Ligation-Enhanced π-Hole···π Interactions Involving Isocyanides

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

Ligation-Enhanced π‑Hole···π Interactions Involving Isocyanides: Effect of π‑Hole···π Noncovalent Bonding on Conformational Stabilization of Acyclic Diaminocarbene Ligands Alexander S. Mikherdov, Mikhail A. Kinzhalov, Alexander S. Novikov, Vadim P. Boyarskiy,* Irina A. Boyarskaya, Margarita S. Avdontceva, and Vadim Yu. Kukushkin* Saint Petersburg State University, 7/9 Universitetskaya Nab., Saint Petersburg, 199034, Russian Federation S Supporting Information *

ABSTRACT: The reaction of cis-[PdCl2(CNXyl)2] (Xyl = 2,6Me2C6H3) with the aminoazoles [1H-imidazol-2-amine (1), 4H1,2,4-triazol-3-amine (2), 1H-tetrazol-5-amine (3), 1H-benzimidazol-2-amine (4), 1-alkyl-1H-benzimidazol-2-amines, where alkyl = Me (5), Et (6)] in a 2:1 ratio in the presence of a base in CHCl3 at RT proceeds regioselectively and leads to the binuclear diaminocarbene complexes [(ClPdCNXyl)2{μ-C(N-azolyl)N(Xyl) CNXyl}] (7−12; 73−91%). Compounds 7−12 were characterized by C, H, N elemental analyses, high-resolution ESI+-MS, Fourier transform infrared spectroscopy, 1D (1H, 13C) and 2D (1H,1H-COSY, 1H,1H-NOESY, 1H,13C-HSQC, 1H,13C-HMBC) NMR spectroscopies, and X-ray diffraction (XRDn). Inspection of the XRDn data and results of the Hirshfeld surface analysis suggest the presence in all six structures of intramolecular πholeisocyanide···πarene interactions between the electrophilic C atom of the isocyanide moiety and the neighboring arene ring. These interactions also result in distortion of the Pd−CN−Xyl fragment from the linearity. Results of density functional theory calculations [M06/MWB28 (Pd) and 6-31G* (other atoms) level of theory] for model structures of 7−9 followed by the topological analysis of the electron density distribution within the framework of Bader’s theory (QTAIM method) reveal the presence of these weak interactions also in a CHCl3 solution, and their calculated strength is 1.9−2.2 kcal/mol. The natural bond orbital analysis of 7−9 revealed that π(C−C)Xyl → π*(C−N)isocyanide charge transfer (CT) takes place along with the intramolecular π-holeisocyanide···πarene interactions. The observed π(C−C)Xyl → π*(C−N)isocyanide CT is due to ligation of the isocyanide to the metal center, whereas in the cases of the uncomplexed p-CNC6H4NC and CNXyl species, the effects of CT are negligible. Available CCDC data were processed from the perspective of isocyanide-involving π-hole···π interactions, disclosed the role of metal coordination in the π-hole donor ability of isocyanides, and verified the π-holeisocyanide···πarene interaction effect on the stabilization of the in-conformation in metal-bound acyclic diaminocarbenes.

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INTRODUCTION Scientific interest in noncovalent interactions has grown exponentially during the past decade in light of emerging evidence that these interactions can control structures and properties of supramolecular systems and thus play key roles in various chemical, physical, and biochemical processes. Most recent reports devoted to weak interactions typically deal with two large groups of their donor participants, namely, σ-hole donors (participating in hydrogen,1 halogen,2 chalcogen,3 pnictogen,4 tetrel,5 etc. bonding) and π-hole donors (capable of forming anion−π,6 lone pair−π,7 and π−π contacts8). A πhole is usually defined as the region of positive electrostatic potential in place of an empty antibonding π*-orbital, typically located perpendicularly to a framework. Noncovalent interactions involving π-hole donors are incomparably less studied than those of σ-hole species. The well-established types of π-hole donors include compounds featuring one-and-a half and double bonds such as acyl carbons (Figure 1, A),9 electrophilically activated arenes © XXXX American Chemical Society

Figure 1. Common types of π-hole donors.

(B),10 nitro compounds (C),11 and several other less abundant classes.11c,12 The π-hole acceptors usually include anions and neutral molecules bearing lone pairs (lp) that form anion−π, lp−π contacts and also less studied π-systems featuring πhole···π contacts. Nevertheless, the latter type of π-hole interactions still has significant application in catalysis,13 material science,14 and chemical and biological recognition.15 Usually it is rather typical for polyfluoroarene−arene8,13,14 systems due to the weak electrostatic interaction between Received: April 15, 2018

A

DOI: 10.1021/acs.inorgchem.8b01027 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

interactions affect structure and reactivity of M-ADCs, we continued our work devoted to M-ADCs in which, by varying nucleophile type, one can easily obtain compounds featuring various types of noncovalent bonding. Previously we reported the reactions of cis-[PdCl2(CNR)2] (R = Xyl, Cy) with thiazole-2-amines and thiadiazole-2-amines (Figure 3, A)

polyfluoroarenes (positive electrostatic surface potential; Figure 2, D) and arenes (negative electrostatic surface potential).

Figure 2. π-Hole···π interaction involving polyfluoroarenes (left) and CNR (right) with aromatic ring systems.

Figure 3. Previously17b,c (A) and currently (B) studied nucleophiles.

furnishing the PdII-ADC species with a S-center that serves as chalcogen bond (CB) donor.17b,c In this work, we employed another type of reacting heterocycle, namely, those where the S-center was replaced by the NH group [1H-imidazol-2-amine (1), 4H-1,2,4-triazol-3-amine (2), 1H-tetrazol-5-amine (3), 1Hbenzimidazol-2-amine (4); B] or by the NAlk moiety [1methyl-1H-benzimidazol-2-amine (5), 1-ethyl-1H-benzimidazol-2-amine (6)]. (i) Synthetic Approach. The reaction any one of 1−6 with cis-[PdCl2(CNXyl)2] in a 1:2 ratio in the presence of Et3N in CHCl3 at RT proceeds regioselectively for 24 h and gives only one binuclear aminocarbene product (7−12; Scheme 1).

Despite the increasing amount of research into noncovalent contacts of π-hole donors, some types, in particular π-hole···π interactions involving isocyanide functionality (E), are virtually unknown, and the effect of coordination of isocyanides on their π-hole donor ability has never been studied. In this work, we integrate our diverse projects focused on metal-activated isocyanides (for our review see ref 16), CNRderived acyclic diaminocarbenes (ADCs),17 and noncovalent interactions in transition metal complexes.17b,c,18 Extending our previous findings concerning the effects of noncovalent interactions on PdII-ADC systems,17b,c,18a,f,l we have now established both experimentally and theoretically that metalbound isocyanides can behave as π-hole donors toward neighboring arenes, and these π-holeisocyanide···πarene interactions affect the spatial structure of ADC ligands stabilizing their conformations. We also processed the available CCDC data from the perspective of π-hole···π interactions in CNR-based systems and uncovered the role of metal coordination in the πhole donor ability of isocyanides. This work is the first recognition of the π-hole donor ability in compounds featuring the triple bonded isocyanide moiety. Our work is summarized as follows. First, we studied the coupling between cis-[PdCl2(CNXyl)2] (Xyl = 2,6-Me2C6H3) with aminoazoles that leads to binuclear diaminocarbene complexes bearing two ligated CNXyl’s. Upon analysis of noncovalent bonding in their X-ray structures (XRD), we recognized hitherto unknown π-hole···π interactions between the electrophilic isocyanide C atom and the neighboring Xyl ring acting as a π-hole acceptor. The availability of these contacts was additionally confirmed theoretically by the Hirshfeld surface analysis, the topological analysis of the electron density distribution within the framework of Bader’s approach (QTAIM), and the natural bond orbital (NBO) analysis. Second, we undertook an extensive CCDC search for similar interactions, systematized their types, and found that the involvement of CNR species in π-hole···π interactions is a relatively common but overlooked phenomenon. Third, we established that the coordination of CNRs to metal centers increases their ability to serve as π-hole donors toward arenes and that in many instances this type of interaction determines the spatial conformation of M-ADCs. These results are discussed below.

Scheme 1. Reaction between cis-[PdCl2(CNXyl)2] and Aminoazoles 1−6

Similarly to the coupling with the thiazoles,17b this reaction proceeds through the intermediate C,N-chelated mononuclear carbene complex (boxed on Scheme 1), which upon deprotonation could serve as a nucleophile itself and react with the other isocyanide complex.17j We did not succeed to isolate this intermediate; however, its appearance in the reaction mixture was confirmed by HRESI+-MS. In contrast to binuclear species based on thiazol-2-amines, complexes 7− 12 are stable at RT or even upon reflux at C2H4Cl2 for 2 days, and they are not subject to the isomerization.17b Complexes 7−12 were isolated by evaporation of the reaction mixtures to dryness in air at RT, whereupon the solid residues were redissolved in a CH2Cl2−Me2CO mixture, and the formed solutions were subjected to fractional crystallization upon slow evaporation at RT.

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RESULTS AND DISCUSSION Coupling between Metal-bound CNXyl and Aminoazoles. Being interested in exploring how noncovalent B

DOI: 10.1021/acs.inorgchem.8b01027 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Fragment of the 1H,13C-HMBC (left) and 1H,1H-NOESY (right) spectra of 8.

(ii) Characterization of 7−12. The pure complexes were obtained as pale yellow solids and characterized by HRESI+MS, Fourier transform infrared (FTIR) spectroscopy, 1D (1H, 13 C{1H}) and 2D (1H,1H-COSY, 1H,1H-NOESY, 1H,13CHSQC, 1H,13C-HMBC) NMR spectroscopies. The complexes gave satisfactory C, H, N elemental analyses. In addition, solidstate structures of all complexes were elucidated by singlecrystal XRD. In each case, the HRESI+ mass-spectrum exhibits a fragmentation pattern corresponding to [M − Cl]+ with the characteristic isotopic distribution. The FTIR spectra display two separated (7−10) or partially overlapped (11, 12) strong and broad ν(CN) bands in the range 2190−2210 cm−1 from both isocyanide ligands. The spectra also exhibit three ν(C N) strong bands near 1590, 1470, and 1450 cm−1. The 1H NMR spectra display a set of overlapped and individual signals in the δ 7.25−6.05 range assigned to the 12 aromatic C−H protons in the meta- and para-positions of the Xyl groups and signals of the Me groups as four resolved singlets in the δ 2.41−2.05 interval. Strong downfield shift of the N−H signals (δ 12.17 for 7; 13.24 for 8; 14.74 for 9; 12.75 for 10) is probably due to intermolecular HB;19 the HBs were also observed in the XRD structures of 7−10 (see later and Supporting Information). In the 13C{1H} NMR spectra, two signals of the carbons of the NCN fragments were observed near δ 195 and 162, and the chemical shift of these signals fall in the range specific for Pd− Ccarbene (δ 160−224) in M-ADCs.17a,b,d−m,20 These resonances exhibit similar chemical shift values as for the relevant binuclear complexes derived from the reaction with thiazol-2-amines and pyridin-2-amines.17b,j With the 1H,13C-HMBC and 1H,1H-NOESY NMR experiments we assigned signals from the carbene atoms and Xyl rings of 7−12. Moreover, the 1H,1H-NOESY NMR experiment also allows the determination of the spatial structure of the complexes in a solution. In the 1H,13C-HMBC spectra of 7−12, three long-range correlations between the NCN carbons and the methyl protons of the Xyl groups B and C were observed (Figure 4, left). The C1 atom may correlate only with the methyl protons from the Xyl group B, while the other NCN carbon (C2) may have correlation with the Xyl groups the both B and C. Hence, C1 signal is located near δ 195, whereas the C2 carbon resonates at ca. δ 162. The signals of the Xyl groups A and D were assigned by NOESY experiment (Figure 4, right), viz. we found in the

1

H,1H-NOESY spectra of 7−12 three through-space interactions between the methyl protons in the Xyl group A and B, B and C, C and D, thus indicating that all four Xyl substituents are adjacent. The assignments of the other 1H and 13C resonances were performed by the 1H,1H-COSY, 1H,1H-NOESY, 1H,13CHSQC, 1H,13C-HMBC NMR. The crystal data, data collection parameters, and structure refinement data for 7−12 are given in Table S1 (Supporting Information). The plot of structure 7 is given in Figure 5; all

Figure 5. View of 7 with the atomic numbering scheme. Thermal ellipsoids are drawn at the 50% probability level. Dotted lines indicate the C···C and N−H···Cl contacts.

theoretical calculations (see later) were performed for this structure. Views of the other structures (8−12) are provided in Figures S1−S2 (Supporting Information). In all these complexes, atoms were numbered similarly, and they tabulated in Table 1 for comparison purposes. The geometry of these complexes is in agreement with the structures proposed from the solution NMR experiments, and it is similar to the relevant binuclear aminocarbene complexes.17b,j In the cases of 7−9, the complex molecules lie in one plane with exception of the Xyl groups. Incorporation of the alkyl substituents to the heterocyclic N atom in the structures of 11 and 12 leads to the twisting between the alkyl group and Cl2 presumably due the repulsive interactions. In 7−12, both metal centers adopt a slightly distorted square-planar geometry and the angles around the PdII centers are in the 78.93(12)− C

DOI: 10.1021/acs.inorgchem.8b01027 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Selected Bond Lengths (Å) and Angles (deg) for 7−12 bond lengths

7

8

9

10

11

12

d(Pd1−C1) d(Pd2−C2) d(C3−N4) d(C4−N5) angles

2.008(4) 1.980(4) 1.148(5) 1.152(5) 7

2.036(3) 1.995(3) 1.144(5) 1.147(4) 8

2.041(3) 1.985(3) 1.147(4) 1.148(4) 9

2.011(3) 2.036(2) 1.152(3) 1.143(4) 10

2.018(4) 1.994(3) 1.145(5) 1.152(4) 11

2.010(3) 1.981(4) 1.148(5) 1.150(5) 12

∠(Pd1−C3−N4) ∠(C3−N4−C5) ∠(Pd2−C4−N5) ∠(C4−N5−C29)

166.9(4) 171.6(5) 165.4(4) 168.0(4)

169.4(3) 175.6(3) 170.3(3) 175.1(3)

170.5(3) 178.5(3) 166.9(3) 176.4(3)

169.2(2) 167.3(3) 170.45(19) 170.0(3)

173.1(3) 174.5(4) 170.3(4) 173.5(4)

167.6(3) 175.5(4) 172.9 (3) 176.8(4)

Table 2. Distances (Å) and Angles (deg) for Two Types of Noncovalent Contacts in the Structures of 7−12 intramolecular π-hole···π interactions d(C3···C13)

∠(N≡C···C)

7 8 9 10 11 12

2.943(6) 2.976(5) 2.983(4) 2.926(3) 2.998(3) 2.902(5)

104.2(3) 101.6(3) 100.5(2) 101.32(17) 102.9(3) 102.7(3)

Bondi’s vdW25 Rowland’s vdW26

3.40 3.54

90° 90°

compound

intramolecular HB

d(C4···C21) 2.875(6) 2.989(5) 2.929(4) 2.916(3) 2.950(6) 3.005(5) Comparison 3.40 3.54

∠(N≡C···C)

d(H···Cl)

d(N···Cl)

∠(N−H···Cl)

105.7(3) 100.8(3) 104.6(3) 102.12(16) 96.0(3) 99.4(3)

2.34(5) 2.40(5) 2.24(5) 2.24(5)

3.084(4) 3.025(3) 2.993(3) 3.031(2)

144(1) 147(4) 144(1) 147(1)

90° 90°

2.95 2.86

3.30 3.40

180° 180°

for transition metals carbonyl complexes where metal-ligated carbon monoxide acts as π-hole donor and interacts with a lone pair.12e,f Other evidence of π-holeisocyanide···πarene interactions in 7−12 is the deviation of the Pd−CN and CN−Xyl fragments from the linearity (Table 1). If nonlinearity of the CN−Xyl fragment could be at least partially explained by π-backdonation from metal to ligand, which is rather unusual for PdII isocyanide complexes,16 the reason for distortion of the Pd− CN linkage is probably a charge transfer from the π-system of the Xyl substituent by diaminocarbene fragment to the π*orbital of the isocyanide moiety. This hypothesis was clarified by theoretical calculations (see later). It is noteworthy that such loss of linearity was also found in the systems featuring the M− CO group (M = Pd, Pt, Ni, Fe) and attributed to the mixing between empty and filled orbitals.12e,f (ii) Processing of CCDC Data for π-Hole···π Interactions Involving CNRs. The CCDC search allows the verification of approximately 640 examples of isocyanides featuring the πholeisocyanide···π contacts. However, to the best of our knowledge, the only recognized example of such contact has been provided by Colapietro et al.28 who solved the crystal structure of p-diisocyanobenzene (p-CNC6H4NC) and attributed the identified short contacts to a charge transfer interaction involving π-orbital of the benzene ring and π*-orbital of the isocyanide moiety. Our processing of available CCDC data clearly indicates that noncovalent interactions of the isocyanide CN group with various π-systems are significantly more abundant for ligated rather than for uncomplexed CNR species. We found more than 615 examples of π-hole···π interactions with M−CNRs and only 25 examples of those with metal-free CNRs. It is believed that a broader availability of π-holeisocyanide···π contacts is, at least partially, due to the electrophilic activation of the CN group by metal centers that leads to enhancement of an electrophilic area (π-hole) on the isocyanide C atom. However,

79.69(12)° range, which is close to those previously observed in the structurally related binuclear aminocarbene complexes derived from thiazol-2-amines or pyridin-2-amine [78.25(9)− 80.78(11)°].17b,j The CN bond lengths of the two isocyanide ligands are in the range of 1.145−1.152 Å that is typical for the CN triple bonds in the related isocyanide palladium complexes, e.g., [PdX2(CNXyl)2] (1.145−1.156 Å; X = Cl,21 Br22). The Pd−C distances are longer than those for the (pyridin-2amine)-based binuclear aminocarbene complex [(ClPdCNXyl) 2 {μ-C(N-pyridinyl)N(Xyl)CNXyl}] (1.988(2) and 1.977(2) Å).17j The C−N bond lengths in the N1−C1−N2 fragment are similar, and they are intermediate between the typical double and single bonds reflecting the diaminocarbene nature. In the N2−C2−N3 fragment, one C2− N2 bond is a single bond (1.452(8) Å),23 whereas the other C2−N3 bond is a typical double bond (1.260(9) Å).23 The same situation was observed for the binuclear complexes derived from the reaction with thiazol-2-amines and pyridine-2amines. All other bond lengths in 7−12 are usual, and their values agree with those reported for relevant palladium(II) carbene and isocyanide complexes.21,24 Recognition of π-Hole···π Contacts Involving CNR Species. (i) Noncovalent Interactions in 7−12. The XRD data indicate the presence of intramolecular πholeisocyanide···πarene interactions for 7−12 accompanied by N− H···Cl HBs in the cases of 7−10 (for discussion of these HBs see Supporting Information). The geometric parameters for these noncovalent contacts are given in Table 2. The distances between C atoms of the CN groups and the neighboring Xyl rings (2.875(6)−3.005(5) Å, Table 2) are substantially less than the sums of Rowland’s (3.54 Å26) or even Bondi’s (3.40 Å25) vdW radii. We assume that in these cases the isocyanide C atom has a π-hole,27 and it acts as an acceptor of electron density, whereas the π-system of the Xyl ring acts as a donor. The relevant type of π-hole interactions involving triple bond moiety has recently been described by Echeverriá D

DOI: 10.1021/acs.inorgchem.8b01027 Inorg. Chem. XXXX, XXX, XXX−XXX

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the acceptor CN group in p-CNC6H4NC and the donor Me groups in the xylyl radical of CNXyl. Theoretical Consideration of π-Hole···π Interactions Involving CNRs. (i) Hirshfeld Surface Analysis for the XRD Structure of 7. The Hirshfeld surface represents an area where molecules (or some fragments of molecules) come into contacts, and we have used this technique for the visualization of π-holeisocyanide···πarene interactions involving two Xyl groups in 7. The Hirshfeld surfaces for two PdII-ligated Xyl groups in 7 were generated by CrystalExplorer 3.1 program34 based on the results of the XRD study (Figure 8). The normalized contact

broader abundance of XRD structures for ligated rather than for uncomplexed CNRs should be also be taken into account. Analysis of 40 structures of M-CNRs with the shortest (less than 3.0 Å) π-hole···π contacts reveals the distortion of M− CN fragment from the linearity (175−156°, Figure S3) as it was found for 7−12. Curiously, the shortest contact (2.831(5) Å; CCDC code: IFABAU) is accompanied by the highest deviation from the linearity (156.6(3)°). We also found 44 examples of both neutral and deprotonated M-ADCs featuring the π-holeisocyanide···πarene contacts with isocyanide ligands. In most of these structures (27 out of 44; Table S3, Supporting Information), π-holeisocyanide···πarene separations are intramolecular, and they link the isocyanide C atom and the neighboring aryl substituent in the ADC-ligand, thus stabilizing the in-conformation of M-ADC (Figure 6). It is

Figure 8. Hirshfeld surfaces for two PdII-ligated CNXyl groups in 7. Thermal ellipsoids are drawn at the 50% probability level; all hydrogen atoms are omitted for clarity.

distances, dnorm,35 based on Bondi’s vdW radii,25 were mapped into the Hirshfeld surface. In the color scale, negative values of dnorm are visualized by the red color indicating contacts shorter than the sum of vdW radii. The white color denotes intermolecular distances close to vdW contacts with dnorm equal to zero. In turn, contacts longer than the sum of vdW radii with positive dnorm values are colored in blue. Thus, the π-holeisocyanide···πarene interactions involving two groups in 7 are visualized in Figure 8 by red circle areas. However, the Hirshfeld surface analysis does not answer the question on energies of these noncovalent interactions, and therefore, the DFT calculations and QTAIM analysis should be further performed to study the nature of these contacts more deeply. (ii) The QTAIM and NBO Analyses for Optimized Structures of 7−9 in a CHCl3 Solution. In order to confirm or disprove the hypothesis on the existence of the πholeisocyanide···πarene interactions also in a CHCl3 solution, for 7−9, we carried out DFT calculations and performed topological analysis of the electron density distribution within the framework of Bader’s theory (QTAIM method).36 We successfully used this approach upon studies of noncovalent interactions in transition metal complexes.17b,c,18a−m The results are summarized in Table 3. The atomic basins of electron density gradient lines map and the contour line diagram of the Laplacian distribution ∇2ρ(r), bond paths, and selected zero-flux surfaces for the π-holeisocyanide···πarene interactions in 7 are shown in Figure 9. To visualize studied noncovalent interactions, reduced density gradient (RDG) analysis37 was carried out, and RDG isosurface for 7 was plotted (Figure 9, bottom). The Poincare−Hopf relationship in all cases is satisfied, and thus all critical points have been found. The QTAIM analysis demonstrates the presence of appropriate bond critical points (BCPs) (3, −1) for the C··· C noncovalent interactions between the isocyanide C atoms and the π-system of the Xyl rings in the optimized equilibrium geometries of 7−9 in a CHCl3 solution. Additionally, the QTAIM analysis for optimized geometries of 7−9 in a CHCl3 solution reveals the presence of BCPs (3,−1) for four C−H···C

Figure 6. Stabilization of the in-conformation by intramolecular πhole···π interaction in M-ADCs.

of note that carbene ligands in different conformations could exhibit different electronic and steric properties,29 and all these affect the catalytic activity of these highly potent catalysts ADC-ligands: AuI-catalyzed cyclization reactions;30 ADC and NHC-ligands: metathesis reactions with Grubbs II-like catalysts31or the photophysical properties of carbene species.32 (iii) Noncovalent Interactions in the XRD Structures of Uncomplexed p-CNC6H4NC and CNXyl. As indicated by Colapietro et al.,28 the crystal structure of p-diisocyanobenzene (ZZZIZA, Figure 7, left) exhibits contacts between the

Figure 7. XRD structures of uncomplexed p-CNC6H4NC (ZZZIZA; left) and CNXyl (EBONAK; right). Dotted lines indicate the C···C and N···C noncovalent contacts.

electrophilic isocyanide C atom and the ipso-C atom of the benzene ring, and also π−π stacking; the latter was not recognized in the original work.28 The analysis of crystallographic data for the uncomplexed CNXyl (EBONAK) does not indicate any contacts involving the C atom of the isocyanide group, and only weak intermolecular π−π stacking interactions (see Supporting Information)33 supported by the Nisocyanide··· CXyl dispersion interactions were identified (Figure 7, right). The different intermolecular interactions patterns in the two metal-free isocyanides are probably because of the different substituent effects in the aromatic rings of the CN group, viz. E

DOI: 10.1021/acs.inorgchem.8b01027 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 3. Values of the Density of All Electrons − ρ(r), Laplacian of Electron Density − ∇2ρ(r), Energy Density − Hb, potential energy density − V(r), and Lagrangian Kinetic Energy − G(r) (Hartree) at the Bond Critical Points (3, −1), Corresponding to Noncovalent Interactions in Optimized Equilibrium Geometries of 7−9 (M06/6-31G* Levels of Theory, MWB28 Pseudopotentials on the Pd Atoms) and in Dimer of Metal-free CNXyl and p-CNC6H4NC (Experimental XRD Geometries) As Well As Energies for These Contacts Eint (kcal/mol), Defined by Two Approachesa contact

ρ(r)

C3···C13 C4···C21

0.012 0.011

C3···C13 C4···C21

0.012 0.011

C3···C13 C4···C21

0.013 0.011

Ciso···CAr CAr···CAr

0.004 0.004

Niso···CXyl CXyl···CXyl

0.004 0.005

∇2ρ(r)

Hb

V(r)

G(r)

7 in a CHCl3 Solution 0.002 −0.006 0.008 0.002 −0.006 0.007 8 in a CHCl3 Solution 0.040 0.002 −0.006 0.008 0.035 0.002 −0.006 0.007 9 in a CHCl3 Solution 0.040 0.002 −0.006 0.008 0.035 0.002 −0.006 0.007 Dimer of Metal-free p-CNC6H4NC in the Solid State (CCDC code: ZZZIZA) 0.012 0.001 −0.002 0.002 0.014 0.001 −0.002 0.003 Dimer of Metal-free CNXyl in the Solid State (CCDC code: EBONAK) 0.013 0.000 −0.002 0.003 0.014 0.001 −0.002 0.003 0.040 0.035

Eintb

Eintc

1.9 1.9

2.2 1.9

1.9 1.9

2.2 1.9

1.9 1.9

2.2 1.9

0.6 0.6

0.5 0.8

0.6 0.6

0.8 0.8

a

We carried out additional DFT calculations and performed topological analysis of the electron density distribution within the framework of Bader’s theory (QTAIM method) for all discussed model structures using popular dispersion-corrected hybrid functional ωB97XD. The obtained results are presented in Table S6 (Supporting Information), the obtained values of the density of all electrons, Laplacian of electron density, energy density, potential energy density, and Lagrangian kinetic energy at the bond critical points (3, −1), corresponding to noncovalent interactions C···C and N··· C are almost the same as we previously obtained with M06 functional. Thus, the estimated energies for the studied noncovalent interactions are almost independent of the DFT functional used. bEint = −V(r)/2. cEint = 0.429G(r)).

In 7−9, the NBO analysis43 revealed intramolecular charge transfer from Xyl to the isocyanide CN moieties. The secondorder perturbation theory analysis indicates that this intramolecular charge transfer is mainly associated with the π(C− C)Xyl → π*(C−N)isocyanide transitions of the isocyanide, and the total E(2) are 1.66/1.98 (7), 1.65/1.61 (8), and 1.49/2.17 (9) kcal/mol for C3···C13/C4···C21 contacts, respectively. The calculated NBO atomic charges on the appropriate CXyl, Cisocyanide, and Nisocyanide atoms are 0.14−0.15, 0.43−0.46, and −(0.37−0.39), respectively. The NBO data are in agreement with the QTAIM analysis and additionally confirm the πholeisocyanide···πarene nature of these noncovalent interactions. Finally, the geometry optimization of 7−9 leads to nonlinear distortion of the Pd−CN (166.1−169.8°) and CN−Xyl (174.4−178.0°) fragments, similarly to the XRD structures, thus indicating that this effect is not because of a crystal packing effect. In addition, both the QTAIM and NBO analyses also confirm the presence of π-hole···π interactions between the isocyanide moieties and the Xyl ring by diaminocarbene fragments. These interactions are attractive by nature but not repulsive. Thus, the DFT calculations for the optimized geometry of studied species reveal that the charge transfer π(C−C)Xyl → π*(C−N)isocyanide is a probable reason for the distortion of the Pd−CN−C fragments. However, in real systems steric and/or crystal packing factors can also take place. (iii) Theoretical Estimate of the Ligation and Substitution Effects on the π-Hole Donor Ability of CNRs. To clarify the difference between PdII-bound CNXyl and uncomplexed CNXyl and p-CNC6H4NC, we decided to carry out the Hirshfeld surface analysis, QTAIM and NBO analyses for dimers of CNXyl (EBONAK) and p-CNC6H4NC (ZZZIZA) using their experimental XRD geometries. The Hirshfield surface analysis did not reveal the short contacts with negative values of dnorm corresponding to π-holeisocyanide···πarene inter-

noncovalent interactions between the CH3 groups in the Xyl substituents and the π-systems of the neighboring Xyl rings. Notably that these intramolecular contacts C−H···C are absent in appropriate experimental XRD structures (their geometrical parameters does not fulfill the IUPAC criterion for HBs: C···C and H···C distances are greater than the sum of the Rowland26 and Bondi25 vdW radii of these atoms; for details about these C−H···C noncovalent interaction see Supporting Information). The low magnitude of the electron density (0.011−0.013 hartree), positive values of the Laplacian (0.035−0.040 hartree), and close to zero energy density (0.002 hartree) in these BCPs are typical for noncovalent interactions.38 The energies for these contacts were defined according to the procedures proposed by Espinosa et al.39 and Vener et al.40 (Table 3). The strength of π-holeisocyanide···πarene interactions of ligated isocyanides with Xyl rings is in the range of 1.9−2.2 kcal/mol, which is comparable with contacts of such π-hole donors as metal-ligated carbon monoxide,12e,f nitro compounds,11b SO3,12d and acyl carbons (0.5−5.0 kcal/mol).9 It is noteworthy that the energy of π-hole···π interactions with polyfluoroarenes is much higher due to the stronger electrophilic activation of the aromatic systems (5−20 kcal/ mol).8c,d,13d,14a,b,e,f,41 The balance between the Lagrangian kinetic energy G(r) and potential energy density V(r) at the BCPs (3, −1) reveals the nature of these interactions. If the ratio −G(r)/V(r) > 1 is satisfied, then the nature of appropriate interaction is purely noncovalent, in case the −G(r)/V(r) < 1 some covalent component takes place.42 On the basis of this criterion, one can state that the covalent contribution in all intramolecular interactions discussed above is absent. The negligible values of the Wiberg bond indices for C···C contacts in optimized structures of 7−9 (0.01) computed by using the NBO partitioning scheme43 additionally confirm the electrostatic nature of these interactions. F

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stacking and Nisocyanide···CXyl dispersion interactions in CNXyl and for the Cisocyanide···Carene π-hole···π and CXyl···CXyl π−π stacking contacts in p-CNC6H4NC. The energies of these contacts are very small (0.6−0.8 kcal/mol, Table 3). In contrast to the structures of 7−9 featuring PdII-coordinated CNXyl, the NBO analysis for the dimer of CNXyl revealed the reverse pattern of charge transfer, viz. from π(C−N) of the isocyanide moiety to π*(C−C) of the Xyl ring (Figure 11, A), but its total

Figure 11. Charge transfer for metal-free CNXyl (A), p-CNC6H4NC (B), and PdII-ligated CNXyl (C).

energy is negligible (total E(2) = 0.10 kcal/mol) compared with the charge transfer for 7−9 (3.26−3.66 kcal/mol). In the case of p-CNC6H4NC, the charge transfer direction (π(C−C)ar → π*(C−N)isocyanide, B) is the same as for 7−9 (C), but its energy is still incomparable with metal-bound isocyanide (total E(2) = 0.18 kcal/mol). The calculated NBO atomic charges on the appropriate atoms in the dimers of CNXyl and pCNC6H4NC are also different than those in the CNXyl ligand being more negative, viz. −0.55 on Nisocyanide, 0.29 on Cisocyanide for CNXyl, and −0.56 on Nisocyanide, 0.32 on Cisocyanide for CNC6H4NC, respectively. The results of DFT calculation for systems with uncomplexed CNRs (CNXyl and p-CNC6H4NC) and Pdcoordinated CNXyl demonstrate that modification of substituents nature in CNR and, to a greater degree, the coordination to the metal-center leads to dramatic changes in the electronic properties of the CN moiety switching the pattern of charge transfer and turning it from π-hole acceptor to π-hole donor. Notably that related example of an enhancement of π-hole donor ability of heteroarenes upon coordination has previously been detected theoretically44 and experimentally45 for anion−π interactions in pyridine, pyrazine, and tetrazine silver(I) species.

Figure 9. Atomic basins of electron density gradient lines map (top), contour line diagram of the Laplacian distribution ∇2ρ(r), bond paths and selected zero-flux surfaces (middle), and RDG isosurface (bottom) referring to intramolecular π-holeisocyanide···πarene interaction between C4···C21 in 7. Bond critical points (3, −1) are shown in blue, nuclear critical points (3, −3) − in pale brown, ring critical points (3, +1) − in orange, cage critical points (3, +3) − in light green. Length units − Å, RDG isosurface values are given in Hartree.

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actions in XRD structures of metal-f ree CNXyl (EBONAK) and p-CNC6H4NC (ZZZIZA) (Figure S5, Supporting Information). The QTAIM analysis reveals the presence of bond critical points (BCPs, Figure 10) (3, −1) for the CXyl···CXyl π−π

CONCLUSIONS The results of this work could be considered from at least four perspectives. First, by inspection of the XRD structures (Figure 4) of the diaminocarbene complexes [(ClPdCNXyl)2{μ-C(Nazolyl)N(Xyl)CNXyl}] (7−12) obtained in this work (Scheme 1), we have verified previously unknown πholeisocyanide···πarene interactions (Figure 2) and confirmed their presence by Hirshfeld surface analysis (Figure 8). The contacts, which are substantially closer than even the sum of Bondi’s vdW radii, are formed between the electrophilic C atom of the isocyanide moiety (π-hole donor) and the neighboring arene ring (π-hole acceptor). In 7−12, the presence of such πholeisocyanide···πarene interactions also leads to distortion of the Pd−CN−Xyl fragment from linearity. Results of DFT calculations followed by the topological analysis of the electron density distribution within the framework of Bader’s theory (QTAIM method) revealed that these contacts were also present in a CHCl3 solution with an estimated strength between 1.9−2.2 kcal/mol, a range similar to that seen for π-

Figure 10. Molecular graph from the QTAIM analysis of the dimers of uncomplexed CNXyl (left) and p-CNC6H4NC (right, experimental XRD geometries EBONAK and ZZZIZA). Bond critical points (3, −1) are shown in orange, nuclear critical points (3, −3) − in violet, ring critical points (3, +1) − in yellow, cage critical points (3, +3) − in light green. The Poincare−Hopf relationship is satisfied. G

DOI: 10.1021/acs.inorgchem.8b01027 Inorg. Chem. XXXX, XXX, XXX−XXX

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syntheses of acyclic diaminocarbenes in certain conformations exhibiting specific useful properties. This project is underway in our group.

hole donors such as metal-ligated carbon monoxide, nitro compounds, SO3, and acyl carbons (0.5−5.0 kcal/mol), but weaker than π-hole interactions of strongly electrophilic polyfluoroarenes (5−20 kcal/mol). Noncovalent interactions involving π-hole donors are incomparably less studied than those of σ-hole species. The currently recognized groups of π-hole donors include compounds bearing one-and-a half and double bonds such as acyl carbons, electrophilically activated arenes, and nitro co mpound s (F i g u r e 1 ). O ur o bse rv at io n o f π holeisocyanide···πarene interactions add to the growing list of available π-hole systems, but even more importantly diversify the range of π-hole donors by documenting a novel class of species featuring a triple bond. These results point to the potential of other compounds bearing electron-deficient triple bonds to behave as π-hole donors, and deserve further studies. Second, the recognition of π-holeisocyanide···πarene interactions in the diaminocarbene complexes stimulated our further interest in this type of noncovalent linkage. By processing the available XRD (CCDC) data, we have verified many more examples of contacts of isocyanides with π-systems (arenes and other π-systems such as alkenes, carbonyl groups, iminyl groups, carbon monoxide, and isocyanides). We systematized types of π-holeisocyanide···π contacts and found that the involvement of CNR species in π-hole···π interactions is a relatively common but overlooked phenomenon. Third, we found that the ligation of CNRs to electrophilic metal centers enhances their π-hole donor ability toward arenes. The NBO analysis of 7−9 revealed that π(C−C)Xyl → π*(C−N)isocyanide charge transfer takes place along the intramolecular π-holeisocyanide···πarene interactions. The observed π(C−C)Xyl → π*(C−N)isocyanide charge transfer is due to coordination of the isocyanide ligand to the metal center, whereas in the cases of uncomplexed p-CNC6H4NC and CNXyl species, the effects of charge transfer are negligible. The DFT calculations for systems with uncomplexed CNRs (CNXyl and p-CNC6H4NC) and Pd-bound CNXyl demonstrate that modifying the nature of substituents in CNR and, to an even greater extent, the coordination to the metal center, leads to dramatic changes in the electronic properties of the CN moiety, turning it from a π-hole acceptor to a π-hole donor and thereby switching the pattern of charge transfer (Figure 11). Moreover, our processing and analysis of CCDC data also indicate that the π-holeisocyanide···π interactions involving isocyanide moieties are significantly more abundant for ligated rather than uncomplexed CNR species. We postulate that a broader availability of π-holeisocyanide···π contacts is at least partially due to the electrophilic activation of the CN group by metal centers. Fourth, the combination of experimental results with the processed XRD (CCDC) data led us to conclude that πholeisocyanide···πarene interactions in many instances determine the spatial conformation of metal-bound acyclic diaminocarbenes (for reviews on catalytic properties of these species see refs 17a), leading to preferential stabilization of the in- rather than the out-conformation (Figure 6). Notably, carbene ligands in different conformations should exhibit different electronic and steric properties, and, consequently, these conformers might exhibit different catalytic activities (for experimentally confirmed examples see refs 30 and 31) and photophysical properties (refs 32). To the best of our knowledge, this work is the first recognition of π-holeisocyanide···π interactions. It justifies further interest in developing π-holeisocyanide···π systems for directed

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EXPERIMENTAL SECTION

Materials and Instrumentation. Solvents, PdCl2, azoles 1−6, and CNXyl were obtained from commercial sources and used as received, apart from chloroform, which was dried by the conventional distillation over calcium chloride. The complex cis-[PdCl2(CNXyl)2]46 was synthesized by the reported procedure. C, H, and N elemental analyses were carried out on a Euro EA 3028 HT CHNSO analyzer. Mass spectra were obtained on a Bruker micrOTOF spectrometer equipped with electrospray ionization (ESI) source, and a mixture of MeOH and CH2Cl2 was used for samples dissolution. The instrument was operated in a positive ion mode using m/z range of 50−3000. The capillary voltage of the ion source was set at −4500 V and the capillary exit at 50−150 V. The nebulizer gas pressure was 1 bar and the drying gas flow 4.0 L/min. The most intensive peak in the isotopic pattern is reported. Infrared spectra were recorded on a Shimadzu IRAffinity-1S FT-IR spectrometer (4000−400 cm−1) in KBr pellets. The 1D (1H, 13 C{1H}) NMR spectra were acquired on a Bruker Avance 400 spectrometer, whereas the 2D (1H,1H-COSY, 1H,1H-NOESY, 1H,13CHSQC, and 1H,13C-HMBC) NMR correlation experiments were recorded on a Bruker Avance III 500 MHz spectrometer. All NMR spectra were measured in CDCl3 at ambient temperature. Synthetic Work. Synthesis of 7−12. A solution of triethylamine [(i) 11 mg, 0.110 mmol in the case of 1·1/2H2SO4 and 3·H2O; (ii) 7.5 mg, 0.075 mmol in the case of 2 and 4−6] in CHCl3 (3 mL) was added to a mixture of cis-[PdCl2(CNXyl)2] (30 mg, 0.075 mmol) and any one of 1H-imidazol-2-amine sulfate (1·1/2H2SO4), 1H-tetrazol-5amine monohydrate (3·H2O), or aminoazoles 2, 4−6 (0.037 mmol) placed in a 10 mL round-bottomed flask; then the reaction mixture was stirred in air at RT for 24 h. The color of the reaction mixture gradually turned from pale yellow to intense green yellow, and the solid cis-[PdCl2(CNXyl)2] was dissolved. The solution was evaporated at RT to dryness, whereupon the solid residue was washed with Me2CO (three 1 mL portions) and redissolved in a CH2Cl2−Me2CO mixture (1:2, v/v), and the formed solution was subjected to crystallization on slow evaporation. The obtained crystals of pure products were filtered off, washed with Et2O (three 1 mL portions), and dried in air at RT. Characterization. The pure complexes were obtained as yellow solids and were characterized by microanalyses (C, H, N), high resolution ESI−MS, FTIR, 1D (1H, 13C{1H}) and 2D (1H,1H-COSY, 1 1 H, H-NOESY, 1H,13C-HSQC, and 1H,13C-HMBC) NMR spectroscopies. Characterization data, plots of the 1H, 13C{1H}, HMBC NMR spectra, and XRD structures for 7−12 are included in Supporting Information. X-ray Structure Determination. Single crystals of 7−10 and 12 were grown from acetone/CH2Cl2 mixtures, whereas crystals of 11 were from CHCl3. The X-ray experiments were conducted on SuperNova, Dual, Cu at zero, Atlas (7 and 11) and Xcalibur, Eos diffractometers (8−10 and 12). The crystals were kept at 100(2) K during data collection. The structures have been solved by the direct methods and refined by means of the SHELXL-2015 program47 incorporated in the OLEX2 program package.48 Empirical absorption correction was applied in CrysAlisPro49 program complex using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. Crystallographic data for these samples have been deposed at Cambridge Crystallographic Data Centre (CCDC 1584521− 1584525, 1812794). Processing of CCDC. Processing of the Cambridge Structure Database (v 5.39) was performed using the ConQuest module (v 1.20). The analysis for π-hole···π interactions involving CNRs was based on six parameters, viz. distance C···X (d1) (X = C, N) and angles: NC···X (a1) and C···X−Y1/Y2 (a2,3) (Figure 12). The distances were restricted by sum of Rowland’s vdW radii and a1-3 angularity was restricted to be greater than 65° and 115°, respectively. H

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Inorganic Chemistry Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support of the synthetic work and compounds characterization from the Russian Science Foundation (Grant 14-43-00017P) is gratefully acknowledged. Theoretical part of this work was conducted under the Russian Foundation for Basic Research Project (16-33-60063). A.S.N. is thankful to the Saint Petersburg State University and Santander Bank for support of his sabbatical on leave at the Instituto Superior Técnico, Universidade de Lisboa (Lisbon, Portugal). Physicochemical studies were performed at the Center for Magnetic Resonance, Center for X-ray Diffraction Studies, Center for Chemical Analysis and Materials Research, and Chemistry Educational Center (all belonging to Saint Petersburg State University).

Figure 12. Parameters taken for processing CCDC data.

In the case of several contacts with one π-system, the shortest contact was selected. Only structures with determined 3D and with no error were included in the search query. In addition to this, powder structures were excluded from the search. To ensure that we have only highquality structures, R-factorwhich represents the agreement between the obtained crystallographic model and the experimental diffraction datawas kept below 0.1. Computational Details. The single point calculations and full geometry optimization have been carried out at DFT level of theory using the M06 functional50 (this functional was specifically developed to describe weak dispersion forces and noncovalent interactions) with the help of the Gaussian-0951 program package. The calculations were performed using the quasi-relativistic Stuttgart pseudopotentials that described 28 core electrons and the appropriate contracted basis sets52 for the palladium atoms and the 6-31G* basis sets for other atoms. No symmetry restrictions have been applied during the geometry optimization. The solvent effects were taken into account using the SMD continuum solvation model by Truhlar et al.53 with CHCl3 as solvent. The Hessian matrix was calculated analytically for the optimized structures in order to prove the location of correct minima (no imaginary frequencies). The topological analysis of the electron density distribution with the help of the atoms in molecules (QTAIM) method developed by Bader36 has been performed by using the Multiwfn program.54 The Cartesian atomic coordinates of the calculated equilibrium structures in CHCl3 solution are presented in Table S7 (Supporting Information).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01027. Experimental details for the synthesis and characterization of the complexes, crystallographic and computational information, NMR spectra (PDF) Accession Codes

CCDC 1584521−1584525 and 1812794 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], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(V.P.B.) E-mail: [email protected]. *(V.Y.K.) E-mail: [email protected]. ORCID

Alexander S. Mikherdov: 0000-0002-6471-5158 Mikhail A. Kinzhalov: 0000-0001-5055-1212 Alexander S. Novikov: 0000-0001-9913-5324 Vadim P. Boyarskiy: 0000-0002-6038-0872 Vadim Yu. Kukushkin: 0000-0002-2253-085X I

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DOI: 10.1021/acs.inorgchem.8b01027 Inorg. Chem. XXXX, XXX, XXX−XXX