Multinuclear Alkali Metal Complexes of a Triphenylene-Based

Aug 15, 2016 - Moreover, the hexalithium complex [LLi6] readily reacted with EII halides (GeCl2·dioxane, SnCl2, PbCl2) through transmetalation to aff...
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Multinuclear Alkali Metal Complexes of a Triphenylene-Based Hexamine and the Transmetalation to Tris(N-heterocyclic tetrylenes) (Ge, Sn, Pb) Fei Zhong,†,§ Xiaodong Yang,†,§ Lingyi Shen,† Yanxia Zhao,† Hongwei Ma,‡ Biao Wu,† and Xiao-Juan Yang*,† †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710069, China ‡ Analysis and Testing Center, Beijing Institute of Technology, Beijing 102488, China S Supporting Information *

ABSTRACT: A C3-symmetric hexamine (LH6) based on the triphenylene and ortho-phenylenediamine (PDAH2) skeletons has been synthesized, and was partially or fully deprotonated upon treatment with alkali metal agents to afford amino−amido or diamido coordination sites. Four alkali metal complexes, the dinuclear [Na2(LH4)(DME)5] (1) and [K2(LH4)(DME)4] (2), trinuclear [K3(LH3)(DME)6] (3), and hexanuclear [Li6(L)(DME)6] (4), were obtained and used in transmetalation/ligand exchange with other metals. The hexalithium salt of the fully deprotonated ligand, [Li6L], reacted with heavier group 14 element halides to yield three tris(N-heterocyclic tetrylenes), the germylene [Ge3(L)] (5), stannylene [Sn3(L)] (6), and plumbylene [Pb3(L)] (7). The synthesis and crystal and electronic structures of these compounds are reported.

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Lappert et al. in as early as 1973.10 Shortly afterward, in 1975, the first N-heterocyclic stannylene was prepared by Veith.11 More recently, a series of NHEs and their transition metal complexes have been reported.12−15 Besides the monocarbenes and tetrylenes, their multiple analogues are also known. For instance, the triphenylene was used as the backbone for tris(Nheterocyclic carbenes) and corresponding multinuclear metal complexes,16−18 while two examples of tris(N-heterocyclic stannylene) based on triphenylene were also reported.19 Nonetheless, tris-NHEs are yet far less explored. In our previous studies on N,N′-donor ligands, we have employed the bidentate α-diimine species, which have proven to be able to stabilize a series of metal−metal-bonded compounds.20,21 Moreover, we synthesized the tetradentate dabqdiH2 ligand (Chart 1), which can be viewed as a hybrid of diimine and diamine.22 In this present work, the ligand used is further expanded to the hexadentate hexamine (LH6) by combining triphenylene and ortho-phenylenediamine (PDAH2) (which can deprotonate and coordinate to main group and transition metals).23 The new rigid, planar LH6 molecule, bearing the π-conjugation system and bulky N-Dipp substituents (Dipp = 2,6-diisopropylphenyl), could be a promising preligand for novel coordination compounds or polymers, such as those with low-valent, low-coordinate metal centers. Herein we report the synthesis of four alkali metal complexes (salts) of

INTRODUCTION The search for polynuclear complexes is of considerable interest because they may benefit from the cooperation of the multiple functional sites and offer more advantages in catalysis, biological processes, and materials science, etc.1,2 Various spacers have been utilized to link the desired coordination donors, and the triphenylene fragment is a good candidate not only because it can provide multiple binding sites but also because it has a π-conjugation property that can facilitate communication of the metal centers.3−5 In recent years, a few discrete or polymeric transition metal (Ni, Cu, etc.) complexes of hexamine, hexathiol, and mixed N,O-donor triphenylene derivatives have been reported, which exhibit interesting electric or optical properties.6,7 However, triphenylene-fused metal complexes are still very rare. N-Heterocyclic carbenes (NHCs) have become a class of the most important ligands since their first synthesis.8 As heavier analogues of NHCs, stable N-heterocyclic tetrylenes (NHEs) featuring an electron-deficient group 14 element display quite similar properties as compared to their light homologues. On the other hand, they also possess significant differences from the NHCs. The central atom in NHEs adopts (ns)2(np)2 valence electron configurations instead of sp2 hybridization in NHCs; the latter species have better electron donation ability while NHEs show stronger π-accepting property. Therefore, the NHEs have received considerable attention in terms of catalysis and supramolecular chemistry.9 Indeed, the earliest stable acyclic dialkyl and diamino tetrylenes were isolated by © XXXX American Chemical Society

Received: July 20, 2016

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DOI: 10.1021/acs.inorgchem.6b01743 Inorg. Chem. XXXX, XXX, XXX−XXX

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crystals. In the complex, two of the secondary amino protons of LH6 were abstracted and replaced by sodium ions, which is also indicated by its 1H NMR spectrum with a singlet at δ = 5.23 ppm corresponding to four protons (by NMR integration). In addition, the IR spectrum revealed the presence of an NH stretch. There are two independent molecules in the asymmetric unit with very close metrical parameters, and one of them is discussed here (Figure 1). It is evidenced that the

Chart 1

the partially or completely deprotonated form of the ligand, LH42−, LH33−, and L6− (complexes 1−4, Chart 1). Since the PDAH2 unit is known to be a good platform for NHCs and NHEs, we utilized the hexamine ligand L to prepare tris-NHEs. Interestingly, the hexalithium salt [Li6L] readily underwent transmetalation with EII halides to afford three tris(Nheterocyclic tetrylenes), [E3(L)] (E = Ge, 5; E = Sn, 6; and E = Pb, 7) (Chart 1). The crystal and electronic structures of 1−7 have been studied by X-ray diffraction and spectral methods as well as DFT computations.

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Figure 1. Molecular structure of 1 (thermal ellipsoids are set at the 30% probability level; isopropyl groups and CH hydrogen atoms have been omitted for clarity). Selected bond lengths (Å): N1−C1, 1.367(7); N2−C2, 1.421(6); N3−C7, 1.353(6); N4−C8, 1.401(6); N5−C13, 1.412(6); N6−C14, 1.425(6); Na1−N1, 2.381(5); Na−O, 2.306(1)−2.996(2).

RESULTS AND DISCUSSION The hexamine LH6 was synthesized by a modified literature procedure of Buchwald−Hartwig Pd-catalyzed cross-coupling amination24 of 2,3,6,7,10,11-hexabromotriphenylene with excess 2,6-diisopropylphenylamine. Its 1H NMR spectrum gives a single, diagnostic signal at δ = 5.23 ppm (C6D6) for the amino NH protons, and the IR spectrum shows the expected absorption at 3357 cm−1. The free ligand LH6 was initially treated with the metalation agents NaH or KH, but no products were isolated. Interestingly, by using Na, K, and n-BuLi, four multinuclear complexes, the dinuclear [Na(DME)3][Na(LH4)(DME)2]·0.5DME·0.75hexane (1) and [K2(LH4)(DME)4] (2), trinuclear [K3(LH3)(DME)6] (3), and hexalithium [Li6(L)(DME)6]·DME·toluene (4), were obtained as air- and moisture-sensitive compounds (Scheme 1). We have also attempted to access other deprotonated states (i.e., LH5− and LH24−) of LH6 by changing the amount of Na, K, or n-BuLi; unfortunately, up to now all of the experiments have been unsuccessful. [Na(DME)3][Na(LH4)(DME)2]·0.5DME·0.75Hexane (1) and [K2(LH4)(DME)4] (2). Reaction of LH6 with 2 equiv of Na metal afforded the dinuclear sodium complex 1 as orange

two deprotonated amine groups are from two different orthophenylenediamine (PDAH2) arms (to yield the amino−amido form) rather than from the same PDAH2 unit (to yield the diamido fragment), as discussed below. Noticeably, the Na1 ion is coordinated by only one nitrogen atom N1 (Na−N: 2.381(5) Å) and two DME molecules, while the other N atom (N2) of the same diamine arm is not coordinating (Na···N: 2.806(5) Å). Nevertheless, the coordinated Na−N bond length compares to those in [{(PDANa2)(THF)2}2] (2.379(2)−2.455(2) Å)25 and our previously reported sodium complex of dipp-dabqdiH2 (2.348(3)− 2.352(2) Å).22 Therefore, it may be proposed that this “PDA” unit is in the amino−amido form; that is, the proton on N1 is removed, but that on N2 remains. This single deprotonation of ortho-diamine has been reported previously in a potassium complex of a bis(diamine)22 and more recently in

Scheme 1. Synthesis of the Alkali Metal Complexes 1−4 and Tris(N-heterocyclic tetrylenes) 5−7

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Inorganic Chemistry the alkali metal salts of a monoanionic (amido−amino)silane.26 Consequently, the other deprotonation site should be on another PDAH2 arm, which can be localized on N3 (C7− N3:1.353(6) Å) by the shortening of the Ctriphenylene−N bond upon deprotonation. The coordinated C1−N1(amido) (1.367(7) Å) is also shorter compared to the “free” C2− N2(amino) (1.421(6) Å), although the difference (0.07 Å) is slightly smaller than that of the Si−N bond in the case of the (amido−amino)silane (ca. 0.1 Å).27 Surprisingly, the other sodium ion (Na2) is not coordinated by the amido N3, but rather is separated from the ligand and surrounded by three DME molecules, which may be due to the strong competition of the chelating solvent DME. Unfortunately, the quality of the structure is not good enough; nevertheless, the main backbone of the compound is unambiguous. The dipotassium analogue [K2(LH4)(DME)4] (2) was obtained in a same way as that for the sodium complex 1 by using 2 equiv of K metal. Its structure is very similar to that of 1, consisting of two K+ ions in different coordination environments (Figure 2). The coordination of the K1 atom

from the larger radius of K+ (r = 1.33 Å) than Na+ (0.98 Å), requesting more interactions around K.28 Notably, the C−N bond lengths of the other, uncoordinated amino−amido unit (N3, N4) are close to each other (N3−C7, 1.373(9); N4−C8, 1.368(4) Å), suggesting a possible distribution of the proton on the two nitrogen atoms to result in an averaged state of fast exchange. On the other hand, like complex 1, the 1H NMR spectrum of 2 demonstrates the presence of four equivalent protons at δ = 5.24 ppm, suggesting that the four amine protons are indistinguishable in solution. This may result from the fast exchange of the protons in solution within the NMR time scale, while in the solid state they are more “fixed”. To further understand the electronic structure of the complexes and the selectivity of the deprotonation sites, DFT computations were carried out at the B3LYP level with 6-31G* basis sets. Simplified model compounds, wherein the N-dipp substituents on the ligand were replaced by phenyl groups and the coordinated DME molecules were simplified to H2O, were used for the evaluation of the ligand and complexes 1−4 (see Supporting Information for details). The optimized structures of ligand L′H6 and complexes [Na2(L′H4)(H2O)5] (1a) and [K2(L′H4)(H2O)4] (2a) are very close to the X-ray results (Figure S10). An examination of double deprotonation of the hexamine LH6 demonstrated that removing two protons from two “PDAH2” arms is energetically more favored (by 147.75 kJ mol−1) than taking both protons from the same one PDAH2. This is consistent with the crystal structures of 1 and 2. Moreover, according to the surface electrostatic potential map for 2, the aryl rings show considerable negative charges (Figure S11) upon deprotonation and delocalization of the electrons, which may explain the η6-coordination of an arene ring to the K2 ion. [K3(LH3)(DME)6]·DME (3). When 3 equiv of K was used to react with ligand LH6, the trinuclear complex 3 was obtained as reddish orange crystals, in which three amino groups of LH6 were deprotonated as indicated by the 1H NMR (5.24 ppm, 3H) and IR spectrum. As in the dinuclear analogues 1 and 2, in the tripotassium complex 3 the deprotonation occurs on each diamine arm to yield three “PDAH−” units, with shortened N2−C2 (1.365(4) Å), N4−C8 (1.364(4) Å), and N6−C14 (1.359(5) Å) bonds (Figure 3). DFT calculations also revealed that this mode is favored (see SI). There are also two distinct coordination modes for the three K+ ions. Two of them (K1 and K2) are each chelated by a “PDAH” arm of the (LH3)3− ligand, as well as two DME molecules to complete an octahedral environment. In this case, the potassium ion is coordinated by both of the amino and amido N atoms, which is different from complexes 1 and 2 (by only the amido N). The K−N bond lengths (2.776(3)−3.024(4) Å, av 2.862 Å) are somewhat longer than that in 2 (2.765(7) Å), but they all fall in the range of K−N bonding in similar compounds.29 Moreover, the K1 and K2 atoms deviate dramatically from the plane defined by the six N atoms, with vertical distances of 1.974 and 1.979 Å, respectively. This is similar to the potassium complex with bis(amino−amido) ligand [dipp−dabqdi]2− reported by us22 and might be caused by the presence of one proton. In addition, once again, as the situation of the K2 atom in complex 2, the third potassium ion (K3) in 3 does not bond to the nitrogen atoms but is η4-coordinated by one aryl ring of the triphenylene scaffold. [Li6(L)(DME)6]·DME·Toluene (4). When the lithium agent n-BuLi (6 equiv) was reacted with LH6, the fully deprotonated

Figure 2. Molecular structure of 2 (thermal ellipsoids are set at the 30% probability level; isopropyl groups and CH hydrogen atoms have been omitted for clarity). Selected bond lengths (Å): N1−C1, 1.392(10); N2−C2, 1.356(10); N3−C7, 1.373(9); N4−C8, 1.368(10); N5−C13, 1.422(11); N6−C14, 1.412(11); K1−N2, 2.769(6); K−O, 2.613(9)−2.963(10).

resembles that of Na1 in complex 1, with the amido N2 atom of a “PDAH−” unit and two DME molecules (but not with the “amino” N1, 3.972 Å), and the difference lies in that it is in further contact with two carbon atoms of a nearby aryl ring. The K1−N2 bond length of 2.769(7) Å compares with those reported for the PDA complex [{(PDAK 2 )(THF) 3 } 2 ] (2.763(5) Å).25 The other potassium (K2) is also away from the amino−amido N,N-donors as Na2 in 1. However, unlike the completely solvated sodium ion, the K2 atom is η6coordinated by one aryl ring beside two DME molecules. This is similar to the compound Ph3CK·THF·PMDTA, in which the K+ cation favors η6-interaction with the more chargedelocalized phenyl ring. The K−C(aryl) distances (3.120(8)− 3.353(9) Å) in the current complex 2 are comparable to those of the latter compound (ca. 2.892(15)−3.663(8) Å).27 The difference between the Na and K complexes 1 and 2 may arise C

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coordinated by a “PDA2−” arm of the (L)6− ligand in the η4fashion through the C2N2 moiety. They are located out of the ligand plane (by 1.619 and 1.677 Å, respectively) with slightly longer Li−N bond lengths (2.094(8)−2.147(9) Å), consistent with the π-coordination to the diamido C2N2 moiety (Li−C: 2.270(8)−2.309(8) Å). All of these Li−N bond lengths are in the normal range observed for Li−N(amido) bonds.31−33 The last Li atom (Li6), once again as in the cases of complexes 1−3, does not interact with the amido nitrogen donors of the completely deprotonated ligand. Instead, it is η6-coordinated by the carbon atoms of one aryl ring of the triphenylene scaffold with Li−C distances of 2.255(8)−2.534(8) Å, which compare well to reported arene−lithium interactions.34 The coordination sphere of each lithium ion is completed by one DME molecule, demonstrating the tendency of lithium to interact with electron-rich molecules.35 Synthesis and Structures of the Tris(tetrylenes) 5−7. Generally, the N-heterocyclic tetrylenes of the heavier group 14 elements E (Ge, Sn, and Pb) were conveniently prepared by transmetalation of the lithium salts with EII halides. The same protocol was applied for the synthesis of tris(tetrylenes) 5−7. The hexalithiated species (in situ synthesized by free ligand LH6 and n-BuLi) was treated with GeCl2·dioxane, SnCl2, or PbCl2, and three deeply colored products, the yellow [Ge3(L)]· toluene (5) and red [Sn3(L)]·2toluene (6) and [Pb3(L)]· 6toluene (7) were isolated as air- and moisture-sensitive but thermally stable compounds (Scheme 1). [Ge3(L)]·Toluene (5). Single crystals of the tris-germylene 5 were obtained by slow evaporation of saturated toluene solution of the crude product. X-ray diffraction analysis confirms the presence of three Ge centers (Figure 5) with

Figure 3. Molecular structure of 3 (thermal ellipsoids are set at the 30% probability level; isopropyl groups and CH hydrogen atoms have been omitted for clarity). Selected bond lengths (Å): N1−C1, 1.408(4); N2−C2, 1.365(4); N3−C7, 1.439(4); N4−C8, 1.364(4); N5−C13, 1.432(5); N6−C14, 1.359(5); K1−N3, 2.864(3); K1−N4, 2.776(3); K2−N5, 3.024(4); K2−N6, 2.788(4); K−O, 2.608(5)− 3.016(5).

ligand (L)6− was obtained as the hexanuclear lithium complex 4. Correspondingly, the 1H NMR spectrum reveals the absence of NH resonances, which is again proved by the disappearance of the NH stretch in the IR spectrum. As reported in the literature, the structures of lithium compounds often do not follow classical bonding considerations.30 In complex 4, the six lithium ions display three different coordination modes (Figure 4). Three of them (Li1, Li3, and Li5) are located in the ligand plane and are each chelated by the N,N′-donors of one diamido (Li−N: 1.934(8)−2.016(8) Å), generating three five-membered rings which are slightly puckered about the N···N vector (vertical distance of Li to ligand plane: 0.688, 0.657, and 0.533 Å). Another two of the lithium atoms (Li2 and Li4) are each

Figure 5. Molecular structure of 5 (thermal ellipsoids are set at the 30% probability level; isopropyl groups, hydrogen atoms, and lattice solvent molecules have been omitted for clarity). Selected bond lengths (Å) and angles (deg): Ge1−N1, 1.867(4); Ge1−N2, 1.866(4); Ge2−N3, 1.860(4); Ge2−N4, 1.866(4); Ge3−N5, 1.863(4); Ge3− N6, 1.876(4); N1−Ge1−N2, 84.67(16); N3−Ge2−N4, 84.62(16); N5−Ge3−N6, 84.92(16).

slightly varying Ge−N (1.860−1.876 Å) bond lengths, which are comparable to those previously reported for benzannulated and related NHGes.12 The Ge atoms (without solvation) are each chelated by two N atoms in a “PDA” arm of the (L)6− ligand, and the five-membered rings show a planar geometry. The Dipp substituents in 5 are oriented almost in a perpendicular manner to the central triphenylene plane. No

Figure 4. Molecular structure of 4 (thermal ellipsoids are set at the 30% probability level; isopropyl groups and CH hydrogen atoms have been omitted for clarity). Selected bond lengths (Å): N1−C1, 1.399(5); N2−C2, 1.409(5); N3−C7, 1.389(5); N4−C8, 1.399(5); N5−C13, 1.377(5); N6−C14, 1.372(6); Li−N, 1.988(8)−2.144(9); Li−O, 1.962(7)−2.613(11). D

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suitable for X-ray diffraction study (Figure 7). The metric parameters of 7 do not differ significantly from those reported

intermolecular interactions between molecules of 5 have been observed in the solid state, as in the case of benzannulated Nheterocyclic germylene with the bulky N,N′-Dipp substituents (which may be viewed as one-third of compound 5).12b However, intermolecular C···Ge interactions were observed between the benzannulated NHGe molecules with the smaller neopentyl groups.36 This difference might be caused by the steric hindrance of the Dipp groups. [Sn3(L)]·2Toluene (6). Reaction of the lithium salt [Li6L] with SnCl2 proceeded through transmetalation at each arm of the hexamido molecule to give the tris-stannylene 6 in 90% yield. Crystals of 6 were grown by storage of its toluene solution at −10 °C. The molecular structure of 6 is shown in Figure 6. The average Sn−N bond length of 2.065 Å and the

Figure 7. Molecular structure of 7 (thermal ellipsoids are set at the 30% probability level; isopropyl groups, hydrogen atoms, and lattice solvent molecules have been omitted for clarity). Selected bond lengths (Å) and angles (deg): Pb1−N1, 2.165(9); Pb1−N2, 2.191(9); Pb2−N3, 2.166(9); Pb2−N4, 2.197(9); Pb3−N5, 2.171(9); Pb3−N6, 2.179(9); N2−Pb1−N1, 75.8(3); N3−Pb2−N4, 74.9(3); N6−Pb3− N5, 75.7(3).

for related benzannulated N-heterocyclic plumbylenes.14 The average N−Pb−N angle (75.5°) is more compressed compared to those of 5 (84.7°) and 6 (78.3°). It is clear that, as the E atom becomes heavier, the N−E−N bond angle is more acute in the compounds. This trend is common in tetrylenes and is attributed to the relativistic effect.37 Notably, in contrast to the related germylene 5 and stannylene 6, which are discrete monomers in the solid state, the heaviest analogue 7 exists as a polymer with incontrovertible contacts of the Pb atoms to the aryl rings of Dipp substituents on adjacent molecules in the η6- or η3-fashion (Figure 8). The Pb···C(aryl) contacts fall in the range from 3.49 to 3.68 Å, with Pb···centroid distances to the η6-arene of 3.45, 3.29, and 3.27 Å, respectively, for the three Pb atoms, which are similar to those reported for benzannulated NHPbs.14 These intermolecular Pb−C interactions account for the poor solubility of 7 and can be attributed to the enhanced Lewis acidity of the empty p-orbital of the

Figure 6. Molecular structure of 6 (thermal ellipsoids are set at the 30% probability level; isopropyl groups, hydrogen atoms, and lattice solvent molecules have been omitted for clarity). Selected bond lengths (Å) and angles (deg): Sn1−N1, 2.069(3); Sn1−N2, 2.066(4); Sn2−N3, 2.066(4); Sn2−N4, 2.073(4); Sn3−N5, 2.071(4); Sn3−N6, 2.061(4); N2−Sn1−N1, 78.22(14); N3−Sn2−N4, 78.31(14); N6− Sn3−N5, 78.30(14).

N−Sn−N angle of 78.3° fall in the normal range previously reported for benzannulated NHSns12b,13a and tin analogues of Arduengo carbenes.13c The structure of 6 is similar to the related triphenylene-based tris(N-heterocyclic stannylenes) except that, in the latter compounds, the substituents on nitrogen are benzhydrylamine or mesitylene groups (instead of Dipp).18 Like in the germylene analogue 5, the Sn3L compound is monomeric without contacts to adjacent molecules (the shortest intermolecular Sn···C distance measures 3.84 Å), which could also be attributed to the sterically demanding Dipp substituents and the weak Lewis acidity of tin that prevent the intermolecular Sn···N, Sn···C, and Sn···Sn interactions commonly observed in the solid state of stannylenes with less bulkier groups. 12b,13a However, there are weak Sn···C interactions in the benzannulated NHSns with Dipp substituents.12b In the 119Sn NMR spectrum of 6, only one sharp resonance was observed at δ = 235.4 ppm in C6D6 (Figure S9). The chemical shift is similar to those previously observed for benzhydryl-substituted (δ = 226.4 ppm) and mesitylsubstituted (δ = 257.1) tris-stannylenes,18 and the Dippsubstituted benzannulated NHSn (δ = 216.1 ppm),12b confirming the formation of tris-NHSn. [Pb3(L)]·6Toluene (7). The tris-plumbylene 7 was obtained by a similar reaction of [Li6L] and PbCl2 with a yield of up to 67%. Storage at −35 °C produced blue crystals that were

Figure 8. Intermolecular Pb···C(aryl) interactions in 7. E

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Figure 9. Calculated frontier molecular orbitals of 5a−7a.

to the hypochromic shift of diaryl tetrylenes.39 Generally, the n−p (lone pair orbital to π-symmetric unoccupied orbital) transition of tetrylenes is assigned to be responsible for the visible region absorption.39b,40 However, TD-DFT calculations on 5a−7a demonstrated that the lowest-energy band corresponds to the transition from HOMO (both metal- and ligand-based π orbital) to LUMO (metal-based π-symmetric unoccupied orbital) instead of the n−p transitions which feature higher-energy gaps (5.3 eV for 5a, 5.8 eV for 6a, 6.2 eV for 7a) (Table S3). This transition mode of the tetrylenes was also reported for the unsaturated N-heterocyclic tetrylenes (HCRN)2EII (E = Si, Ge, Sn).41 The λmax (421 nm) of 5 in the visible region is longer than that (350 nm) of unsaturated Nheterocyclic germylene (HCRN)2Ge,42 which could be caused by the extended π-conjugation in the former. The λmax (512 nm) of the stannylene 6 is close to that for the known trisstannylene analogues with triphenylene backbone (504 nm).

plumbylene and larger atom radius of Pb compared to those of the germylene and stannylene.15c DFT calculations (PBE1PBE-SVP) were carried out on model compounds 5a, 6a, and 7a (see SI for details) to elucidate the electronic properties of the tetrylenes. The results demonstrate that the lone pair (σ) orbital at the center E atom is HOMO − 6 for 5a, HOMO − 7 for 6a, and HOMO − 6 for 7a (Figure 9). This lowering of energy of the lone pair orbital was also reported for similar tetrylenes, and the “inert s-pair effect” may be responsible for it.38 On the other hand, the πsymmetric unoccupied orbital at center E is LUMO for the three molecules. The energy of the lone pair orbital reflects the weakening of σ donation ability from the germylene 5a down to plumbylene 7a, while the LUMO orbital shows the expected tendency of decrease for the three tetrylenes, indicating increasing π-accepting ability. However, the tin and lead compounds have very close energy levels, which is similar to the reported calculation results on tetrylenes.38 UV−vis spectra of compound 5−7 were recorded in toluene solution. As shown in Figure 10, the tetrylenes feature two

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CONCLUSION In summary, the C 3 -symmetric hexamine (LH 6) with triphenylene and Dipp-PDAH2 fragments undergoes partial or complete deprotonation to form alkali metal derivatives. Complexes 1−3 (with the doubly or triply deprotonated ligand) feature the amino−amido mode (i.e., only one proton of the PDAH2 unit is removed), while the hexalithium complex 4 shows the tris-diamido form of the ligand. These alkali metal salts can serve as starting materials for the preparation of new multinuclear metal complexes, as demonstrated by the facile synthesis of the tris(N-heterocyclic tetrylenes) 5, 6, and 7 from the lithium salt of the hexamido ligand and EII halides. We are currently carrying out the transmetalation reactions of the alkali metal salts with other main group and transition metals, and will report the results in due course.

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Figure 10. UV−vis spectra of 5−7 in toluene.

EXPERIMENTAL SECTION

General Procedures. All reactions were performed under an inert atmosphere of argon or nitrogen gas using standard Schlenk vacuum line and glovebox techniques. Solvents were dried by refluxing over and distillation from sodium/benzophenone (hexane, toluene, THF, and DME) or calcium hydride (dichloromethane). Sodium metal, potassium metal, n-butyl lithium (hexane solution), tin(II) chloride (anhydrous), and lead(II) chloride were purchased from Alfa Aesar, and the germanium(II) chloride dioxane complex was purchased from Aldrich. The 1H and 13C NMR spectra were recorded on a Bruker Avance III-400 spectrometer at 298 K, and 7Li, 119Sn NMR spectra

broad absorption bands. The peak in the UV region at about 300 nm is similar to each other and can be attributed to the π−π* transition of the ligand backbone, which is confirmed by time-dependent density functional theory (TD-DFT) calculations (see SI for details). The absorption band in the visible region (5, 421 nm; 6, 512 nm; 7, 580 nm) shows a dramatic bathochromic shift on descending group 14, which is opposite F

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Synthesis of 4. A solution of n-butyl lithium (0.50 mL, 1.2 mmol; 2.5 M in hexanes) was added dropwise to a cold (0 °C) solution of LH6 (0.26 g, 0.2 mmol) in DME (20 mL) under stirring. After warming to room temperature, the resultant dark yellow-green solution was stirred for 24 h. Removal of volatiles in vacuo followed by recrystallization from DME (3 mL) at room temperature for several days yielded yellow-green crystals of 4. Yield: 0.18 g, 70%. This compound is extremely air-sensitive and decomposes under prolonged vacuum at room temperature. Consequently, no satisfactory analytical data were obtained. Nevertheless, the 1H NMR clearly demonstrates the disappearance of the NH signal, while the 7Li NMR spectrum shows a single peak at 1.06 ppm, suggesting that the six lithium centers are equivalent in solution. IR (KBr, ν/cm−1): 2958, 2925, 2867, 1471, 1338, 1255, 1130, 850, 782, 740, 690, 553, 414. Synthesis of 5. Ligand LH6 (0.26 g, 0.2 mmol) was dissolved in THF (8 mL), and a solution of 2.5 M n-BuLi in hexanes was added at −35 °C. The clear pale yellow solution was allowed to warm to room temperature and stirred for 10 h. Then, GeCl2·dioxane (0.6 mmol) was added at 0 °C, and the reaction mixture was stirred at ambient temperature for another 10 h. Subsequently, all solvents were removed in vacuo, and the residue was extracted by toluene (10 mL). The extract was filtered, concentrated, and stored at room temperature for several days to afford the germylene 5 as yellow crystals. Yield: 0.12 g, 40%. 1H NMR (C6D6, 298 K): δ 0.83 (d, J = 6.8 Hz, 36H, CH3), 1.24 (d, J = 6.8 Hz, 36H, CH3), 2.97 (sept, J = 6.6 Hz, 12H, CH(CH3)2), 7.26 (m, 18H, CH-aryl-dipp), 7.40 (s, 6H, CH-aryl-triphenylene). 13 C{1H} NMR (C6D6, 298 K): δ 24.0, 26.0, 28.4, 104.1, 123.7, 123.8, 128.9, 137.1, 142.2, 145.3. IR (KBr, ν/cm−1): 2950, 2926, 2860, 1511, 1430, 1321, 1260, 1201, 797, 684, 501. Anal. Calcd for C90H108N6Ge3 (1491.78): C, 72.46; H, 7.30; N, 5.63. Found: C, 72.28; H, 7.46; N, 5.35. Synthesis of 6. Compound 6 was synthesized in a similar manner to that of 5, by employing SnCl2 (0.11 g, 0.6 mmol). Red crystals of the product were isolated after cooling the concentrated toluene solution to −18 °C. Yield: 0.32 g, 90%. 1H NMR (C6D6, 298 K): δ 0.80 (d, J = 6.4 Hz 36H, CH3), 1.22 (d, J = 6.8 Hz, 36H, CH3), 3.01 (m, 12H, CH(CH3)2), 7.26 (m, 18H, CH-aryl-dipp), 7.37 (s, 6H, CHaryl-triphenylene). 13C{1H} NMR (C6D6, 298 K): δ 24.4, 26.4, 28.0, 123.8, 125.4, 126.8, 128.2, 129.0, 137.6, 145.1. IR (KBr, ν/cm−1): 2954, 2923, 2862, 1510, 1434, 1321, 1265, 1200, 793, 694, 505, 422. Anal. Calcd for C90H108N6Sn3 (1629.99): C, 66.32; H, 6.68; N, 5.16. Found: C, 66.54; H, 6.41; N, 4.92. Synthesis of 7. In a similar manner, the plumbylene 7 was synthesized by using PbCl2 (0.16 g, 0.6 mmol) as blue crystals. Yield: 0.33 g, 67%. 1H NMR (C6D6, 298 K): δ 0.81 (d, 36H, J = 4.0 Hz, CH3), 1.22 (d, J = 5.6 Hz, 36H, CH3), 3.04 (sept, J = 5.0 Hz, 12H, CH(CH3)2), 7.25 (m, 18H, CH-aryl-dipp), 7.37 (s, 6H, CH-aryltriphenylene). 13C{1H} NMR (C6D6, 298 K): δ 24.7, 26.4, 27.2, 108.7, 123.6, 126.6, 128.9, 141.9, 146.5, 153.2. IR (KBr, ν/cm−1): 2954, 2922, 2858, 1512, 1456, 1323, 1259, 1200, 789, 677, 500, 449. Anal. Calcd for C90H108N6Pb3 (1895.46): C, 57.03; H, 5.74; N, 4.43. Found: C, 57.37; H, 6.02; N, 4.26. DFT Calculations. DFT computations were carried out in order to gain more insight into the electronic structure of the ligand and complexes. The structure optimization and NBO bonding analysis were done at the B3LYP level with the 6-31G* basis sets using the Gaussian 09 program. The model compounds L′H6, [Na2(L′H4)(H2O)5] (1a), [K2(L′H4)(H2O)4] (2a), [K3(L’H3)(H2O)6] (3a), and [Li6(L′)(H2O)6] (4a), wherein the N-dipp substituents on the ligand were replaced by phenyl groups and the coordinated DME molecules were simplified to H2O, were used for the evaluation of ligand LH6 and the complexes. For the tetrylenes 5−7, DFT calculations (PBE1PBE-SVP) were carried out on the model molecules 5a−7a with the Dipp substituents on the ligand replaced by hydrogen atoms (all attempts to optimize the geometry of the full molecules met with failure). Moreover, TD-DFT calculations were performed for the optimized structures of 5a−7a. X-ray Crystal Structure Determination. Diffraction data for compounds 1−7 were collected on a Bruker SMART APEX II diffractometer at low temperature (100 K) with graphite-monochro-

were recorded on a Bruker Avance III-700 spectrometer, with an external standard of 0.6 M LiCl in methanol-d4 (for 7Li) and 0.5 M Me4Sn in benzene-d6 (for 119Sn). Benzene-d6 was dried over Na/K alloy. IR spectra were recorded in THF solution using potassium bromide plates as infrared transmission window on a Nicolet AVATAR 360 FT-IR spectrometer. UV−vis measurements were performed on a Cary-100 spectrophotometer using cuvettes from Ajilent Technologies. Synthesis of LH6. The starting material 2,3,6,7,10,11-hexabromotriphenylene was prepared according to the literature method.43 The ligand LH6 was synthesized by modifying the literature procedure of Buchwald−Hartwig Pd-catalyzed cross-coupling amination of 2,3,6,7,10,11-hexabromotriphenylene with the aromatic amine under basic conditions.24 The reaction was conducted with excess 2,6diisopropylphenylamine (7.5 equiv) in refluxing toluene for 3 days (instead of with 6 equiv of the amine in o-xylene at 143 °C for 24 h). The mixture was concentrated and the residue recrystallized from nhexane to yield an off-white crystalline material. 1H NMR (C6D6, 298 K): δ 0.85 (d, J = 6.8 Hz, 36H, CH3), 1.23 (d, J = 6.4 Hz, 36H, CH3), 3.21 (m, 12H, CH(CH3)2), 5.23 (s, 6H, NH), 7.09 (s, 6H, CH-aryltriphenylene), 7.17 (p-CH-aryl-dipp, overlapped with the signal of solvent C6D6), 7.31 (t, J = 7.6 Hz, 6H, m-CH-aryl-dipp). 13C{1H} NMR (C6D6, 298 K): δ 23.9, 24.8, 28.6, 107.6, 123.9, 124.6, 126.9,127.8, 136.6, 145.2. IR (KBr, ν/cm−1): 3357 (NH), 2960, 2925, 2867, 1616, 1511, 1440, 1328, 1199, 1137, 1097, 794, 553, 414. Anal. Calcd for C90H114N6 (1279.91): C, 84.46; H, 8.98; N, 6.57. Found: C, 84.81; H, 9.28; N, 6.29. Synthesis of 1. Sodium complex 1 was obtained by reaction of LH6 (0.26 g, 0.2 mmol) with 2 equiv of sodium metal (0.01 g, 0.4 mmol) in DME (20 mL). The mixture was stirred at room temperature (rt) for several days, and the volatiles were removed in vacuo. Recrystallization from DME (3 mL) at room temperature overnight yielded orange crystals of 1 suitable for X-ray crystallographic analysis. Yield: 0.15 g, 60%. 1H NMR (C6D6, 298 K): δ 0.85 (d, J = 5.2 Hz, 36H, CH3), 1.23 (d, J = 5.2 Hz, 36H, CH3), 3.21 (m, 12H, CH(CH3)2), 5.23 (s, 4H, NH), 7.09 (s, 6H, CH-aryltriphenylene), 7.18 (p-CH-aryl-dipp, overlapped with the signal of solvent C6D6), 7.31 (t, J = 7.2 Hz, 6H, m-CH-aryl-dipp). 13C{1H} NMR (C6D6, 298 K): δ 23.5, 24.5, 28.2, 107.3, 123.6, 124.2, 126.6, 127.5, 136.3, 144.9. IR (KBr, ν/cm−1): 3357 (NH), 2960, 2925, 2867, 1440, 1328, 1265, 1195, 1045, 744, 553, 414. Anal. Calcd for C220H324N12Na4O20 (3548.95): C, 74.45; H, 9.20; N, 4.74. Found: C, 74.77; H, 9.58; N, 4.53. Synthesis of 2. Freshly cut potassium metal (0.016 g, 0.40 mmol) was added to a solution of LH6 (0.26 g, 0.20 mmol) in DME (30 mL), and the reaction mixture was stirred at ambient temperature for several days. The resulting mixture was filtered and the filtrate concentrated to about 5 mL. Slow evaporation at ambient temperature afforded the product as dark red crystals (0.19 g, 78%). 1H NMR (C6D6, 298 K): δ 0.85 (d, J = 6.4 Hz, 36H, CH3), 1.23 (d, J = 6.4 Hz, 36H, CH3), 3.21 (m, 12H, CH(CH3)2), 5.24 (s, 4H, NH), 7.09 (s, 6H, CH-aryltriphenylene), 7.17 (p-CH-aryl-dipp, overlapped with the signal of solvent C6D6), 7.31 (t, J = 7.6 Hz, 6H, m-CH-aryl-dipp). 13C{1H} NMR (C6D6, 298 K): δ 23.5, 24.5, 28.2, 107.3, 123.6, 124.2, 126.6, 127.5, 136.3, 144.8. IR (KBr, ν/cm−1): 3353 (NH), 2960, 2925, 2869, 1467, 1328, 1263, 1135, 1047, 794, 553, 414. Anal. Calcd for C106H152N6K2O8 (1716.57): C, 74.17; H, 8.93; N, 4.90. Found: C, 74.17; H, 8.85; N, 4.63. Synthesis of 3. Complex 3 was prepared by a procedure similar to that employed for 2, from LH6 (0.26 g, 0.20 mmol) and 3 equiv of K (0.02 g, 0.40 mmol), as dark red crystals (crystal yield: 0.17 g, 66%). 1 H NMR (C6D6, 298 K): δ 0.85 (d, J = 5.6 Hz, 36H, CH3), 1.22 (d, J = 5.2 Hz, 36H, CH3), 3.21 (m, 12H, CH(CH3)2), 5.24 (s, 3H, NH), 7.09 (s, 6H, CH-aryl-triphenylene), 7.17 (p-CH-aryl-dipp, overlapped with the signal of solvent C6D6), 7.31 (t, J = 6.8 Hz, 6H, m-CH-aryldipp). 13C{1H} NMR (C6D6, 298 K): δ 23.5, 24.5, 28.2, 107.3, 123.6, 124.2, 126.6, 127.5, 136.3, 144.9. IR (KBr, ν/cm−1): 3355 (NH), 2958, 2923, 2867, 1465, 1328, 1270, 1135, 1085, 555, 414. Anal. Calcd for C114H171N6K3O12 (1934.91): C, 70.76; H, 8.91; N, 4.34. Found: C, 71.12; H, 9.27; N, 4.07. G

DOI: 10.1021/acs.inorgchem.6b01743 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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mated Mo Kα radiation (λ = 0.71073 Å). Absorption correction, structure solution, and refinement were done by using the SADABS,44 SHELXS-2014, and SHELXL-2014 programs.45 Hydrogen atoms bonded to carbon were included in idealized geometric positions with thermal parameters equivalent to 1.2 times those of the atom to which they were attached. More details of the refinement are given in the Supporting Information. Crystallographic data for 1−7 are listed in Tables S1 and S2. CCDC numbers 1450599−1450602 (1−4) and 1457409−1457411 (5−7).

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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.6b01743. Experimental details, crystallographic data, and characterization data (PDF) Crystallographic data for 1 (CIF) Crystallographic data for 2 (CIF) Crystallographic data for 3 (CIF) Crystallographic data for 4 (CIF) Crystallographic data for 5 (CIF) Crystallographic data for 6 (CIF) Crystallographic data for 7 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions §

F.Z. and X.Y. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21273170). REFERENCES

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DOI: 10.1021/acs.inorgchem.6b01743 Inorg. Chem. XXXX, XXX, XXX−XXX