOs result essentially from bonding interactions of dx2−y2 and dxz orbitals at Pd atoms that are counterbalanced by populated antibonding MOs (Figure 9).31 Apart from all other orbital interactions that enable spin coupling through the intricate σ and π backbone of the molecule, these Pd−Pd orbital interactions procure a direct channel for spin coupling between the two centers, which rationalizes the diamagnetic nature of 4. The situation of the Pd−Pd interaction is not exceptional and can be paralleled with the case of Co2(CO)8 reported by Reinhold et al., for which the mere hypothesis of a stabilizing Co−Co covalent bond was ruled out by QTAIM investigations carried out on experimentally and theoretically built electronic density maps. Furthermore, situations wherein spin coupling occurs between nonbonded metal centers14,33 are legion. Worthy of note is the case of the crystal state dimer of a Ni,Pd lantern type complex, in which spin crossover behavior involving two remote Ni(II) centers occurs through noncovalently interacting and distant Pt atoms (Pt−Pt distance ∼3.08 Å).2a Finally, spin-unrestricted computation of the fictitious triplet state (expectation value S2 = 2.008) of 4 indicates that residual α spin density (α spin density − β spin density) is not centered at the Pd atoms but rather is delocalized over Pd and Cr atoms as well as over atoms C1 and C3 (cf. the Supporting Information). Recently, Braunstein et al.34 have studied related Pd(I) species, for which they reported that the electron localization function (ELF)35 of a typical Pd(I)−Pd(I) bond in such μ-allylbridged species possesses valence basins of ∼0.30−0.20 electron between the two Pd(I) centers. In the case of 7, the value of the corresponding valence basin was 0.22 electron for the Pd−Pd bond (cf. the Supporting Information). In the case of 4, the ELF valence basin for Pd−Pd as well as for Pd−Cr and C11−Pd valence basins was 0.18 electron (cf. Supporting Information), which corroborates the lower covalent character of the Pd−Pd interaction inferred from NCI analysis in comparison to 7. Quantum theory of atoms in molecules (QTAIM)26 analyses were carried out with molecular geometries optimized at the (ZORA)-PBE0-Ddsc/all-electron TZP level, for which the

Figure 8. ADFview2013 plot of noncovalent interaction (NCI) regions materialized by reduced density gradient isosurfaces (cutoff value s = 0.02 au, ρ = 0.05 au) colored according to the sign of the signed density λ2ρ27 (red and blue colors are associated with negatively and positively signed terms) for a gas-phase relaxed singlet ground state model of 7. Calculations were performed with gas phase singlet ground state geometry optimized at the ZORA-PBE0-Ddsc/all-electron TZP level. A narrow covalent through can be noticed in the red attractive reduced density gradient isosurface surrounding the Pd−Pd segment.

Given the uncertainty of the existence of a covalent bond arising from QTAIM and NCI analyses, the question of the singlet spin state of 4 (inferred from EPR spectroscopy) was addressed by performing an interfragment orbital interaction analysis (Figure 9) to trace Pd−Pd orbital interactions. It revealed nonbonding situations (two orbital−four-electron) between Pd atoms that imply π-type orbitals lying rather deep in energy below the highest occupied molecular orbital 3060


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Organometallics electron density was recomputed with the same functional without the ZORA, using regular all-electron TZP basis sets for all elements except metals, for which frozen cores were used (up to 3d). Table 1 gives the topological bond critical points (BCPs) of interest located in 4 and 7. No BCP (3,−1) was located for the Cr−Pd segment in 4.7,8 The properties of the BCPs in 4 and 7 associated with the Pd−Pd bond path are almost identical. The electron density at BCP ρ is of the same order, as is the value of the −1/4∇2ρ density Laplacian, which suggests that the interpalladium interaction has roughly the same nature in 4 and 7: i.e., a closed-shell interaction.26a The BCP at bond path C11−Pd in complex 4 bears values of ρ and −1/4∇2ρ very akin to those of the bond paths associated with C3−Pd1 and C1−Pd1, which leads one to consider the C11− Pd interaction like a typical coordination bond of ionic character, in the considered static representation of 4. As was mentioned above, 13C NMR data revealed the unhindered rotation of the Cr(CO)3 rotor in solution, which strongly suggests that the C11−Pd bonding interaction, i.e. an incipient semibridging CO situation,36 is energetically very weak. The C4−C3 bond path provides a good internal reference for a socalled shared interaction typical of covalent bonds: −1/4∇2ρ > 0 and ρ > 10−2 au. ETS-NOCV analyses25 are particularly useful for evaluating the orbital components of intra- and intermolecular bonding interactions when the contribution of covalence is weak although not fully absent as in hemichelates. Such analyses provide a dichotomy of intramolecular donor−acceptor relationships between molecular fragments chosen for their assumed contribution to an interaction of interest. Furthermore, ETS-NOCV analysis may provide a symmetry-ordered decomposition of bonding interactions of σ, π, and δ type, for which energetic contributions to the total orbital interaction energy for a particular fragment interaction scheme can be weighed. This method of analysis was used here with compound 4 to address the nature of the interaction between the bis-palladium bis-allyl unit and the Cr(CO)3 moieties. The two Cr(CO)3 moieties were treated as parts of one fictitious molecular fragment (Figure 10). The total interaction energy ΔEint between the prepared fragments of the bis-palladium bis-allyl unit and the pair of Cr(CO)3 units was found to amount to −199.1 kcal/mol: that is, around −100 kcal/mol per Cr(CO)3 moiety. The associated orbital interaction energy component ΔEorb is −353.7 kcal/ mol: i.e., around 175 kcal/mol per Cr(CO)3 moiety. Plots of deformation density isosurfaces in the case of 4, within the chosen fragmentation scheme, mostly provide information on the bonding of the Cr(CO)3 moieties to the phenyl moieties. However, they reveal an interesting contribution of various atomic centers to the minute buildup of electron density in the Pd−Cr−C11 triangle. This buildup of electron density is materialized in Figure 10 by a banana-shaped deformation density isosurface shown in blue in Δρ1: that is, a volume of the interatomic space that, upon interaction of the considered fragments, receives electron density from the red density isosurfaces located mostly at the Cr atoms and at C11. Of course, in Δρ1 the density donation is also directed toward the Pd and σ type bonding orbitals, ensuring the coordination of the arene ligands to the Cr center. Significant back-donation from Pd-centered orbitals toward arene−Cr bonds and Crcentered orbitals is also noticed in density deformation isosurfaces Δρ3 and Δρ5 (full drawings of other significant density deformation isosurfaces are provided in the Supporting

Figure 10. Selected ETS-NOCV deformation densities Δρ and the associated orbital interaction energy ΔEorb (in kcal/mol) for the interaction of closed-shell prepared Cr(CO)3 moieties and the bispalladium bis-allyl fragment of 4 in a singlet ground state gas phase geometry optimized at the ZORA-PBE-D3(BJ)/all-electron QZ4P level. Red and blue isosurfaces materialize regions where charge density is depleted and built up, respectively: electron density transfer operates from red areas to blue ones upon interaction. The deformation density isosurface contour was set to 0.004 e/bohr.3

Information). This back-donation of electron density to the Cr center is energetically dominant according to the ETS-NOCV analysis (ΔEorb‑3 + ΔEorb‑5 > ΔEorb‑1) and might explain the shift of the C−O bond stretching mode to lower energies in the IR spectrum of 4 in comparison to 1. In other words, hemichelation of the Pd(I) center entails a weak Cr,C11−Pd donor−acceptor interaction through the semibridging CO interaction and a concomitant overall enrichment in electron density at the Cr centers: noncovalent attractive interactions, however, dominate the picture by conferring cohesion to the molecular structure. Opposite natural charges qCr and qPd (Table 1) provide an intuitive picture of the role played by electrostatics in 4, as in other hemichelates of Pd(II) and Rh(I).7,8

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CONCLUSION In this article we demonstrate that, upon deprotonation of a 3phenylprop-1-ene-based ligand such as 1, the formation of a Pd(II) hemichelate is not favored a priori. Even though the dynamic behavior of the isolated π-allylic Pd(II) complexes 3a−c in solution could not be solved, theoretical modeling on 3a suggests that the isolated complex can readily be subject to conformational exchange and haptotropic displacement of the Pd(II) center to form a short-lived hemichelate. This dynamic behavior of 3a,b might be at the origin of the propensity of these complexes to disproportionate into homoleptic 4 and 5a,b possibly by a reductive mechanism. To the best of our knowledge, the structure of 4 is the first of the kind for this class of μ-allyl-bridged bis-Pd(I) complexes. The bis-hemichelate bears structural and electronic characteristics very akin 3061


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Organometallics to those of previously reported Pd(II) hemichelates.7,8 The Cr(CO)3 moieties, although not innocent in the stabilization of the complex, do not establish strong covalent bonds with the vicinal Pd(I) centers. We propose that, in solution, complex 4 possesses a time-averaged formal 14-valence-electron configuration at each Pd(I) center due to the unhindered fast rotation of the Cr(CO)3 moieties, which continuously disrupts the weak contribution of the incipient donor−acceptor semibridging carbonyl−palladium interaction. The ELF, NCI, and QTAIM analyses of 4 suggest that the Pd−Pd interaction is predominantly a closed-shell interaction. To the best of our knowledge, the nature of the Pd−Pd d9−d9 interaction37 in bisμ-allyl complexes has not been investigated yet with a wide scope that includes noncovalent interactions and treats their role in molecular cohesion. Previous studies by Kostić et al.17b (Fenske−Hall method), Yamamoto et al.17a (Extended Hückel MO theory) and Hazari et al.16b (DFT) considered the Pd(I)− Pd(I) interaction from the partial, albeit classical, point of view of covalence through MO interactions. Because it fell out of the scope of this paper, the treatment of the Pd(I)−Pd(I) interaction was not extended to other cases. Complex 4 raises the question of the influence of the coordination environment on the bonding within the Pd(I)−Pd(I) unit in a way similar to the case of the diamagnetic cationic bis-μ-butadiene Pd(I)− Pd(I) complex reported by Kurosawa and co-workers, in which an unusually long Pd(I)−Pd(I) distance (d ≈ 3.2 Å) was observed.38 Investigations should certainly be extended to other examples of bis-μ-allyl-bridged Pd(I)−Pd(I) complexes to gauge the weight of covalence in the bis-palladium(I) unit as well as the influence of explicit donor ligands such as in 7 that may reinstate some covalent character into the Pd−Pd interaction by a lifting of nonbonding Pd−Pd orbital interactions. Finally, the unexpected formation of 4 validates the concept of hemichelation7,8 as a reasonable alternative coordination mode for the stabilization of electron-unsaturated metal centers, which is the topic of ongoing research.

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with Grimme’s DFT-D3(BJ) implementation of dispersion with a Becke−Johnson (BJ) damping function.3 Corminboeuf’s PBE043Ddsc29 hybrid functional was also used in AIM and NCI analyses. Within the PBE scheme, electron correlation was treated within the local density approximation (LDA) in the PW9244 parametrization. Unless otherwise stated, all computations were carried out using scalar relativistic corrections within the zeroth order regular approximation (ZORA) for relativistic effects30 with ad hoc all-electron (AE) polarized triple-ζ (TZP) Slater type basis sets. QTAIM analyses were carried out without scalar relativistic corrections for basis sets starting from geometries optimized at the (ZORA) PBE0-Ddsc/allelectron TZP level. Geometry optimizations by energy gradient minimization were carried out in all cases with grid accuracy comprised between 4.5 and 7.5, an energy gradient convergence criterion of 10−3 au, and a tight to very tight SCF convergence criterion. Counterpoise correction for basis set superposition error (BSSE) was neglected throughout this study. ETS-NOCV analyses as well as calculations of vibrational modes were performed with optimized geometries using ADF2013 subroutines. Vibrational modes were analytically computed to verify that the optimized geometries were related to energy minima: statistical thermodynamic data at 298.15 K were extracted for further determination of enthalpies and variations of Gibbs free enthalpies by conventional methods. Natural population analyses (NPA) as well as Wiberg index determinations were performed with geometries of models relaxed at the (ZORA) PBE-D3(BJ) level using all-electron TZP or QZ4P basis sets with the GENNBO45 6.0 module of ADF. QTAIM and NCI analyses were carried out using the modules embedded within ADF2013. Representations of molecular structures and isosurfaces were produced with ADFview 2013. X-ray Diffraction Analyses. Acquisition and processing parameters are displayed in Table S1 (cf. the Supporting Information). Reflections were collected with Nonius KappaCCD and APEX diffractometers equipped with an Oxford Cryosystem liquid N2 device, using Mo Kα radiation (λ = 0.71073 Å). The crystal−detector distance was 38 mm. The cell parameters were determined (APEX2 software46) from reflections taken from 3 sets of 12 frames, each at 10 s exposure. The structures were solved by direct methods using the program SHELXS-97.47 The refinement and all further calculations were carried out using SHELXL-97.48 The crystal structures acquired with the Nonius Kappa CCD instrument were solved using SIR-9749 and refined with SHELXL-97.48 The refinement and all further calculations were carried out using SHELXL-97.48 The H atoms were included in calculated positions and treated as riding atoms using SHELXL default parameters. The non-H atoms were refined anisotropically, using weighted full-matrix least squares on F2. A semiempirical absorption correction was applied using SADABS in APEX2.46 Synthesis of 1-[Tricarbonyl(η6-phenyl)chromium(0)]allyl (1).15 A mixture of Cr(CO)6 (7.0 g, 31.81 mmol), allylbenzene (7.6 mL, 57.2 mmol), THF (15 mL), and di-n-butyl ether (150 mL) was gently refluxed for 144 h under a permanent flow of argon. The resulting orange-red solution was cooled to room temperature, filtered through Celite, and evaporated to dryness. The resulting residue was dissolved in dichloromethane, and silica gel was added to the resulting solution, which was evaporated to dryness. The resulting coated silica gel was loaded on the top of a silica gel column packed in n-pentane. Flash chromatographic separation was performed by eluting the complex with a 1:1 mixture of CH2Cl2 and pentane. The resulting yellow solution was stripped of solvents under reduced pressure to afford a yellow oil. Further trituration in cold n-pentane afforded a canary yellow solid, which was filtered and dried under reduced pressure (5.5 g, 21.6 mmol, 68%). Anal. Calcd for C12H10CrO3: C, 56.70; H, 3.97. Found: C, 56.78; H, 3.95. IR (cm−1) ν(CO): 1951 (s), 1851 (vs). 1H NMR (300 MHz, C6D6, 298 K): δ 5.54 (ddt, J = 16.8, 10.0, 6.7 Hz, 1H, H11), 4.87 (dq, J = 10.1, 1.3 Hz, 1H, H12a), 4.78 (dd, J = 17.0, 1.6 Hz, 1H, H12b), 4.44 (t, J = 6.4 Hz, 2H, H3, H5), 4.36−4.31 (m, 2H, H2, H6), 4.29−4.21 (m, 1H, H4), 2.58 (dt, J = 6.8, 1.4 Hz, 2H, H10). 13C NMR (101 MHz, C6D6, 298 K): δ 233.54 (C8, C9, C7), 135.08 (C11), 117.9 (C12), 111.01 (C1), 93.4 (C3, C5), 92.5 (C2, C6),

EXPERIMENTAL SECTION

General Considerations. All experiments were carried out under a dry argon atmosphere using standard Schlenk techniques or in an argon-filled glovebox when necessary. n-Butyllithium was purchased from Aldrich Chemical Co. as a 1.6 M solution in hexanes, hexacarbonylchromium was purchased from ABCR, and 3-phenylprop-1-ene (99%) was purchased from Aldrich Chemical Co. Celite 545 was purchased from VWR Prolabo. 4-(tert-Butyl)-2-(p-N,Ndimethylaminophenyl)pyridine was prepared according to a literature procedure.39 The palladacycles used in this study were prepared according to literature procedures.40 Anhydrous tetrahydrofuran and diethyl ether were distilled from purple solutions of Na/benzophenone under argon. All other solvents were distilled over sodium or CaH2 under argon. Deuterated solvents were dried over sodium or CaH2 and purified by trap-to-trap techniques, degassed by freeze−pump−thaw cycles, and stored under argon. 1H and 13C NMR spectra were obtained on Bruker DPX 300 and 400, Avance I 500, and Avance III 600 spectrometers. Chemical shifts (expressed in parts per million) were referenced against solvent peaks. Full NMR spectral assignments are provided in the Supporting Information; the atomic numbering scheme used for NMR assignments is detailed in the Supporting Information. Infrared spectra of powdered amorphous samples were acquired with a Fourier transform-IR Bruker alpha spectrometer using an ATR solid state sample cell. Computational Details. Computations were performed with methods of the density functional theory: i.e., the Perdew−Burke− Ernzerhof (PBE) GGA functional41 implemented in the Amsterdam Density Functional package34,42 (ADF2013 version) and augmented 3062


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resulting solution was filtered through Celite and the filtrate cooled just above the freezing point of benzene for several hours. The supernatant was removed, and the reddish crystals of 4 were dried under vacuum. Stirring compound 4 in toluene for several hours resulted in the production of large amounts of palladium black, compound 1, and minute amounts of compound 6, which was essentially characterized by structural X-ray diffraction analysis. Anal. Calcd for C24H18Cr2O6Pd2•0.5C6H6: C, 42.60; H, 3.18. Found: C, 42.79; H, 2.99. IR (cm−1) ν(CO): 1928 (s), 1874 (s), 1835 (vs). 1H NMR (600 MHz, C6D6, 298 K): δ 5.81−5.72 (m, 2H, H11), 5.56 (d, J = 15.8 Hz, 2H, H10), 4.76−4.70 (m, 4H, H2, H6), 4.54 (t, J = 6.5 Hz, 4H, H3, H5), 4.32 (tt, J = 6.2, 1.0 Hz, 2H, H4), 1.99−1.94 (m, 4H, H12). 13C NMR (151 MHz, C6D6, 298 K): δ 233.6 (C8, C9, C7), 132.2 (C11), 127.6 (C10), 106.3 (C1), 93.1 (C3, C5), 90.8 (C4), 90.3 (C2, C6), 32.3 (C12). HRMS-ESI (m/z): [M/2]+ calcd for C12H9CrO3Pd, 358.8992; found, 358.8990. Data for compound 6 are as follows. HRMS-ESI (m/z): [M + 1Na]+ calcd for C24H18Cr2O6, 528.9806; found, 528.9808.

90.3 (C4), 38.7 (C10). HRMS-ESI (m/z): [M]+ calcd for C12H10CrO3, 254.0035; found, 254.0039. General Procedure for the Synthesis of Compounds 3a−c. Complex 1 was dissolved in THF (5 mL), and a solution of n-BuLi in hexanes was added at −40 °C under argon. The resulting solution was transferred after 30 min via cannula to another Schlenk vessel containing a THF (3 mL) solution of the corresponding μ-chlorobridged palladacycle:, i.e., 2a−c. The resulting solution was stirred for 30 min while the temperature was slowly raised to −20 °C. At this temperature the solvent was removed and the residue extracted with cold diethyl ether. Finally, filtration over Celite and recrystallization from a dichloromethane/pentane mixture led to the expected yellowish solid. Synthesis of Compound 3a. Complex 1 (0.200 g, 0.79 mmol), nBuLi (0.54 mL, 0.87 mmol), 2a (0.226 g, 0.41 mmol): bimetallic compound 3a (0.220 g, 0.22 mmol, 57% yield). Anal. Calcd for C21H21CrNO3Pd•0.4CH2Cl2: C, 48.70; H, 4.16; N, 2.65. Found: C, 48.63; H, 4.15; N, 2.29. IR (cm−1) ν(CO): 1950 (s), 1896 (s) 1860 (vs). 1H NMR (600 MHz, C7D8, 223 K): δ 7.82 (dd, J = 5.7, 2.9 Hz, 1H, H14), 7.22−7.17 (m, 2H, H15, H16), 7.05 (d, J = 5.7 Hz, 1H, H17), 5.30 (td, J = 12.2, 7.1 Hz, 1H, H11), 4.60 (q, J = 5.6 Hz, 2H, H3, H5), 4.18−4.13 (m, 3H, H2, H4, H6), 3.19 (d, J = 10.3 Hz, 1H, H19a), 3.04− 2.97 (m, 3H, H10, H12a, H19b), 2.88 (d, J = 11.9 Hz, 1H, H12b), 1.89 (s, 3H, H21), 1.68 (s, 3H, H20). 13C NMR (151 MHz, C7D8, 223 K): δ 234.2 (C9, C8, C7), 161.5 (C13), 148.5 (C18), 140.3 (C14), 126.0 (C15), 124.6 (C16), 122.4 (C17), 113.4 (C11), 113.0 (C1), 94.4 (C3, C5), 90.1 (C6), 87.5 (C4), 85.1 (C2), 75.0 (C10), 72.4 (C19), 50.4 (C20), 49.9 (C21), 45.9 (C12). HRMS-ESI (m/z): [M + 1H]+ calcd for C21H21CrNO3Pd, 494.0034; found, 494.0035. Synthesis of Compound 3b. Complex 1 (0.200 g, 0.79 mmol), nBuLi (0.54 mL, 0.87 mmol), 2b (0.241 g, 0.41 mmol): bimetallic compound 3b (0.171 g, 0.33 mmol, 42% yield). Anal. Calcd for C21H20CrFNO3Pd: C, 49.28; H, 3.94; N, 2.74. Found: C, 49.18; H, 4.08; N, 2.70. IR (cm−1) ν(CO): 1959 (s), 1940 (s) 1858 (vs). 1H NMR (600 MHz, C7D8, 233 K): δ 7.66 (dd, J = 7.9, 2.6 Hz, 1H, H14), 6.89−6.84 (m, 1H, H16), 6.82 (d, J = 5.1 Hz, 1H, H17), 5.17 (td, J = 12.3, 7.1 Hz, 1H, H11), 4.59−4.55 (m, 2H, H3, H5), 4.16 (t, J = 6.2 Hz, 1H, H4), 4.14 (d, J = 6.8 Hz, 2H, H2, H6), 3.06 (d, J = 13.4 Hz, 1H, H19a), 2.97−2.85 (m, 3H, H10, H12a, H19b), 2.72 (dt, J = 11.8, 1.4 Hz, 1H, H12b), 1.84 (s, 3H, H20), 1.62 (s, 3H, H21). 13C NMR (151 MHz, C7D8, 233 K): δ 233.9 (C9, C8, C7), 164.5 (d, J = 1.6 Hz, C13), 160.9 (d, J = 248.2 Hz, C15), 143.8 (d, J = 2.2 Hz, C18), 125.9 (d, J = 15.3 Hz, C14), 123.0 (d, J = 7.1 Hz, C17), 113.2 (C11), 112.5 (C1), 110.7 (d, J = 22.2 Hz, C16), 94.1 (C3), 94.09 (C5), 90.2 (C6), 87.7 (C4), 85.2 (C2), 75.5 (C10), 71.7 (C19), 50.2 (C20), 49.8 (C21), 46.2 (C12). 19F NMR (282 MHz, C6D6): δ −118.5. HRMS-ESI (m/z): [M + 1H]+ calcd for C21H20CrFNO3Pd, 511.9940; found, 511.9963. Synthesis of Compound 3c. Complex 1 (0.200 g, 0.79 mmol), nBuLi (0.54 mL, 0.87 mmol), 2c (0.317 g, 0.40 mmol): bimetallic compound 3c (0.247 g, 0.40 mmol, 51% yield). Anal. Calcd for C29H30CrN2O3Pd: C, 56.82; H, 4.93; N, 4.54. Found: C, 56.66; H, 5.03; N, 4.77. IR (cm−1) ν(CO): 1951 (s), 1866 (vs). 1H NMR (600 MHz, C7D8, 233 K): δ 8.26 (d, J = 5.9 Hz, 1H, H13), 7.78 (d, J = 8.7 Hz, 1H, H19), 7.68 (d, J = 2.0 Hz, 1H, H16), 6.48 (dd, J = 8.6, 2.6 Hz, 1H, H20), 6.44 (dd, J = 5.9, 2.1 Hz, 1H, H14), 6.15 (d, J = 2.5 Hz, 1H, H22), 5.94 (q, J = 11.3, 10.8 Hz, 1H, H11), 5.08 (d, J = 6.9 Hz, 1H, H2), 4.55 (q, J = 6.3 Hz, 1H, H5), 4.45 (d, J = 6.3 Hz, 1H, H6), 4.35 (t, J = 6.6 Hz, 1H, H3), 4.18 (t, J = 6.3 Hz, 1H, H4), 3.72 (d, J = 8.2 Hz, 1H, H12a), 3.43 (d, J = 14.0 Hz, 1H, H12b), 3.15 (d, J = 10.9 Hz, 1H, H10), 2.51 (s, 6H, H29, H28), 1.13 (s, 9H, H25, H26, H27). 13C NMR (151 MHz, C7D8, 233 K): δ 234.4 (C8, C9, C7), 166.8 (C23), 166.0 (C17), 161.7 (C15), 154.1 (C13), 150.3 (C21), 135.8 (C18), 124.6 (C19), 119.4 (C22), 118.1 (C14), 116.8 (C1), 116.4 (C11), 114.7 (C16), 108.0 (C20), 95.5 (C3), 94.8 (C5), 92.1 (C6), 88.8 (C2), 87.3 (C4), 69.0 (C12), 55.1 (C10), 40.5 (C29, C28), 34.9 (C24), 29.9 (C25, C26, C27). HRMS-ESI (m/z): [M + 1Na]+ calcd for C29H30CrN2O3Pd, 635.0589; found, 635.0573. General Procedure for the Formation of 4. Optimal yields were obtained by stirring 3a,b (0.200 g, ca. 0.4 mmol) for 12 h in dry benzene (5 mL) to give 4 (0.032 g, 0.04 mmol, 22% yield). The

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Tables, figures, and CIF and XYZ files giving acquisition and refinement parameters for the structures of 3a−c, 4, 5a,b, and 6 and the associated lists of geometrical parameters, energies of all singlet ground state geometries of relevant species, computed vibrational modes, QTAIM, ETS-NOCV, and ELF plots, all computed molecule Cartesian coordinates in a format for convenient visualization, and crystallographic data for all relevant structures disclosed herein. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00349. Corresponding Author

*J.-P.D.: fax, (+33) 03 68 85 00 01; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. Sylvie Choua is acknowledged for explorative EPR investigations on compound 4. This work was supported by the National Research Agency (ANR project WEAKINTERMET-2DA), the Laboratory of Excellence (LABEX) “Chemistry of Complex Systems”, the University of Strasbourg, and the Centre National de la Recherche Scientifique.

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REFERENCES

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