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

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Ubiquity of cis-Halide → Isocyanide Direct Interligand Interaction in Organometallic Complexes Niccolò Bartalucci,† Leonardo Belpassi,‡ Fabio Marchetti,*,† Guido Pampaloni,† Stefano Zacchini,§ and Gianluca Ciancaleoni*,† †

Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Giuseppe Moruzzi 13, Pisa 56124, Italy Istituto di Scienze e Tecnologie Molecolari del CNR (CNR-ISTM), c/o Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, via Elce di Sotto 8, Perugia I-06123, Italy § Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy Downloaded via STOCKHOLM UNIV on November 15, 2018 at 20:26:03 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: We recently reported a density functional theory (DFT) analysis of the Nb(V)−C bond in various NbCl5(L) complexes, discovering that the carbon ligand L receives electronic density from the metal (classical back-donation) and from the chlorides in the cis position (direct interligand interaction). Here we report the synthesis and the structural characterization of two new coordination compounds of niobium pentahalides, i.e., NbX5(CNXyl) (X = Cl, Br; Xyl = 2,6C6H3Me2), and the corresponding DFT analyses of the Nb(V)−C bond using the Natural Orbitals for Chemical ValenceCharge Displacement (NOCV-CD) approach, confirming the presence of a cis-halide → isocyanide direct interligand interaction. To verify whether the latter is limited to Nb complexes or not, we performed a NOCV-CD analysis on a series of several organometallic complexes based on Ti(IV), Nb(V), Ta(V), Rh(III), Pd(II), and Au(III), all of which bear one halide ligand and m-xylyl-isocyanide in a mutual cis position, revealing that the cis-halide → isocyanide interaction is always present.

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INTRODUCTION The Dewar−Chatt−Duncanson (DCD) model,1,2 which 60 years ago described the coordination bond between coinage metals and an olefin in terms of σ donation and π backdonation, has had a crucial importance in coordination chemistry and beyond. Indeed, the DCD model has been used as a framework to interpret spectroscopic results,3 to rationally design catalysts,3−6 but also applied to main block elements, i.e., to describe the bond between phosphinidenes or selenium and N-heterocyclic carbenes (NHC) (Scheme 1).7−10 With time, the coordination bond has become progressively clearer, and also carbonyl complexes called “not classical”11 (wherein the metal is often considered unable to

back-donate) have been recently demonstrated to possess a not negligible back-donation component, together with a strong polarization effect that originates their “not classical” behavior.12,13 Also for high-valent transition-metal complexes, the general idea was that back-donation was suppressed by the formal d0 electronic configuration. In some cases, it has been found that the angle between a NHC ligand and a halide in the cis mutual position was smaller than 90°,14−17 as if a direct interligand interaction could be present, possibly as a consequence of the absence of back-donation.15 In order to shed some light on this topic, some of us theoretically analyzed [NbCln(L′)m]x systems (n = 5 or 6; m = 1 or 2; L′ = NHC, CO, CN, or CNH; x = +1, 0, or −1),18 using the charge displacement (CD) approach.5,19−21 According to the results, Nb(V) is capable of back-donating electronic density to the NHC from its d orbitals (partially filled up by the halide electrons), whereas the Cl → NHC direct interaction was absent. Substituting the NHC ligand with CO, the Cl → L interaction becomes clearly visible, even overcoming the importance of the Nb → CO back-donation, whereas the use of the simplest isocyanide, CNH, led to the coexistence of the Cl → L and Nb → L interactions.

Scheme 1. Examples of Application for the DCD Model

Received: July 24, 2018

© XXXX American Chemical Society

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DOI: 10.1021/acs.inorgchem.8b02088 Inorg. Chem. XXXX, XXX, XXX−XXX

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upfield shift upon coordination to the niobium center. The Nb spectra consist of a single resonance at −124 ppm (1) and 479 ppm (2). As a comparison, compounds NbX5(NHC) [NHC = 1,3-bis(2,6-diisopropyl-phenyl)imidazol-2-ylidene], in benzene solution, give rise to 93Nb NMR resonances at −318 ppm (X = Cl) and −65 ppm (X = Br).15,16 On the other hand, the 93Nb chemical shift values reported for a variety of NbX5(L) complexes (L = neutral chalcogen ligand) fall within the ranges −10/250 ppm (X = Cl) and 750/775 ppm (X = Br), respectively.24,25 Compounds 1−2 display low solubility in benzene and toluene, while they are well soluble (albeit not indefinitely stable, see below) in chlorinated solvents. Indeed, a CD2Cl2 mixture of NbCl5 and CNXyl could be NMR characterized at RT soon after the mixing of the reactants at −30 °C (see Experimental Section for details), but, under these conditions, the 93Nb NMR spectrum clearly indicated the presence of the [NbCl6]− anion (typical sharp resonance close to 0 ppm24−28) and of a minor amount of 1 (−131 ppm). This outcome suggests that the relatively higher polarity of dichloromethane (respect to benzene, at least) favors the formation of ionic coordination compound(s), [NbCl4(CNXyl)n][NbCl6] (n = 1 or 2) rather than the neutral 1. Moreover, the possible generation of [NbCl4(CNXyl)][NbCl6], containing a pentacoordinated cation, could be related to the steric hindrance exerted by the CNXyl ligand. In general, NbX5(L) and [NbX4(L)2][NbX6] can be viewed as isomers originated, respectively, from symmetric and asymmetric cleavage of the dinuclear structure of NbX5 by addition of L (Scheme 3b).23,29−31 Only one of the two modes is usually observed depending on the nature of L; however, isomerism between the two kind of species was previously detected in solution for NHC carbene derivatives.16,32 When 1−2 were stored in dichloromethane at ambient temperature, progressive degradation occurred over 24 h, affording green-colored substances. Subsequent IR analysis on the residue originated from 1 pointed out the disappearance of the CN functional group and the occurrence of new absorptions around 1650 cm−1 (probably caused by a lowering of the carbon−nitrogen bond order), while the 1H NMR spectrum revealed the presence of a complicated mixture of species. According to these data, it is presumable that the isocyanide moiety in 1−2 undergoes unclean addition reactions involving the solvent. Views of the molecular structures of 1 and 2 are shown in Figure 1 and Figure 2, respectively, while relevant bonding parameters are reported in Table 1. Compounds 1−2 represent very rare cases of crystallographically characterized coordination adducts of a niobium pentahalide with a neutral carbon ligand, the only other example being NbCl 5 (NHC′) (NHC′ = 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene).15 On the other hand, 1 is related to the previously reported Nb(V) complex NbCp*Cl4(CNXyl).33 The Nb(1) centers display approximately octahedral coordination and are slightly displaced [0.321 and 0.350 Å for 1 and 2, respectively, toward the apical X(1)] from the plane individuated by the four halides. This configuration is common with other NbX5L complexes (L = monodentate neutral ligand).15,30,32−34 The Nb(1)−C(1) distance [2.319(5) and 2.329(12) Å for 1 and 2, respectively] is significantly longer than both Nb(V)alkylidene moieties35 and even classical Nb(V)-alkyl σbonds;36−38 for instance, the longest Nb−C distance in NbCl2Me3 measures 2.152(4) Å.36 At the same time,

Then we decided to start an experimental study on [NbX5(CNXyl)] systems, choosing as ligand the m-xylylisocyanide (L) for its stabilizing properties, likely due to the possibility of accepting more back-donation and delocalize the electronic density on the aromatic ring. Both complexes [NbCl5(CNXyl)] (1) and [NbBr5(CNXyl)] (2) have been successfully synthesized and characterized by IR and NMR spectroscopy and X-ray crystallography. The theoretical analysis by Natural Orbitals for Chemical Valence-Charge Displacement (NOCV-CD)22 confirms and quantifies the presence of Cl → L and M → L interactions. Finally, the scope of the theoretical study has been extended to several organometallic complexes found in the literature (Scheme 2), all of them bearing one halide ligand and L in the

93

Scheme 2. Structures of the Metal Complexes Selected for the Present Study

mutual cis position. Our results demonstrate that the cis-halide → isocyanide interaction is always present, even if its importance depends on the nature of the metal.

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RESULTS AND DISCUSSION Complexes [NbX5(CNXyl)] (X = Cl, 1; X = Br, 2) were obtained by allowing the dimeric halides NbX523 to react with one equivalent of xylyl-isocyanide in toluene (Scheme 3a). Scheme 3. Symmetric (a) and Asymmetric (b) Cleavage of the Dinuclear Structure of NbX5 (X = Cl, Br) upon Addition of a Neutral Ligand L

The products were isolated as yellow/orange solids, and characterized by elemental analysis, IR and NMR spectroscopy, and X-ray diffraction. In the IR spectra of 1 and 2 (solid state), the absorption due to the CN bond falls at 2221 and 2209 cm−1, respectively, i.e., at a considerably higher wavenumber value compared to the noncoordinated isocyanide (2121 cm−1). On the other hand, the 1H NMR resonances (C6D6 solution) of CNXyl experience a significant B

DOI: 10.1021/acs.inorgchem.8b02088 Inorg. Chem. XXXX, XXX, XXX−XXX

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the back-donation,13 but to different extents in the case of 1 and 2 (see later). Moreover, the isonitrile carbon C(1) displays C···X contacts with the four equatorial halide ligands [3.049 and 3.141 Å for 1 and 2, respectively] which are within the sum of the van der Waals radii of the respective atoms [X = Cl, sum = 3.45 Å; X = Br, sum = 3.55 Å].39 This feature is not uncommon in [MCln(L)] complexes33,40,41 or in similar complexes in which a carbene is present instead of the isonitrile.14−17,42 Interestingly, the crystal packing of 1 and 2 is quite different: in 2, the bromine in trans position with respect to L (Brtrans) is involved in intermolecular short contacts with the Brtrans of other two molecules of 2 (Brtrans···Brtrans = 3.626 Å, see Supporting Information), whereas in 1 Cltrans is not involved in any short contact, but all the Clcis interact with the methyl hydrogens of L belonging to a different molecule through weak hydrogen bonds (Supporting Information). Bond Analysis. A typical strategy of bond analysis is the fragmentation of the adduct into two moieties. The comparison of the electron density of the adduct with the sum of the electron densities of the fragments provides information about their interaction. Unfortunately, the Kohn− Sham orbitals of the adduct or the fragments are not able to provide such information (Supporting Information), whereas charge displacement, among other methods, was already demonstrated to be an effective strategy.13,18−20 In particular, the latter is able to quantify the different electronic fluxes (ligand → metal and metal → ligand, for example) and associate each of them to a specific DCD component, especially if the adduct presents an appropriate symmetry.21 For example, in our previous contribution,18 the Nb−C bond components were decomposed by symmetry. In the case of nonsymmetric adducts or fragments, the NOCV-CD approach22 (see Computational Details) can be used. It is important to remember here that also in the NOCV framework the difference of electron density between the adduct and its fragments (Δρ′) can be decomposed into contributions (Δρk′, k ≥ 0), each one describing the electron rearrangement that occurs upon the adduct formation. This produces three-dimensional functions (see for example Figure 3a) with depletion regions (red-colored), where the electronic density is lower than in the fragments, or accumulation regions (blue-colored), where the electronic density is higher than in the fragments. This implies a displacement of electronic density from the former to the latter. Furthermore, most of these contributions show very low eigenvalues and they are negligible. Generally, only the first three or four contributions are enough to completely characterize the interaction between the fragments (see Computational Details and Supporting Information), and each of them can be associated with a specific DCD bond component. In this work, the Nb−C1 bond in 1 and 2 have been characterized through both the methods (see below and Supporting Information), and they give comparable results. But since most of the systems discussed in the final section of this work are nonsymmetric, from now on only NOCV-CD results will be discussed. For 1, the fragmentation considered here is [NbCl5] and [CNXyl]. The first component of the interaction between the two fragments (Δρ′0) shows depletions regions on the carbon of L and accumulation regions on the metal and chlorides of the metallic fragment (Figure 3a). Looking at the electron density rearrangement, it is evident that it describes a

Figure 1. Molecular structure of 1 with key atoms labeled. Displacement ellipsoids are at the 50% probability level. Symmetry transformations used to generate equivalent atoms: #1 −x + 1, y, −z + 3/2; #2 −x + 1, y, z; #3 x, y, −z + 3/2.

Figure 2. Molecular structure of 2 with key atoms labeled. Displacement ellipsoids are at the 50% probability level. Symmetry transformations used to generate equivalent atoms: #1 −x + 1, y, −z + 3/2; #2 x, y, −z + 3/2; #3 −x + 1, y, z.

Table 1. Selected Bond Distances (Å) and Angles (deg) for 1 and 2a Nb(1)−X(1) Nb(1)−X(2) Nb(1)−C(1) C(1)−N(1) C(2)−N(1) X(1)−Nb(1)−C(1) X(2)−Nb(1)−X(2)#2 X(2)#1−Nb(1)−X(2)#3 Nb(1)−C(1)−N(1) C(1)−N(1)−C(2)

1 (X = Cl)

2 (X = Br)

2.2738(11) 2.3257(6) 2.319(5) 1.143(6) 1.400(5) 180.0 164.12(3) 164.12(3 180.0 180.0

2.3937(16) 2.4639(7) 2.329(12) 1.114(16) 1.393(14) 180.0 163.68(6) 163.68(6) 180.0 180.0

Symmetry transformations used to generate equivalent atoms: #1 −x + 1, y, −z + 3/2; #2 x, y, −z + 3/2; #3 −x + 1, y, z. a

Nb(1)−C(1) as found in 1 and 2 is slightly longer than the analogous Nb(V)−CCNXyl interaction of NbCp*Cl4−(CNXyl) [2.245(10) Å] and slightly shorter than the Nb(V)-−CNHC′ contact of NbCl5(NHC′) [2.396(12) Å]. The Nb(1)−C(1)− N(1) and C(1)−N(1)−C(2) angles are exactly 180.0°, since the molecules are located on mirror planes. The C(1)−N(1) contacts [1.143(6) and 1.114(16) Å for 1 and 2, respectively] are in agreement with a triple bond.33 Interestingly, in the “free” ligand the C(1)−N(1) bond length is 1.160 Å, indicating that in both complexes the polarization overcomes C

DOI: 10.1021/acs.inorgchem.8b02088 Inorg. Chem. XXXX, XXX, XXX−XXX

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(Eint = −21.6). Summing up all the back-donation contributions, the total orbital stabilization (Eorb,back) is −8.1 kcal/mol, which is 37% of Eint. Now the NOCVs decomposition of the total electron density deformation can be separately integrated along the Nb−C1 axis (z, see Supporting Information), in order to obtain quantitative information about the interactions. The integration gives the Charge Displacement functions (Δqk), which measure, at each point, the amount of electronic charge that crosses the plane perpendicular to the z axis. It is positive if the electronic density moves from right to left and negative for the opposite direction. For example, the Δq0 function, obtained integrating Δρ′0 along z, is always positive, indicating that in this case the flux is always from right (the ligand) to left (the metallic fragment), in agreement with the previous association with the M ← L σ donation. In the interfragment region (isoboundary), Δq0 assumes the value CT(Δρ′0) of 0.181 e, which is the quantification of the σ donation. For the Δq1 function, obtained integrating Δρ′1 along z, it is negative in the interfragment region, indicating a flux from the metallic fragment to the ligand, but it becomes positive in the ligand region. This indicates a polarization of the ligand by the metal fragment, which is positive and electrostatically attracts the electronic density of the isocyanide. At the isoboundary, Δq1 assumes the value of CT(Δρ′0) = −0.023 e, giving a quantification of the Clcis → C1N1 direct interaction component. Expanding the analysis to all the contributions, it results that CT(Δρ′k) is −0.038 and −0.007 e for k = 2 and 3, respectively (Figure 3b). It is important to underline here that the d orbitals of the Nb(V) are not completely empty when the [NbCl5(L)] complex is considered. Indeed, by using the NBO method43 to calculate atomic charges, orbital populations and orbital− orbital interactions, each d orbital is occupied by 0.80−0.94 e, and the niobium carries a partial charge (qNb) of 0.29 (Table 2). According to the NBO analysis of donor/acceptor (D →

Figure 3. (a) Isodensity surfaces (±0.0010 e/au) for the most relevant Δρ′k (k = 0−3) for complex 1; (b) total NOCV-CD curve and its most relevant components for the Nb−C1 bond in the complex 1. Black dots indicate the z position of the atomic nuclei. A yellow vertical band indicates the boundary between the [NbCl5] and L fragments.

contribution with local axial “symmetry”. Combining the two observations, we can associate Δρ′0 with the M ← L σ donation, combined with a strong polarization contribution, with the electronic density of the M-Cl bonds repelled by the lone pair of the cyanide to the chlorine atoms. Δρ′1 shows a depletion region on all the Clcis, whose shape resembles a full p orbital (lone pair) slightly deformed in its orientation, and an accumulation region on the triple bond, whose shape resembles the π* orbital. No contribution is visible on the metal. In this case, the local symmetry of the contribution is clearly planar. Therefore, Δρ′1 describes a displacement of electronic density from the chlorines to the carbon of the isocyanide, without passing through the metal. For this, it can be associated with a Clcis → C1N1 direct interaction, already described in our previous contribution.18 On the other hand, Δρ′2 describes a standard M → L π back-donation, with a depletion region on the metal, whose shape resembles the dxz orbital of niobium, and an accumulation on the π* orbital of C1N1. The phenyl ring is strongly polarized, especially in the ortho and para positions. Finally, Δρ′3 describes a similar type of back-donation, whose electronic flux starts from Cltrans, passes through the dyz orbital of the metal and ends up on the π* orbital of C1N1. Interestingly, in this component the polarization of the triple bond (C → N) is opposite with respect to Δρ′ and Δρ′2 (C ← N). Notably, the symmetry-based method is not able to decompose the interligand direct interaction and the backdonation components (see ref 18 and Supporting Information), since both of them belong to the same irreducible representation, while the NOCV method succeeds. In the NOCV-CD framework, each component can be associated with an orbital energy contribution: for 1, Eorb,k is −27.3, −4.1, −3.4, and −0.6 kcal/mol for k = 0, 1, 2, and 3, respectively

Table 2. CT (in e) Obtained from the NOCV-CD Analysis, Eint, Eback,tot (in kcal/mol) and te NPA charge of Nb (in e) for Different Halogen Atoms X in [NbX5(L)] Complexes X

CT(Δρ′0)

CT(Δρ′1)

CT(Δρ′2)

CT(Δρ′3)

Eint

Eback,tot

F Cl Br I

0.131 0.181 0.186 0.188

−0.002 −0.023 −0.041 −0.061

−0.010 −0.038 −0.053 −0.071

−0.002 −0.008 −0.004 −0.007

−24.6 −21.6 −20.8 −20.1

−2.6 −8.0 −10.1 −13.5

A) pairs, the orbitals able to interact with the σ*(C1N1) are the four σ(Nb−Clcis) and the lone pairs of Cltrans (Supporting Information), in agreement with the NOCV results. For compound 2, the DFT-computed C1−N1 bond distance is 1.169 Å, irrespective of the imposition of a symmetry or not and similarly to the value obtained for 1 (1.167 Å). On the other hand, and as already noted before, the experimental C1− N1 bond distance is lower for 2 than for 1, 1.114 and 1.143 Å, respectively. Such an effect can be likely due to the different crystal packing in the two crystals, and, in particular, to the Br···Br contacts highlighted in the crystal packing (see above). Indeed, the lone pairs of Xtrans are involved in the backdonation (Figure 3a and Figure 4a). If the lone pairs of 2 are involved in other interactions (as in the solid state), they will contribute to a lesser extent to the back-donation, leading to a D

DOI: 10.1021/acs.inorgchem.8b02088 Inorg. Chem. XXXX, XXX, XXX−XXX

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(Figure 4b). It can be highlighted that the back-donation component is larger than in the case of 1 (−0.098 e), as Eorb,back is (−10.1 kcal/mol, 48% of Eint). This fact leads to a slightly longer C1−N1 bond. In the case of 2, the NBO analysis reveals that σ*(C1N1) receives electronic density from the lone pairs of Brcis and Brtrans, but also from σ(Nb−Brcis) and σ(Nb−Br trans) (Supporting Information). Expanding the scope of the investigation to [NbF5(L)] (3) and [NbI5(L)] (4), the trends are confirmed: the backdonation and Eorb,back increase as the mass of the halogen increases (Table 2). This effect can be reasonably explained through the electronegativity of the halogen: the fluorine fills up the Nb-X orbitals to a lesser extent, and, at the same time, it is less prone to donate electronic energy directly to the triple bond, and then all the back-donation components are depressed (see Supporting Information). On the contrary, the iodine is more prone to share its electrons with both the niobium (increasing the standard back-donation) and with the triple bond (increasing the direct interligand interaction). A confirmation of this rationalization comes from the NBO data (Supporting Information): it is interesting to note that the charge on the metal goes from 2.10 to −0.46 going from the fluorine to the iodine. Parallely, the donor−acceptor (D → A) natural bond orbital interaction between the lone pair of the cis-halide and the CN σ* orbital increases as the mass of the halogen increases, with the corresponding second-order perturbation stabilization energies (ENBO(2)) equal to 2.44, 3.63, and 7.66 kcal/mol for fluorine, chlorine, and bromine, respectively. For the iodine, the value of ENBO(2) decreases to 2.24 kcal/mol because the main D → A orbital interaction becomes that between the Nb−I bond orbital and the CN σ* orbital (ENBO(2) = 18.36 kcal/mol). An interesting consideration arises from Table 2: one would think that the more electronegative halides, enhancing qNb, would greatly favor the interaction of the metal with a Lewis base, as L is, but the effect on Eint is not large, as the more favorable electrostatic Nb-L interaction is balanced by the higher repulsion between the lone pair of L and those of X and a smaller amount of back-donation (Supporting Information). It would be interesting to compare our newly synthesized systems with other characterized [M(L)(L′)Cln] complexes.

Figure 4. (a) Isodensity surfaces (±0.0010 e/au) for the most relevant Δρ′k (k = 0−3) for complex 2; (b) total NOCV-CD curve and its most relevant components for the Nb−C1 bond in the complex 2. Black dots indicate the z position of the atomic nuclei. A yellow vertical band indicates the boundary between the [NbBr5] and L fragments.

shorter C1−N1 bond distance. On the other hand, analyzing a single, isolated molecule (DFT scenario), the lone pairs of Brtrans are available for the back-donation and cause a lengthening of C1−N1. Considering a single, isolated molecule, the Nb−C1 bond in 2 can be described through the same pattern already described for 1 (Figure 4a). In particular, Brcis interacts directly with the triple bond as Clcis in the previous case, demonstrating that such interligand direct interaction exists also for other halogen atoms. From the quantitative point of view, the integrated values of the contributions are CT(Δρ0) = 0.186 e, CT(Δρ1) = −0.041 e, CT(Δρ2) = −0.053 e, and CT(Δρ3) = −0.004 e

Table 3. Comparison between Geometrical Parameters (Experimental, exp, and Theoretical, th) and Frequencies of the C1 N1 Bond of the Different Complexesa C1−N1b

C1−N1−Cipso

Clax−M−C1

υ̃C1N1c

complex

exp

th

exp

th

exp

th

exp

th

1 2 3 4 5AuIII 6NbV 7RhIII 8TaV 9TiIV

1.143 1.114

180.0 180.0

88.8b 75.6 87.6 75.4b 83.3

82.3 82.2 81.6 81.7 88.5 75.0 85.9 75.7 83.2b

2221 2209

178.9 176.7 171.4 177.8 178.3

179.8 179.8 180.0 180.0 176.1 178.5 175 178.8 179.6

82.1 81.8

1.142 1.138 1.156 1.139 1.147

1.167 1.169 1.166 1.171 1.165 1.168 1.180 1.167 1.168

2174 2164 2190 2143 2204 2180 2107 2186 2169

10PdII

1.144

1.173

177.9

174.0

87.1

84.7

2268d 2217e 2190e 2225 2210e 2200f 2229d 2212d

2136

a Bond lengths are expressed in Å, angles in degrees, frequencies in cm−1. bC1−N1 bond distance for free L: 1.160 Å (experimental); 1.180 Å (theoretical). cυ̃ C1N1 for free L: 2121 cm−1 (experimental); 2089 cm−1 (theoretical). dIn KBr. eIn Nujol. fIn CD2Cl2. gAverage value.

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Figure 5. Isodensity surfaces (±0.0010 e/au) for selected Δρ′k relative to the Clax → C1N1 direct interaction for complexes 5AuIII-10PdII.

Figure 6. (a) NOCV-CD curves for the Nb−C1 bond (only total back-donation) in the complexes 1−10PdII. Black dots indicate the z position of the atomic nuclei. A yellow vertical band indicates the boundaries between the two fragments (CTback,tot), and the light blue vertical band indicates the C1−N1 midpoints (CTCN); (b) correlation between the stretching frequency of C1N1 and the π polarization of C1N1.

classical” behavior. This is not uncommon for the complexes of L, as a isocyanide is less prone to accept back-donation than a carbonyl and polarization easily predominates over backdonation. 13 Some exceptions can be found with Re complexes.48 The analysis of the NOCV components for all the considered complexes reveals that in all the cases the Clcis → C1N1 direct interaction exists (Figure 5). For all the systems considered here, Δρ′0 is relative to the σ donation, Δρ′1 to the back-donation component in the plane perpendicular to the phenyl ring and Δρ′2 to the back-donation component in the plane of the phenyl ring. In some cases, the Clcis → C1N1 direct interaction is the only visible component, as in the case of d0 metal complexes 6NbV, 8TaV, and 9TiIV. For the metals with the d atomic orbitals formally populated, the M → L component is always clearly visible, and the fluxes cannot be separated as well as in the previous cases. In some cases, a depletion on Cltrans is present, too (5AuIII, 7RhIII, and 10PdII).

The complexes have been selected from the literature with three criteria: (i) the presence of L; (ii) the presence of at least one chloride in cis position to L; (iii) a general structure that allows the easy fragmentation of the complex in [M(L′)Cln] and L. The result is the following list of complexes: [Au(L)Cl3] (5AuIII),44 [Cp*Nb(L)Cl4] (6NbV),33 [Rh(PEt3)2(L)Cl3] (7RhIII),40 [Cp*Ta(L)Cl4] (8TaV),45 [Ti(L)Cl4]2 (9TiIV),46 and [Pd(L)2Cl2] (10PdII) (Scheme 2).41 The list spans sufficiently in terms of atomic weight, number of oxidation, and electronic configuration of the metal center, giving the possibility to check whether the Clax → C1N1 direct interaction is an unique case or it is of general occurrence. First, all of the selected complexes can be compared in terms of geometry, both experimental and DFT-optimized (see Table 3). Remarkably, for all the systems the experimental value of υ̃ C1N1 is higher than that of the free ligand (2121 cm−1) and a C1−N1 bond distance that is shorter than that of the free ligand (1.160 Å47), indication that all of them have a “nonF

DOI: 10.1021/acs.inorgchem.8b02088 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry The total back-donation for 1−10PdII can be compared summing up all the relevant components that involve the π* orbital of the triple bond and integrating the resultant Δρ′back,tot along the M−C1 axis (Figure 6a). The range is quite wide, from almost −0.286 (7RhIII) to −0.015 e (3). It is also well visible that in all the cases the C1N1 triple bond is polarized with a C ← N charge shift, coherently with a bond shortening with respect to the isolated L. As previously demonstrated for the carbonyl ligand,13,49 the value of Δq in this point (CTCN) is linearly correlated to the computed stretching frequency of the triple bond (Figure 6b).

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CONCLUSIONS

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EXPERIMENTAL DETAILS

(99+%) was purchased from Strem, while NbBr5 was prepared from NbCl5 according to the literature.50 Solvents (Sigma-Aldrich) were distilled from the appropriate drying agents before use. Infrared spectra were recorded at 298 K on a FT IR-PerkinElmer spectrometer, equipped with UATR sampling accessory. NMR spectra were recorded at 298 K on a Bruker Avance II DRX400 instrument equipped with a BBFO broadband probe. The chemical shifts for 1H and 13C were referenced to the nondeuterated aliquot of the solvent, while the chemical shifts for 93Nb were referenced to external [NEt4][NbCl6]. NMR signals due to a second isomeric form (where it has been possible to detect them) are italicized. Carbon, hydrogen, and nitrogen analyses were performed on a Carlo Erba mod. 1106 instrument. The halide content was determined by the Mohr method51 on solutions prepared by dissolution of the solid in aqueous KOH at boiling temperature, followed by cooling to room temperature and addition of HNO3 up to neutralization. Characterization of Commercial CN(2,6-C6H3Me2) (CNXyl). IR (solid state): ν/cm−1 = 2121 (CN). 1H NMR (C6D6): δ/ppm = 6.76 (t, 3JHH = 7.68 Hz, 1 H, para-CH); 6.62 (d, 3JHH = 7.68 Hz, 2 H, meta-CH); 2.07 ppm (s, 6 H, Me). Synthesis and Characterization of [NbX5(CNXyl)] (X = Cl, 1; X = Br, 2). General procedure. In a Schlenk tube, 2,6dimethylphenylisocyanide (ca. 0.5 mmol) was dried under reduced pressure with P4O10 for 1 h. The tube was then filled with N2, and NbX5 (ca. 0.5 mmol), and toluene (10 mL) were added. The yellow/ orange mixture was allowed to stir at room temperature for 24 h. Afterward, the precipitate was separated and washed with heptane (10 mL). The product was collected as a moisture-sensitive solid. Crystals suitable for X-ray diffraction analysis were obtained from a 1,2dichloroethane (1) or dichloromethane (2) solution layered with hexane and settled aside at −30 °C. [NbCl5(CNXyl)], 1. Dark yellow powder. Yield 105 mg (60%), from NbCl5 (143 mg, 0.529 mmol) and CNXyl(70 mg, 0.533 mmol). Anal. Calcd for C9H9Cl5NNb: C, 26.93; H, 2.26; N, 3.49; Cl, 44.17. Found: C, 25.88; H, 2.22; N, 3.16; Cl, 44.70. IR (solid state): ν/cm−1 = 2958w, 2922w, 2221w-m (CN), 1587w-m, 1475w-m, 1383m, 1285w-m, 1264w-m, 1171w-m, 1083w-m, 989w-m, 870m-s, 839s, 806m-s, 787vs, 716m. 1H NMR (C6D6): δ/ppm = 6.60 (t, 3JHH = 7.66 Hz, 1 H, para-CH); 6.32 (d, 3JHH = 7.66 Hz, 2 H, meta-CH); 1.82 (s,6 H, Me). 93Nb NMR (C6D6): δ/ppm = −124 (Δν1/2 = 426 Hz). [NbBr5(CNXyl)], 2. Orange powder. Yield 309 mg (87%), from NbBr5 (280 mg, 0.569 mmol) and CNXyl (75 mg, 0.57 mmol). Anal. Calcd for C9H9Br5NNb: C, 17.33; H, 1.45; N, 2.25; Br, 64.07. Found: C, 17.53; H, 1.51; N, 2.37; Br, 64.50. IR (solid state): ν/cm−1 = 2980w, 2948w, 2918w, 2209m-s (CN), 1968w, 1886w-m, 1803w, 1685w, 1587m-sh, 1496w, 1474m-s-sh, 1441w-m, 1380s, 1315w, 1301w-m, 1283m, 1263m-s, 1170m, 1082m-br, 986w-m, 806m-s, 784vs, 715w-m. 1H NMR (C6D6): δ/ppm = 6.61 (t, 3JHH = 7.79 Hz, 1 H, para-CH); 6.34 (d, 3JHH = 7.79 Hz, 2 H, meta-CH); 1.91 (m,6 H, Me). 93Nb NMR (C6D6): δ/ppm = 479 (Δν1/2 = 757 Hz). Reaction of NbCl5 with CNXyl in Dichloromethane. A solution of 2,6-dimethylphenylisocyanide (52 mg, 0.392 mmol) in CD2Cl2 (4 mL) was cooled to ca. − 30 °C and then treated with NbCl5 (103 mg, 0.380 mmol). The mixture was stirred at −30 °C for 10 min. An aliquot of the resulting yellow solution was readily analyzed by NMR at ambient temperature. 1H NMR (CD2Cl2): δ/ ppm = 7.44,7.39, 7.27, 7.23 (m, arom CH); 2.56, 2.52 (s, Me). 13C NMR{1H} (CD2Cl2): δ/ppm = 156.8 (CN); 137.5, 132.7, 129.4, 124.1, 122.1, 18.9 (aromatics). 93Nb NMR (CD2Cl2): δ/ppm = 7 (s, Δν1/2 = 156 Hz, NbCl6−); −60 (s, Δν1/2 = 426 Hz, NbCl4+); −132 (s, Δν1/2 = 448 Hz, major). In a different experiment, CNXyl (0.75 mmol) and NbCl5 (0.75 mmol) were allowed to react in CH2Cl2 (15 mL) at room temperature for 24 h. The volatiles were removed under a vacuum, and the residue was washed with hexane (30 mL), obtaining a green solid that was dried in vacuo at room temperature. IR (solid state): ν/ cm−1 = 3270w-br, 2964w, 2337w, 1652s, 1644vs, 1635vs, 1623vs, 1587m-s, 1521w-m-br, 1472m, 1445w-m, 1404w, 1481m, 1360m-br, 1338m, 1285w, 1261m, 1191w-br, 1170w, 1156w, 1088w-m-br, 1031w-m-br, 1015w-m, 992w, 939m-br, 849s-br, 807vs-br, 786vs,

In this paper, the synthesis and the structural characterization of the first examples of coordination adducts of niobium pentahalides with an isocyanide ligand have been described. In the two hexacoordinated complexes, the CN triple bond is shorter than in the uncoordinated ligand (higher stretching frequency), and the halides in cis position show intramolecular contacts with the metal-bound carbon. The coordinated isocyanide appears rather activated by the adjacent high valent niobium center, and the complexes slowly decompose in chlorinated solvents, where the formation of ionic adducts becomes favorable. The Nb−C bond has been theoretically analyzed in the two cases by using the NOCV-CD framework and the NBO analysis. According to the results, the lengthening of the triple bond is due to the high polarization effect of the metallic fragment; nonetheless the isocyanide accepts a non-negligible amount of electronic density. The latter partially comes from the niobium itself (M → L standard back-donation) or, more precisely, from the molecular orbitals in which the metal is involved, as σ(Nb-Xcis) and σ(Nb-Xtrans), or from the lone pairs of Xcis and Xtrans (interligand direct interaction). The theoretical analysis of other two, not synthesized, complexes, i.e., [NbF5(CNXyl)] and [NbI5(CNXyl)], reveals that the back-donation increases as the mass of the halogen increases, assuming an important role in the stabilization of the complex itself. Finally, the scope of this study has been widened considering other organometallic complexes structurally characterized in the literature, all of which bearing CNXyl and halides in the cis mutual position. In all of the cases, the NOCV-CD analysis highlighted the presence of the same bond components found in the case of [NbX5(L)], independently on the electronic configuration of the metal and, therefore, the amount of the M → L back-donation. As previously suggested by some of us,18 the presence of the halidecis → C interligand direct interaction seems to be peculiar of the σ-bonded triple bond. Indeed, this was recognized for the related carbonyl derivative, but not the NHC one. We believe that the detailed description of the metal−ligand bond is a useful method to better understand the stability and the reactivity of organometallic complexes.

Synthesis and Characterization of Compounds. Warning! The metal reactants used in this work are highly moisture-sensitive; thus rigorously anhydrous conditions were required for the reaction procedures. The reaction vessels were oven-dried at 150 °C prior to use, evacuated (10−2 mmHg), and then filled with argon. Organic reactants (Sigma-Aldrich, TCI Europe) were of the highest purity available and were stored under argon as received. Metal products were stored in sealed glass tubes under argon atmosphere. NbCl5 G

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Inorganic Chemistry 772vs 1H NMR (CD2Cl2): δ/ppm = 7.41, 7.29−6.75, 3.81−3.74, 2.82, 2.60−1.92. 93Nb NMR (CD2Cl2): no signals detected. X-ray Crystallography. Crystal data and collection details for 1 and 2 are reported in Table 4. Data were recorded on a Bruker APEX

Computational methods; additional figures and tables; experimental IR and NMR spectra (PDF) Optimized geometries (XYZ) Accession Codes

Table 4. Crystal Data and Measurement Details for 1 and 2 formula FW T, K λ, Å crystal system space group a, Å b, Å c, Å cell volume, Å3 Z Dc, g·cm−3 μ, mm−1 F(000) crystal size, mm θ limits,° reflections collected independent reflections data/restraints/parameters goodness on fit on F2 R1 (I > 2σ(I)) wR2 (all data) largest diff peak and hole, e Å−3

1

2

C9H9Cl5NNb 401.33 100(2) 0.71073 orthorhombic Cmcm 12.7342(12) 15.3656(14) 7.0711(7) 1383.6(2) 4 1.927 1.807 784 0.18 × 0.13 × 0.11 2.077−25.974 8776 772 [Rint = 0.0279] 772/0/53 1.251 0.0210 0.0470 0.371/−0.984

C9H9Br5NNb 623.63 100(2) 0.71073 orthorhombic Cmcm 13.0763(12) 15.5844(15) 7.2456(7) 1476.6(2) 4 2.805 14.319 1144 0.14 × 0.11 × 0.10 2.033−26.989 7110 901 [Rint = 0.0373] 901/0/52 1.116 0.0447 0.1255 1.638/−3.036

CCDC 1842953 (1) and 1842954 (2) contain 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 [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Corresponding Authors

*(G.C.) E-mail: [email protected]. *(F.M.) E-mail: [email protected]. ORCID

Leonardo Belpassi: 0000-0002-2888-4990 Fabio Marchetti: 0000-0002-3683-8708 Stefano Zacchini: 0000-0003-0739-0518 Gianluca Ciancaleoni: 0000-0001-5113-2351 Notes

The authors declare no competing financial interest.

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REFERENCES

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II diffractometer equipped with a PHOTON100 detector using Mo− Kα radiation. Data were corrected for Lorentz polarization and absorption effects (empirical absorption correction SADABS).52 The structure was solved by direct methods and refined by full-matrix least-squares based on all data using F2.53 Hydrogen atoms were fixed at calculated positions and refined by a riding model. All nonhydrogen atoms were refined with anisotropic displacement parameters.

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COMPUTATIONAL DETAILS The geometry optimizations and bond analysis have been computed with the ADF package (version 2014.09)54 at the DFT level using TZ2P Slater-type basis sets, Becke’s exchange functional 55 in combination with the Lee−Yang−Parr correlation functional,56 frozen-core approximation, and ZORA Hamiltonian to account for scalar relativistic effects.57−59 We employed the numerical integration grid with precision 6.0. All of the studied geometries that have been optimized are local minima, as confirmed by frequency analysis. NBO analysis has been performed using the NBO6 suite of software.43 For a detailed description of the Charge Displacement function analysis and the Natural Orbital for Chemical Valence method, see the Supporting Information.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02088. H

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