Synthesis and Structural Characterization of Tris

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Synthesis and Structural Characterization of Tris(isopropylbenzimidazol-2-ylthio)methyl Zinc Complexes, i [TitmPr Benz]ZnX: Modulation of Transannular Zn−C Interactions Serge Ruccolo, Michael Rauch, and Gerard Parkin* Department of Chemistry, Columbia University, New York, New York 10027, United States S Supporting Information *

ABSTRACT: Tris(1-isopropylbenzimidazol-2-ylthio)methane i ([TitmPr Benz]H) is obtained by treatment of 1-isopropyl-1,3dihydro-2H-benzimidazole-2-thione with NaH followed by i CHI3. The reaction of [TitmPri Benz]H with Me2Zn affords the zinc methyli complex [κ3-TitmPr Benz]ZnMe, which provides access 3 PriBenz to [Titmi Pr Benz]ZnOAr (Ar = p-C6H4Br), [κ -Titm ]ZnH, PriBenz {[TitmPr Benz]Zn}[HB(C ]Zn}[MeB(C6F5)3], 6F5)3], {[Titm PriBenz and {[Titm ]Zn}[BPh4]. X-ray diffraction demonstrates i that the hydridei and methyl complexes [κ3-TitmPr Benz]ZnH 3 Pr Benz 3 and [κ i-Titm ]ZnMe exhibit κ coordination of the i [Titm Pr iBenz ] ligand, whereas [Titm Pr Benz ]ZnOAr and {[TitmPr Benz]Zn}[HB(C6F5)3] exhibit κ4 coordination and i i adopt an atrane motif. The transannular Zn−C distances in [TitmPr Benz]ZnX (X i = H, Me, OAr) and {[TitmPr Benz]Zn}+ span Pr Benz + ]Zn} and i the longest corresponding to a rangei of 2.093(4)−2.367(2) Å, with the shortest corresponding to {[Titm [TitmPr Benz]ZnOAr. The variation is attributed to the Zn−C bond in four-coordinate {[TitmPr Benz]Zn}+ being a two-center− PriBenz two-electron interaction, whereas that in five-coordinate [Titm ]ZnOAr is a component of a three-center−four-electron interaction, in which the HOMO is approximately an spn hybrid lone pair orbital on carbon.

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([TismPr Benz]),11 allow for the construction of metallacarbatranes, as illustrated in Figure 2.12,13 Although [Tptm] and [TitmMe] differ by the presence of six-membered pyridyl and five-membered imidazolyl rings, respectively, a common aspect is that the carbon atoms that are adjacent to the nitrogen atom donors possess only hydrogen atom substituents; as such, the ligands do not possess much steric bulk. Therefore, we report here a more sterically demanding counterpart of [TitmMe] which features benzannulated rings.14

INTRODUCTION C3-symmetric tetradentate tripodal ligands which afford atrane motifs1−4 provide a useful complement to tridentate tripodal ligands that are devoid of a bridgehead donor. Such ligands can be classified by whether the transannular bond corresponds to an M←L, M−X or M→Z interaction according to the covalent bond classification (Figure 1).5 Of these, atranes which feature

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Tris(1-isopropylbenzimidazol-2-ylthio)methane ([TitmPr Benz]H) may be obtained by treatment of 1-isopropyl-1,3-dihydro-2Hbenzimidazole-2-thione15 with NaH followed by reactioni with CHI3 (Scheme 1).16 The molecular structure of [TitmPr Benz]H has been determined by X-ray diffraction, as illustrated in Figure 3, with selected bond lengths and angles given in Table 1. The H−C−S−C torsion angles are 27.8, 34.0, and 178.3°, which indicate that two of the benzimidazolyl groups are rotated about the HC−S bond such that they point “up” and towards the C−H group (i.e., 90°). The corresponding C−S−C−N torsion angles are 6.2, 4.0, and 3.9°, thereby indicating that each

Figure 1. Classification of atranes according to whether the transannular interaction involves an L-, X-, or Z-type binding site.

M←L4,6 and M→Z4,7 dative interactions are more commonly encountered than those which possess M−X interactions. Therefore, it is noteworthy that we have recently synthesized a variety of different carbatranes that feature M−C bonds. Specifically, several tetradentate tripodal ligands that incorporate a central carbon atom, namely tris(2-pyridylthio)methyl ([Tptm]),8,9 tris(1-methylimidazol-2-ylthio)methyl ([TitmMe]),10 and tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl © XXXX American Chemical Society

RESULTS AND DISCUSSION

Received: March 16, 2018

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DOI: 10.1021/acs.organomet.8b00158 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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Figure 3. Molecular structure of [TitmPr Benz]H. Displacement ellipsoids are depicted at the 30% probability level, and hydrogen atoms on benzimidazolyl groups are omitted for clarity. i

Figure 2. Examples of atranes with transannular M−C interactions i ([Tptm],8,9 [TitmMe],10 and [TismPr Benz]11 have been previously reported).

Table 1.i Selected Metrical Data for [TitmPr Benz]H and [TismPr Benz]H i

[TitmPr Benz]Ha

benzimidazolyl group lies approximately in the C−S−C plane. In contrast to the two “up”/one “down” conformations of the PriBenz benzimidazolyl groups of [Titm ]H, those in the closely i related compound [TismPr Benz]H,11a which differs by virtue of the presence of Me2Si rather than S linkers (Figure 2), possess three “up” conformations with H−C−Si−C torsion angles that are all less than 90° (32.5, 49.3, and 51.1°). Access to zinc compounds is provided by the reaction of i [TitmPr Benzi ]H with Me2Zn to afford the methyl complex [κ3-TitmPriBenz]ZnMe (Scheme 1). The hydride counterpart, [κ3-TitmPr Benz]ZnH, has also beeni obtained via a sequence that involves (i) reaction of [κ3-TitmPr Benz]ZnMe with ArOH (Ar = i p-C6H4Br) to afford the aryloxide [TitmPr Benz]ZnOAr, followed by (ii) metathesis with PhSiH 2. 3, as illustrated in Scheme i i Spectroscopically, [κ3-TitmPr Benz]ZnMe and [κ3-TitmPr Benz]ZnH are characterized by 1H NMR signals at δ 0.55 and 5.77 ppm, which are respectively attributable to the zinc methyl and hydride ligands. i Thei molecular structuresi of [κ3-TitmPr Benz]ZnMe, [κ3TitmPr Benz]ZnH, and [TitmPr Benz]ZnOAr have been determined i by X-ray diffraction, which demonstrate that the [TitmPr Benz]ZnX compounds belong to two structural classes. Specifically, i 3 PriBenz the [TitmPr Benz ] ligands of [κ -Titm ]ZnMe (Figure 4) i and [κ3-TitmPr Benz]ZnH (Figure 5) coordinate in a hypodentate17

a

i

[TismPr Benz]Hb

d(C1−X1)/Å d(C1−X2)/Å d(C1−X3)/Å

1.815(3) 1.806(3) 1.793(3)

1.8856(18) 1.8778(18) 1.8898(18)

X1−C1−X2/deg X1−C1−X3/deg X2−C1−X3/deg

106.69(15) 112.62(18) 114.80(18)

115.60(9) 114.34(9) 112.32(9)

H1−C1−X1−Cim1/deg H1−C1−X2−Cim2/deg H1−C1−X3−Cim3/deg C1−X1−Cim1−N12/deg C1−X2−Cim2−N22/deg C1−X3−Cim3−N32/deg

27.79 33.95 178.26 6.87 3.89 4.50

51.09 32.52 49.33 131.99 23.13 115.62

This work (X = S). bReference 11a (X = Si).

κ3 manner, such that one of the benzimidazolyl groups remains i i uncoordinated, whereas the [TitmPr Benz] ligand of [TitmPr Benz]4 ZnOAr (Figure 6) coordinates in a κ manner, thereby resulting in an atrane motif. Precedent for these structural differences is i provided by [TitmMe]ZnX,10 [Tptm]ZnX,8a and [TismPr Benz]ZnX11a,c systems, for which the hydride and methyl complexes exhibit κ3 coordination, while most other derivatives typically exhibit κ4 coordination.

Scheme 1

B


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Organometallics Scheme 2

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Figure 6. Molecular structure of [TitmPr Benz]ZnO-p-C6H4Br. Displacement ellipsoids are depicted at the 30% probability level, and hydrogen atoms are omitted for clarity.

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Figure 7. Variable-temperature 1H NMR spectra of [κ3-TitmPr Benz]ZnMe. Only the resonances assigned to the hydrogens on the C4 atoms are shown.

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Figure 4. Molecular structure of [κ3-TitmPr Benz]ZnMe. Displacement ellipsoids are depicted at the 30% probability level, and hydrogen atoms are omitted for clarity.

solution, but the compounds are fluxional such that low temperatures are required to discern the 2:1 pattern of benzimidazolyl groups, as illustrated respectively in Figures 7 and 8. Although such observations are not surprising since [κ3-Tptm]ZnMe,8a [κ3-Tptm]ZnH,8a and [κ3-TitmMe]ZnMe10 exhibit similar behavior, it is interesting to note that the barrier for exchange within i [κ3-TitmPr Benz]ZnMe is significantly greater than that for the nonbenzannulated counterpart [κ3-TitmMe]ZnMe.18 Assuming that both compounds exhibit a similar exchange mechanism involving an intermediate atrane structure, a possible explanationi for this difference is the greater steric demand of the [TitmPr Benz] ligand, which would destabilize κ4 coordination. A visual illustration of the impact of benzannulation on the steric properties is provided by comparison of the ispace-filling representations of the atrane motifs in [TitmPr Benz]ZnOAr and [TitmMe]ZnOAr (Figure 9). The bulkiness of these ligands may also be quantified in terms of concepts such as the cone angle19,20 and percent buried volume (%Vbur).21−23 The latter corresponds to the portion of the volume of a sphere centered on the metal atom that is buried by overlap with the ligand atoms, i and values for the [κ4-TitmPr Benz] ligand in the zinc complexes

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Figure 5. Molecular structure of [κ3-TitmPr Benz]ZnH. Displacement ellipsoids are depicted at the 30% probability level, and hydrogen atoms on benzimidazolyl groups are omitted for clarity. 1

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H NMR spectroscopic studies demonstrate that [κ3-TitmPr Benz]3 PriBenz ZnMe and [κ -Titm ]ZnH maintain κ3 coordination in C


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abstraction by B(C6F5)3 from [κ3-TitmPr Benz]ZnH and [κ3PriBenz ]ZnMe, respectively, as illustrated in Schemes 2 and 3. Titm i Furthermore, {[TitmPr Benz]Zn}[BPh4] may also be obtained via protolytic cleavage of the Zn−Me bond by [PhNMe2H][BPh4] (Scheme 3). Scheme 3

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Figure 8. Variable-temperature 1H NMR spectra of [κ3-TitmPr Benz]ZnH. Only the resonances assigned to the hydrogens on the C4 atoms are shown.

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The molecular structures of {[TitmPr Benz]Zn}[HB(C6F5)3] PriBenz and {[Titm ]Zn}[MeB(C6F5)3] have been determined by X-ray diffraction, as illustrated for the former in Figure 10. The i existence of {[TitmPr Benz]Zn}[HB(C6F5)3] as an ion pair is noteworthy because [HB(C6F5)3]− often coordinates to metal centers via M···H−B and M···F−C interactions.25,26 Specifically, the shortest Zn···H−B(C6F5)3 interaction for the two crystallographically independent molecules is 4.22 Å (Table 3), which is substantially longer than both the sum of the covalent radii (1.53 Å)27 and van der Waals radii (3.59 Å)28 of zinc and hydrogen; as such, the compound does not exhibit a threecenter−two-electron interaction.29 Likewise, the shortest Zn···F distances of 4.41 and 5.09 Å for the two crystallographically independent molecules are longer than the sum of the van der Waals radii of zinc and fluorine (3.85 Å),28 and so it is also evident that there are no significant Zn···F interactions. The observation that [HB(C6F5)3i]− does not coordinate to the zinc center of the cation {[TitmPr Benz]Zn}+ may be attributed to the sterically demanding nature of the ligand, as evaluated by its cone angle and percent buried volume (Table 2). In addition, while [MeB(C6F5)3]− is also known to exhibit weak interactions with metal centers via M···Me−B and M···F−C interactions,30,31,32 the shortest Zn···H distance between the zinc center and the methyl group of [MeB(C6F5)3]− (4.72 Å) is much greater than the sum of the van der Waals radii (3.59 Å),28 and so it is evident that the anion is also noncoordinating in this system. The value of 2.6 ppm for Δδ(m,p-F), i.e. the difference in chemical shifts of the meta and para fluorines of [MeB(C6F5)3]−, is also consistent with noncoordination in solution.33 In addition, measurement of the translational self-diffusion coefficient (Dt) by using pulsed gradient spin−echo (PGSE)

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Figure 9. Space-filling representations of [TitmPr Benz]Zn and [TitmMe]Zn moieties in the aryloxide (OC6H4Br) derivatives. Color key: zinc, green; nitrogen, blue; sulfur, yellow; carbon, dark gray; hydrogen, light gray. i

Table 2. Cone Angles (θ) and %Vbur Values for [TitmPr Benz], Me PriBenz [Titm ], and [Tism ] Ligands in Zinc Compounds PriBenz

a

]ZnOAr [Titm [TitmMe]ZnOAra i {[TitmPr Benz]Zn}[HB(C6F5)3]b i

{[TitmPr Benz]Zn}[MeB(C6F5)3] i {[TismPr Benz]Zn}[HB(C6F5)3]

d(Zn−C)/Å

θ/deg

%Vbur

ref

2.368 2.606 2.093 2.104 2.107 2.113 2.110

313.0 277.4 326.6 328.2 327.1 333.6 333.7

69.8 65.6 75.3 75.6 75.1 82.5 83.3

this work 10 this work this work 11a

a

Ar = O-p-C6H4Br. bValues for two crystallographically independent molecules.

reported here are presented in Table 2, which also includes the values for related atrane systems. Of particular note, i comparison of the data for [TitmPri Benz]ZnOAr and [TitmMe]ZnOAr indicates that the [TitmPr Benz] ligand is more sterically demanding than is the nonbenzannulated counterpart [TitmMe].24 i In addition to neutral molecules of the type [TitmPr Benz]ZnX, i i the ion pairs {[TitmPr Benz]Zn}[HB(C6F5)3] and {[TitmPr Benz]Zn}[MeB(C6F5)3] may be obtained via hydride and methyl D


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trigonal-monopyramidal geometry, as indicated by (i) τ′ values that are closer to 1.0 and (ii) smaller displacements of the Zn from the [N3] plane towards the carbon. With respect to the latter, i the average displacement for the different forms of {[TitmPr Benz]i Zn}+ is only 0.09 Å, whereas that for {[TismPr Benz]Zn}+ is 0.29 Å. i A comparison of the istructures of [TitmPr Benz]ZnX (X = H, Me, OAr) and {[TitmPr Benz]Zn}+ reveals significant differences in the transannular Zn−C interactions, as summarized in Table 4, i

Table 4. Transannular Zn−C Interactions in [TitmPr Benz]ZnX and Related Compounds d(Zn−C)/Å i

d(Zn−Me)/Å

Four-Coordinate κ3 i 3 Pr Benz [κ -Titm ]ZnH 2.154(4) i [κ3-TitmPr Benz]ZnMe 2.170(2) [κ3-TitmMe]ZnMe 2.166(2) i [κ3-TismPr Benz]ZnH 2.1611(11) i [κ3-TismPr Benz]ZnMe 2.171(3) [κ3-Tptm]ZnH 2.105(3) [κ3-Tptm]ZnMe 2.095(2) [κ3-Tptm]ZnMe 2.098(2) Five-Coordinate κ4 i [TitmPr Benz]ZnOAr 2.367(2) Me [Titm ]ZnOAr 2.607(4) Four-Coordinate κ4 i {[TitmPr Benz]Zn}[HB(C6F5)3]a 2.093(4) 2.104(4) i {[TitmPr Benz]Zn}[MeB(C6F5)3] 2.1068(16) i {[TismPr Benz]Zn}[HB(C6F5)3]a 2.110(4) 2.113(4)

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Figure 10. Molecular structure of {[TitmPr Benz ]Zn}+ in {[TitmPriBenz]i Zn}[HB(C6F5)3]. The structure of {[TitmPr Benz]Zn}+ in {[TitmPr Benz]Zn}[MeB(C6F5)3] is similar. Displacement ellipsoids are depicted at the 30% probability level, and hydrogen atoms are omitted for clarity.

diffusioni 1 H NMR spectroscopy 34,35i indicates that the {[TitmPr Benz]Zn} moiety of {[TitmPr Benz]Zn}[HB(C6F5)3] (4.6 × 10−10 m2 s−1) has a diffusion constant similar to that of i 3 Pr Benz [κ -Titm ]ZnMe (4.9 × 10−10 m2 s−1), which provides further evidence that the counteranions are noncoordinating in this system. i An interesting feature of {[TitmPr Benz]Zn}[HB(C6F5)3] and i {[TitmPr Benz]Zn}[MeB(C6F5)3] is that both metal centers pos8g,36 sess, for zinc, an uncommon trigonal-monopyramidal geometry. i Thus, the coordination geometry of zinc in {[TitmPr Benz]Zn}[HB(C6F5)3] (Table 3) is characterized by four-coordinate τ4 (0.83 and 0.85) and τδ (0.83 and 0.85) geometry indices37 for the two crystallographically independent molecules that compare favorably with the idealized value of 0.85 for a trigonal monopyramid.38 In addition to the τ4 and τδ geometry indices, a τ′ geometry index has also been introduced to differentiate tetrahedral and trigonalmonopyramidal coordination geometries,39 and the τ′ values (1.01 and 1.05) are also in accord with the idealized value for a trigonal monopyramid (1.00). Although zinc compounds which feature trigonal-monopyramidal coordination are not common, i several examples are known,40 as illustrated by {[TismPr Benz]11c Zn}[HB(C6F5)3]. i In this regard, a comparison of the struci tures of {[TitmPr Benz]Zn}+ and {[TismPr Benz]Zn}+ (Table 3) indicates that the former has a slightly more idealized

a

1.978(3) 1.962(2) 1.989(3) 1.957(3) 1.979(2)

ref this work this work 10 11c 11a 8a 8a 8a this work 10 this work this work 11c

Values for two crystallographically independent molecules.

which also includes relevant [TitmMe]ZnX, [Tptm]ZnX, and PriBenz [Tism ]ZnX derivatives for comparison. In addition to there being a large variation in the lengths of the transannular Zn−C bonds, these values are all longer than the corresponding Zn−Me bond lengths, which range from 1.957(3) to 1.989(3) Å, and are similar to the average value for structurally characterized compounds listed in the Cambridge Structural Database (2.01 Å).41 With respect to the transannular Zn−C bond lengths, it is pertinent to note that the values for the neutral five-coordinate κ4-atrane complexes are significantly longer than those in

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Table 3. Metrical Details for {[TitmPr Benz]Zn}[HB(C6F5)3], {[TitmPr Benz]Zn}[MeB(C6F5)3], and {[TismPr Benz]Zn}[HB(C6F5)3] i

d(Zn−N)/Å d(Zn−C)/Å d(Zn···H)/Å τ4 τδ τ′ d(Zn−[N3])/Åe

i

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{[TitmPr Benz]Zn}[HB(C6F5)3]a,b

{[TitmPr Benz]Zn}[MeB(C6F5)3]a

{[TismPr Benz]Zn}[HB(C6F5)3]b,c

1.957(3), 1.959(3), 1.963(3) 1.951(3), 1.952(3), 1.957(3) 2.093(4) 2.104(4) 9.66d 4.22 0.85 0.83 0.85 0.83 1.05 1.01 0.10 0.08

1.9575(14), 1.9672(13), 1.9726(14) 2.1068(16)

1.986(3), 1.989(3), 2.001(3) 1.994(3), 1.998(3), 2.032(3) 2.110(4) 2.113(4) 8.63d 4.80 0.85 0.85 0.84 0.81 1.14 1.11 0.30 0.29

0.83 0.79 0.98 0.090

a

This work. bValues for two crystallographically independent molecules. cReference 11c. dNote that the B−H bond does not point towards the Zn center. eDistance between zinc and the [N3] coordination plane. E


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Organometallics both the neutral four-coordinate κ3 complexes and the cationic four-coordinate atrane complexes. While the origin of the longer Zn−C bonds in the five-coordinate atrane compounds i [TitmPr Benz]ZnOAr and [TitmMe]ZnOAr is associated with the fact that the Zn−C bonds are components of three-center−fourelectron hypervalent interactions,42,43 which are often longer than conventional two-center−two-electron interactions,42,44 it is appropriate to note that shorter Zn−C bonds are observed in [Tptm]ZnX (2.11−2.22 tÅ)8 and the tris(2-pyridonyl)methyl atrane complex [TpomBu ]ZnOC6H4But (2.071(5) Å).12b The origin of the longer bonds for the [TitmMe]ZnX system has been attributed to the presence of a five-membered imidazolyl ring, rather than a six-membered pyridyl ring. Specifically, the different bond angles result in the nitrogen atom donor orbitals of [TitmMe] being directed farther from the [N3] plane and thereby cause the zinc to be displaced from the plane and the Zn−C distance to be lengthened.10 A similar effect is also observed for transannular dative Zn←N amine bonds in atrane complexes, in which the Zn−N bond lengths of tris(2-benzimidazolylmethyl)amine6 zinc complexes (2.27−2.58 Å)45,46 are typically longer than the corresponding values in tris(2-pyridylmethyl)amine complexes (2.11−2.32 Å).47 A noteworthy difference, however, with respect to the variability of Zn←N bond lengths, is that, in contrast to normal covalent M−C bonds, the lengths of dative bonds are known to be highly sensitive to the coordination environment.48,49 Significant bond length changes have also been observed upon oxidation of molecules,50 but it is uncommon to observe large variations for normal covalent bonds in molecules with atoms in the same oxidation state. In order to evaluate the nature of the Zn−C bonding in i [TitmPri Benz]ZnX in more detail, the molecular structures of [κ3Pr Benz PriBenz PriBenz ]ZnMe, [Titm ]ZnOAr, and {[Titm ]Zn}+ Titm have been determined computationally. Density functional theory geometry optimized structures are illustrated in Figure 11, while

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Figure 12. Moleculari orbitals for [κ3-TitmPr Benz]ZnMe, [TitmPr Benz]ZnOAr, and {[TitmPr Benz]Zn}+.

selected molecular orbitals and their compositions are presented in Figure 12 and Table 5, respectively. In each case, the orbital associated with the transannular Zn−C interaction is the HOMO, which possesses substantial carbon character: [κ3i PriBenz TitmPr Benz ]ZnMe (49.0%), [Titm ]ZnOAr (57.7%), and i {[TitmPr Benz]Zn}+ (44.3%). Interestingly, however, whereas the i i orbitals for [κ3-TitmPr Benz]ZnMe and {[TitmPr Benz]Zn}+ possess a significant amount of zinc character (20.5% and 24.4%, respeci tively), the HOMO of [TitmPr Benz]ZnOAr possesses relatively i little zinc character (7.0%). Thus, the HOMO of [TitmPr Benz]ZnOAr is approximately an spn hybrid lone pair orbital on carbon.51 As an alternative to employing a moleculari orbital description, the axial hypervalent interaction of [TitmPr Benz]ZnOAr can be described in terms of resonance involving formally zwitterionic structures,52 namely [C− Zn+−OAr] and [C−Zn+OAr−]. In view of the fact that the molecular orbital associated with the Zn−C interaction is highly localized on carbon, the [C− Zn+−OAr] resonance structure is considered to be the dominant contributor. Although not common, zwitterionic compounds in which carbon bears a formal negative charge are, nevertheless, precedented.53,54 It is particularly interesting to compare the molecular structure i of [TitmPr Benz]ZnOAr with that of the nonbenzannulated counterpart, [TitmMe]ZnOAr. Specifically, despite the fact that i the Zn−C bond length of [TitmPr Benz]ZnOAr (2.367(2) Å) is longer than that of a typical Zn−C bond, it is distinctly shorter than that of [TitmiMe]ZnOAr (2.607(4) Å);55 the Zn−O bond lengths of [TitmPr Benz]ZnOAr (1.9796(16) Å) and [TitmMe]ZnOAr (1.967(2) Å) are, nevertheless, similar. While this difference in Zn−C bond lengths for two similar compounds is unusual, it is in accord with our recent density functional theory geometry optimization calculations on [TitmMe]ZnCl, which have demonstrated that the energy surface associated with varying the Zn−C bond length iis shallow.10 As such, the different Zn−C bond lengths of [TitmPr Benz]ZnOAr and [TitmMe]ZnOAr may be reconciled.

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Figure i11. Geometry-optimized structures of [κ3-TitmPr Benz]ZnMe, i [TitmPr Benz]ZnOAr, and {[TitmPr Benz]Zn}+. Hydrogen atoms are omitted for clarity. F


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Table 5. Zinc and Carbon Compositions of Molecular Orbitals Associated with the Zn−C Interactions in [TitmPr Benz]Zn Derivatives

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Zn 4s Zn 4px Zn 4py Zn 4pz C 2s C 2px C 2py C 2pz

[κ3-TitmPr Benz]ZnMe HOMO −7.39 eV

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[κ3-TitmPr Benz]ZnMe HOMO-5 −10.63 eV

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[TitmPr Benz]ZnOAr HOMO −5.05 eV

{[TitmPr Benz]Zn}+ HOMO −10.60 eV

2.80 1.25 0.00 16.46 3.12 0.11 0.00 45.76

6.87 13.27 0.01 5.77 4.99 25.59 0.03 21.98

2.84 0.34 0.00 3.90 3.70 0.03 0.01 53.92

0.96 0.00 0.00 23.41 2.65 0.00 0.00 41.64

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32.083974.58 Coupling constants are given in hertz. 1-Isopropylbenzimidazole59 and [PhNMe2H][BPh4]60 were obtained by literature methods, and other chemicals were obtained commercially and used as received. The purity of products was established by 1H NMR spectroscopy and elemental analysis, as appropriate. The calculation of the percent buried volumes (%Vbur) and the steric maps was determined by using SambVca 2.0 (https://www.molnac.unisa.it/OMtools/sambvca2.0/),21a for a sphere of radius 3.5 Å about the metal center and Bondi van der Waals radii scaled by a factor of 1.17. Rates of exchange were determined by using gNMR.61 Self-diffusion constants were determined by pulsed gradient spin−echo (PGSE) diffusion NMR spectroscopic experiments employing the Bruker stebpg1s pulse sequence. X-ray Structure Determinations. X-ray diffraction data were collected on a Bruker Apex II diffractometer. The structures were solved by using direct methods and standard difference map techniques and were refined by full-matrix least-squares procedures on F2 with SHELXTL (Version 2014/7).62 Computational Details. Calculations were carried out using DFT as implemented in the Jaguar 8.9 (release 15) suite of ab initio quantum chemistry programs.63 Geometry optimizations were performed with the B3LYP density functional using the LACVP** basis sets and coordinates are provided in the Supporting Information. Molecular orbital analyses were performed with the aid of JIMP2,64 which employs Fenske−Hall calculations and visualization using MOPLOT.65 Synthesis of 1-Isopropyl-1,3-dihydro-2H-benzimidazole-2thione. A solution of 1-isopropylbenzimidazole (1.35 g, 8.4 mmol) in THF (20 mL) was cooled to −78 °C and was treated with BunLi (5.25 mL, 1.6 M in hexanes, 8.4 mmol). The mixture was stirred for 1 h at −78 °C and then treated with sulfur powder (400 mg, 1.6 mmol S8). The mixture was allowed to warm to room temperature while it was stirred for 3 h. After this period, the reaction was quenched with H2O (20 mL), and the organic phase was extracted into CH2Cl2 (50 mL). The organic phase was washed with a saturated aqueous solution of NH4Cl (3 × 50 mL) and then dried with MgSO4. The volatile components were removed in vacuo, and the oily material was dissolved in acetone (50 mL) and treated with hexanes (50 mL). The mixture was filtered, and the filtrate was allowed to stand for 2 days, during which period crystals were deposited. The crystals were isolated by filtration and washed with hexanes to afford 1-isopropyl-1,3-dihydro-2Hbenzimidazole-2-thione (1.0 g, 62%) as a yellow-orange crystalline solid. 1H NMR (CDCl3): 1.61 [d, J = 7 Hz, 6H, C6H4NCH(CH3)2NHCS], 5.59 [sept, J = 7 Hz, 1H, C6H4NCH(CH3)2NHCS], 7.21 [m, 3H, C6H4NCH(CH3)2NHCS], 7.42 [m, 1H, C6H4NCH(CH3)2NHCS], 10.21 [s, 1H, C6H4NCH(CH3)2NHCS]. 13C{1H} NMR (CDCl3): 20.17 [2C, C6H4NCH(CH3)2NHCS], 48.72 [1C, C6H4NCH(CH3)2NHCS], 110.58 [1C, C6H4NCH(CH3)2NHCS], 110.97 [1C, C6H4NCH(CH3)2NHCS], 122.55 [1C, C6H4NCH(CH3)2NHCS], 123.12 [1C, C6H4NCH(CH3)2NHCS], 131.08 [1C, C6H4NCH(CH3)2NHCS], 131.16 [1C, C6H4NCH(CH3)2NHCS], 167.22 [1C, C6H4NCH(CH3)2NHCS].

SUMMARY In summary, tris(1-isopropylbenzimidazol-2-ylthio)methane, i [TitmPr Benz]H, has been synthesized by treatment of 1-isopropyl1,3-dihydro-2H-benzimidazole-2-thione with NaH followed by i CHI3. The reaction of [TitmPriBenz]H with Me2Zn affords the zinc methyl complex [κ3-Titmi Pr Benz]ZnMe, which subsequently provides access to [TitmPri Benz]ZnOAr (Ar = p-C6H4Br),i [κ3PriBenz Pr Benz Titm ]ZnH, {[TitmPr Benz]Zn}[HB(C ]6F5)3], {[Titm PriBenz Zn}[MeB(C6F5)3], and {[Titm i ]Zn}[BPh4]. X-ray diffraction demonstrates that the [TitmPr Benz]ZnX compounds belong to two structural classes that are differentiated according to i whether the [TitmPr Benz] ligand coordinates in a κ3 manner, such that one of the benzimidazolyl groups remains uncoordinated, or a κ4 manner, thereby resulting in an atrane motif. Thus, the i hydridei and methyl complexes [κ3-TitmPr Benz]ZnH andi [κ3TitmPr Benz]ZnMe exhibit κ3 coordination, whereas [TitmPr Benz]PriBenz ZnOAr and {[Titm ]Zn}[HB(C6F5)3]i adopt atrane struci tures. [κ3-TitmPr Benz]ZnH and [κ3-TitmPr Benz]ZnMe are fluxional on the NMR time scale, and the barrier for exchange within i [κ3-TitmPr Benz]ZnMe is significantly greater than that for the nonbenzannulated counterpart, [κ3-TitmMe]ZnMe. The latter observation is attributed to benzannulation providing a more sterically demanding ligand that inhibits access to the atrane i structure for [TitmPr Benz]ZnMe, by comparison to thati for [TitmMe]ZnMe. Comparison of the istructures of [TitmPr Benz]ZnX (X = H, Me, OAr) and {[TitmPr Benz]Zn}+ reveal significant differences in the transannular Zn−C interactions, with that for i [TitmPr Benz]ZnOAr being exceptionally long, which is attributed to it being a component of a three-center−four-electron interaction, with the HOMO being approximately an spn hybrid lone pair orbital. In terms of a valence bond description, the bonding may be described as possessing a significant contribution from a zwitterionic [C− Zn+−OAr] resonance structure.

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General Considerations. All manipulations were performed using a combination of glovebox, high-vacuum, and Schlenk techniques under an argon atmosphere.56 Solvents were purified and degassed by standard procedures. NMR spectra were measured on Bruker AVIII 300, Bruker AVIII 400 SL, and Bruker AVIII 500 spectrometers, of which the Bruker 400 SL instrument was used for variable-temperature studies. 1H NMR chemical shifts are reported in ppm relative to SiMe4 (δ 0) and were referenced internally with respect to the protio solvent impurity (δ 7.16 for C6D5H, 7.26 for CHCl3, 5.32 for CDHCl2, 1.72 for C4HD7O, and 2.08 for C7D7H).57 13C NMR spectra are reported in ppm relative to SiMe4 (δ 0) and were referenced internally with respect to the solvent (δ 128.06 for C6D6, 53.84 for CDHCl2, 67.21 for C4HD7O, and 20.43 for C7D7H).57 19F NMR chemical shifts are reported in ppm relative to CFCl3 (δ 0.0) and were obtained by using the Ξ/100% value of 94.094011.58 11B NMR chemical shifts are reported in ppm relative to BF3.Et2O (δ 0.0) and were obtained by using the Ξ/100% value of

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Synthesis of [TitmPr Benz]H. A solution of 1-isopropyl-1,3-dihydro2H-benzimidazole-2-thione (7.8 g, 41 mmol) in THF (40 mL) was slowly added to a cold (0 °C) suspension of NaH (1.16 g, 48 mmol) in THF (40 mL), thereby resulting in evolution of H2. Once the evolution of H2 had ceased, the mixture was allowed to warm to room temperature G


Article

Organometallics

white solid. The suspension was allowed to stand at −17 °C overnight and filtered. The white solid was washed with benzene (ca. 1 mL) and i dried in vacuo to afford [κ3-TitmPr Benz]ZnH (140 mg, 45%). Crystals of i [κ3-TitmPr Benz]ZnH suitable for X-ray were obtained by vapor diffusion of pentane into a solution in benzene. 1H NMR (C6D6): 0.90 [bs, 18H, (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnH], 4.05 [bs, 3H, (C 6 H 4 N 2 CH(CH3)2CS)3CZnH], 5.77 [s, 1H, (C6H4N2CH(CH3)2CS)3CZnH], 6.94 [bs, 6H, (C6H4N2CH(CH3)2CS)3CZnH], 7.12 [m, 3H, (C6H4N2CH(CH 3 ) 2 CS) 3 CZnH], 8.10 [d, J = 8 Hz, 3H, (C 6 H 4 N 2 CH(CH3)2CS)3CZnH]. 1H NMR (C7D8, 323 K): 0.98 [d, J = 7 Hz, 18H, (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnH], 4.09 [sept, J = 7 Hz, 3H, (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnH], 5.44 [s, 1H, (C 6 H 4 N 2 CH(CH3)2CS)3CZnH], 6.95 [m, 9H, (C6H4N2CH(CH3)2CS)3CZnH], 7.96 [d, J = 8 Hz, 3H, (C6H4N2CH(CH3)2CS)3CZnH]. 13C{1H} NMR (C7D8, 323 K): 20.53 [6C, obscured by solvent, C6H4N2CH(CH3)2CS)3CZnH], 49.54 [3C, C6H4N2CH(CH3)2CS)3CZnH], 111.20 [3C, C6H4N2CH(CH3)2CS)3CZnH], 118.07 [3C, C6H4N2CH(CH3)2CS)3CZnH], 121.65 [3C, C6H4N2CH(CH3)2CS)3CZnH], 122.46 [3C, C6H4N2CH(CH3)2CS)3CZnH], 136.04 [3C, C6H4N2CH(CH 3 ) 2 CS) 3 CZnH], obscured by solvent [3C, C 6 H 4 N 2 CH[4C, C6H4N2CH(CH3)2CS)3CZnH. (CH3)2CS)3CZnH], not observed i Anal. Calcd for [κ3-TitmPr Benz]ZnH·C7H8 (C38H42N6S3Zn): C, 61.3; H, 5.7; N, 11.3. Found: C, 61.3; H, 5.4; N, 11.1.

and stirred for an additional 1 h. After this period, the suspension was cooled to 0 °C and treated slowly with CHI3 (5.25 g, 13.3 mmol), thereby resulting in the formation of a white precipitate. The mixture was allowed to warm to room temperature and stirred for 1 h. Methanol (10 mL) was added slowly to quench the unreacted NaH. The solution obtained was washed with a saturated solution of NH4Cl and extracted into dichloromethane. The organic phase was dried over NaSO4, and the volatile components were removed in vacuo. The resulting brown oil was triturated with THF (10 mL), resulting in the formation of a white solid, which was isolated by filtration and washed with cold THF. The i solid was dried in vacuo for 12 h to give [TitmPr Benz]H as a white powder (1.9 g). A second batch of crystals was isolated from the THF filtrate after evaporation for a week room temperature (0.45 g). Crystals of i [TitmPr Benz]H suitable for X-ray diffraction were obtained from a solution in THF. 1H NMR (CDCl3): 1.58 [d, J = 7 Hz, 18H, (C6H4N2CH(CH3)2CS)3CH], 4.91 [sept, J = 7 Hz, 3H, (C6H4N2CH(CH3)2CS)3CH], 7.20 [dd, J = 6, 3 Hz, 6H, (C6H4N2CH(CH3)2CS)3CH], 7.48 [m, 3H, (C6H4N2CH(CH3)2CS)3CH], 7.55 [s, 1H, (C6H4N2CH(CH3)2CS)3CH], i

7.66 [bm, 3H, (C6H4N2CH(CH3)2CS)3CH]. Anal. Calcd for [TitmPr Benz]H (C31H34N6S3): C, 63.5; H, 5.8; N, 14.3. Found: C, 63.4; H, 5.8; N, 14.1. i

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Synthesis of [κ3-TitmPr Benz]ZnMe. A suspension of [TitmPr Benz]H (500 mg, 0.85 mmol) in toluene (ca. 10 mL) was treated with Me2Zn (126 mg, 1.32 mmol). The mixture was stirred until the evolution of methane ceased and then placed at −17 °C overnight. The resulting suspension was filtered, and the white precipitate was washed with i pentane (10 mL) and then dried in vacuoi to give [κ3-TitmPr Benz]ZnMe (558 mg, 98%). Crystals of [κ3-TitmPr Benz]ZnMe suitable for X-ray diffraction were obtained from a solution in benzene. 1H NMR (C6D6): 0.55 [s, 3H, (C6H4N2CH(CH3)2CS)3CZnCH3], 0.87 [bs, 18H, (C6H4N2CH ( CH 3 ) 2 C S ) 3 C Zn C H 3 ], 4 .06 [bs , 3H , ( C 6 H 4 N 2 C H (CH3)2CS)3CZnCH3], 6.93 [bs, 6H, (C6H4N2CH(CH3)2CS)3CZnCH3], 7.13 [m, 3H, (C6H4N2CH(CH3)2CS)3CZnCH3], 8.08 [bs, 3H, (C6H4N2CH(CH3)2CS)3CZnCH3]. 1H NMR (C7D8): 0.36 [m, 3H, (C6H4N2CH(CH3)2CS)3CZnCH3], 0.91 [br, 18H, (C6H4N2CH(CH3)2CS)3CZnCH3], 4.05 [br, 3H, (C6H4N2CH(CH3)2CS)3CZnCH3], 6.96 [br, 6H, (C6H4N2CH(CH3)2CS)3CZnCH3], 7.10 [br (partially obscured by solvent), 3H, (C6H4N2CH(CH3)2CS)3CZnCH3], 7.99 [br, 3H, (C6H4N2CH(CH3)2CS)3CZnCH3]. 13C{1H} NMR (C7D8, 333 K): −15.61 [1C, C6H4N2CH(CH3)2CS)3CZnCH3], 20.60 [6C, obscured by solvent, C6H4N2CH(CH3)2CS)3CZnCH3], 49.55 [3C, C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnCH 3 ], 111.27 [3C, C 6 H 4 N 2 CH(CH3)2CS)3CZnCH3], 117.90 [3C, C6H4N2CH(CH3)2CS)3CZnCH3], 121.65 [3C, C6H4N2CH(CH3)2CS)3CZnCH3], 122.45 [3C, C6H4N2CH(CH3)2CS)3CZnCH3], 136.21 [3C, C6H4N2CH(CH3)2CS)3CZnCH3], obscured by solvent [3C, C6H4N2CH(CH3)2CS)3CZnCH3], not observed i [4C, C6H4N2CH(CH3)2CS)3CZnCH3]. Anal. Calcd for [κ3-TitmPr Benz]ZnMe·0.33C6H6 (C34H38N6S3Zn): C, 59.0; H, 5.5; N, 12.1. Found: C, 59.3; H, 5.5; N, 12.5.

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Synthesis of {[TitmPr Benz]Zn}[HB(C6F5)3]. A suspension of [κ3i TitmPr Benz]ZnH (20 mg, 0.03 mmol) in benzene (ca. 2 mL) was treated with B(C6F5)3 (15 mg, 0.03 mmol). The mixture was stirred until it became homogeneous and was allowed to stand overnight. Over this period, colorless crystals were deposited. The crystals were isolated i by decantation and were washed with pentane to afford {[TitmPr Benz]Zn}[HB(C6F5)3] (33 mg, 94%), which was suitable for X-ray diffraction. 1H NMR (CD2Cl2): 1.62 [d, J = 7 Hz, 18H, (C6H4N2CH(CH 3 ) 2 CS) 3 CZnHB(C 6 F 5 ) 3 ], 4.54 [sept, J = 7 Hz, 3H, (C6H4N2CH(CH3)2CS)3CZnHB(C6F5)3], 7.35 [m, 3H, (C6H4N2CH(CH 3 ) 2 CS) 3 CZnHB(C 6 F 5 ) 3 ], 7.45 [m, 3H, (C 6 H 4 N 2 CH(CH3)2CS)3CZnHB(C6F5)3], 7.57 [d, J = 8 Hz, 3H (C6H4N2CH(CH3)2CS)3CZnHB(C6F5)3], 7.77 [d, J = 8 Hz, 3H, (C6H4N2CH(CH3)2CS)3CZnHB(C6F5)3]. 13C{1H} NMR (CD2Cl2): 20.98 [6C, (C6H4N2CH(CH3)2CS)3CZnHB(C6F5)3], 51.51 [3C, (C6H4N2CH(CH 3 ) 2 CS) 3 CZnHB(C 6 F 5 ) 3 ], 112.98 [3C, (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnHB(C 6 F 5 ) 3 ], 115.90 [3C, (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnHB(C 6 F 5 ) 3 ], 124.21 [3C, (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnHB(C 6 F 5 ) 3 ], 124.78 [3C, (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnHB(C 6 F 5 ) 3 ], 135.61 [3C, (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnHB(C 6 F 5 ) 3 ], 139.00 [3C, (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnHB(C 6 F 5 ) 3 ], 161.51 [3C, (C 6 H 4 N 2 CH(CH3)2CS)3CZnHB(C6F5)3], not observed [19C, (C6H4N2CH(CH3)2CS)3CZnHB(C6F5)3]. 19F NMR (CD2Cl2): −167.86 [m, (C6H4N2CH(CH3)2CS)3CZnHB(C6F5)3], −165.03 [m, (C6H4N2CH(CH 3 ) 2 CS) 3 CZnHB(C 6 F 5 ) 3 ], −134.09 [m, (C 6 H 4 N 2 CH(CH3)2CS)3CZnHB(C6F5)3]. 11B NMR (CD2Cl2) −25.51 [d, J = 92 Hz, i(C6H4N2CH(CH3)2CS)3CZnHB(C6F5)3]. Anal. Calcd for {[TitmPr Benz]Zn}[HB(C6F5)3]·1.5C6H6 (C58H43BF15N6S3Zn): C, 54.4; H, 3.4; N, 6.6. Found: C, 54.1; H, 3.1; N, 6.4.

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(13 mg, Synthesis of [TitmPr Benz]ZnO-p-C6H4Br. p-Bromophenol i 0.075 mmol) was added to a suspension of [κ3-TitmPr Benz]ZnMe (50 mg, 0.075 mmol) in toluene (ca. 2 mL), thereby resulting in the immediate evolution of methane. The mixture was shaken until gas evolution ceased, and the mixture was filtered. The filtrate was placed at −17 °C overnight, thereby depositing colorless crystals. The crystals were isolated by idecantation, washed with pentane, and dried in vacuo to afford i[Titm Pr Benz ]ZnO-p-C 6 H4 Br (37 mg, 60%). Crystals of [TitmPr Benz]ZnO-p-C6H4Br suitable for X-ray diffraction were obtained by vapor diffusion of pentane into a solution in benzene. 1H NMR (C6D6): 0.90 [d, J = 7 Hz, 18H, (C6H4N2CH(CH3)2CS)3CZnOC6H4Br], 4.12 [sept, J = 7 Hz, 3H, (C6H4N2CH(CH3)2CS)3CZnOC6H4Br], 6.7−7.2 [several multiplets, 13 H, (C6H4N2CH(CH3)2CS)3CZnOC6H4Br], 9.23 [bs, 3H, (C6H4N2CH(CH3)2CS)3CZnOC6H4Br].

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Synthesis of {[TitmPr Benz]Zn}[MeB(C6F5)3]. (a) B(C6F5i )3 (76 mg, 0.15 mmol) was added to a suspension of [κ3-TitmPr Benz]ZnMe (100 mg, 0.15 mmol) in benzene (ca. 2 mL), and the mixture was stirred until it became homogeneous. The solution was allowed to stand at room temperature overnight, during which period colorless crystals were deposited. The mother liquor was decanted,i and the solid obtained was washed with pentane to afford {[TitmPr Benz]Zn}[MeB(C6F5)3] (157 mg, 89%) as crystals suitable for X-ray diffraction. 3 (b) B(C 6F5)3 (114 mg, 0.22 mmol) was added to a suspension of [κ i Pr Benz ]ZnMe (150 mg, 0.23 mmol) in toluene (ca. 2 mL), and the Titm mixture was stirred until it became homogeneous. The mixture was allowed to stand overnight, during which period a white solid was deposited. The iprecipitate was isolated and washed with pentane to afford {[TitmPr Benz]Zn}[MeB(C6F5)3] (170 mg, 65%). 1H NMR (CD2Cl2): 0.49 [s, 3H, (C6H4N2CH(CH3)2CS)3CZnCH3B(C6F5)3], 1.62 [d, J = 7 Hz, 18H, (C6H4N2CH(CH3)2CS)3CZnCH3B(C6F5)3],

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Synthesis of [κ3-TitmPr Benz]ZnH. p-Bromophenol (100 mg, i 0.58 mmol) was added to a suspension of [κ3-TitmPr Benz]ZnMe (315 mg, 0.47 mmol) in toluene (3 mL), resulting in the immediate evolution of methane. The mixture was agitated until the gas evolution ceased and a solution was obtained. Phenylsilane (30 μg, 0.28 mmol) was added to the rapidly stirred solution, resulting in the immediate precipitation of a H


Organometallics

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4.54 [sept, J = 7 Hz, 3H, (C6H4N2CH(CH3)2CS)3CZnCH3B(C6F5)3], 7.36 [m, 3H, (C6H4N2CH(CH3)2CS)3CZnCH3B(C6F5)3], 7.45 [m, 3H, (C6H4N2CH(CH3)2CS)3CZnCH3B(C6F5)3], 7.57 [d, J = 8 Hz, 3H (C6H4N2CH(CH3)2CS)3CZnCH3B(C6F5)3], 7.78 [d, J = 8 Hz, 3H (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnCH 3 B(C 6 F 5 ) 3 ]. 13 C{ 1 H} NMR (CD2Cl2): 20.97 [s, 6C, (C6H4N2CH(CH3)2CS)3CZnCH3B(C6F5)3], 51.51 [s, 3C, (C6H4N2CH(CH3)2CS)3CZnCH3B(C6F5)3], 112.96 [s, 3C, (C6H4N2CH(CH3)2CS)3CZnCH3B(C6F5)3], 115.91 [s, 3C, (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnCH 3 B(C 6 F 5 ) 3 ], 124.20 [s, 3C, (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnCH 3 B(C 6 F 5 ) 3 ], 124.78 [s, 3C, (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnCH 3 B(C 6 F 5 ) 3 ], 135.63 [s, 3C, (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnCH 3 B(C 6 F 5 ) 3 ], 139.02 [s, 3C, (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnCH 3 B(C 6 F 5 ) 3 ], 161.52 [s, 3C, (C6H4N2CH(CH3)2CS)3CZnCH3B(C6F5)3], not observed [20C, (C6H4N2CH(CH3)2CS)3CZnCH3B(C6F5)3]. 19F NMR (CD2Cl2): −133.26 [d, J = 24 Hz, (C6H4N2CH(CH3)2CS)3CZnCH3B(C6F5)3], −165.53 [t, J = 20 Hz, (C6H4N2CH(CH3)2CS)3CZnCH3B(C6F5)3], −168.07 [m, (C6H4N2CH(CH3)2CS)3CZnCH3B(C6F5)3]. 11B NMR (CD2Cl2) −14.99 [s, (C6H4Ni 2CH(CH3)2CS)3CZnCH3B(C6F5)3]. Anal. Calcd for {[Titm P r B e n z ]Zn}[MeB(C 6 F 5 ) 3 ]·0.5C 6 H 6 (C53H39BF15N6S3Zn): C, 52.3; H, 3.2; N, 6.9. Found: C, 52.1; H, 3.1; N, 6.8.

AUTHOR INFORMATION

Corresponding Author

*E-mail for G.P.: [email protected]. ORCID

Gerard Parkin: 0000-0003-1925-0547 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (CHE-1058987 and CHE-1465095) for support of this research. M.R. acknowledges the National Science Foundation for a Graduate Research Fellowship under Grant No. DGE-16-44869.

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REFERENCES

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Synthesis of {[TitmPr Benz]Zn}[BPh4]. A suspension of [κ3i TitmPr Benz]ZnMe (90 mg, 0.14 mmol) in benzene (ca. 2 mL) was treated with [PhNMe2H][BPh4] (60 mg, 0.14 mmol), and the mixture was stirred for 30 min. Over this period, the reactants dissolved and a white precipitate was deposited. The mixture was filtered, and the solid obtainedi was washed with pentane and dried in vacuo to afford {[TitmPr Benz]Zn}[BPh4] (68 mg, 52%). 1H NMR (THF-d8): 1.55 [d, J = 7 Hz, 18H, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 4.57 [sept, J = 7 Hz, 3H, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 6.68 [t, J = 7 Hz, 4H, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 6.84 [t, J = 7, 8H, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 7.31 [m, 11H, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 7.38 [t, J = 8 Hz, 3H, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 7.66 [d, J = 8 Hz, 3H, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 8.03 [d, J = 8 Hz, 3H, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4]. 13C{1H} NMR (THF-d8): 20.50 [s, 6C, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 51.40 [s, 3C (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 113.17 [s, 3C, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 116.56 [s, 3C, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 121.49 [s, 4C, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 124.08 [s, 3C, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 124.49 [s, 3C, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 125.42 [1:1:1:1 q, J = 3 Hz, 8C, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 136.30 [s, 3C, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 137.08 [s, 8C, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 139.89 [s, 3C, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 161.32 [s, 3C, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4], 165.12 [q, J = 50 Hz, 4C, (C6H4N2CH(CH 3 ) 2 CS) 3 CZnB(Ph) 4 ], not observed [1C, (C 6 H 4 N 2 CH(CH 3 ) 2 CS) 3 CZnB(Ph) 4 ]. 11 B NMR (THF-d 8 ): −6.49 [m, (C6H4N2CH(CH3)2CS)3CZnB(Ph)4].

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00158. NMR spectra (PDF) Cartesian coordinates for geometry optimized structures (XYZ) Accession Codes

CCDC 1829806−1829811 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. I


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i

%Vbur value for [TitmPr Benz] in {[TitmPr Benz]Zn}+ is larger than that in i

i

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Organometallics

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A.; Boucekkine, A.; Carpentier, J. F.; Kirillov, E. Organometallics 2016, 35, 258−276. (g) Reference 25i. (32) [BPh4]− is also known to interact with metal centers, typically via η6 coordination. See, for example, ref 30 and: (a) Strauss, S. H. Chem. Rev. 1993, 93, 927−942. (b) Chen, Y.; Sui-Seng, C.; Zargarian, D. Angew. Chem., Int. Ed. 2005, 44, 7721−7725. (c) Perez-Torrente, J. J.; Angoy, M.; Gomez-Bautista, D.; Palacios, A.; Jimenez, M. V.; Modrego, F. J.; Castarlenas, R.; Lahoz, F. J.; Oro, L. A. Dalton Trans. 2014, 43, 14778−14786. (33) Values of Δδ(m,p-F) in the range 3−6 ppm are considered to be indicative of coordination. See: Horton, A. D. Organometallics 1996, 15, 2675−2677. (34) (a) Macchioni, A.; Ciancaleoni, G.; Zuccaccia, C.; Zuccaccia, D. Chem. Soc. Rev. 2008, 37, 479−489. (b) Zuccaccia, D.; Macchioni, A. Organometallics 2005, 24, 3476−3486. (c) Zuccaccia, C.; Bellachioma, G.; Ciancaleoni, G.; Macchioni, A.; Zuccaccia, D. Inorg. Chim. Acta 2010, 363, 595−600. (35) (a) Kharlamov, S. V.; Latypov, S. K. Russ. Chem. Rev. 2010, 79, 635−653. (b) Pregosin, P. S. Pure Appl. Chem. 2009, 81, 615−633. (c) Li, D.; Keresztes, I.; Hopson, R.; Williard, P. G. Acc. Chem. Res. 2009, 42, 270−280. (d) Cohen, Y.; Avram, L.; Frish, L. Angew. Chem., Int. Ed. 2005, 44, 520−554. (e) Parella, T. Magn. Reson. Chem. 1998, 36, 467− 495. (36) Das, U. K.; Bobak, J.; Fowler, C.; Hann, S. E.; Petten, C. F.; Dawe, L. N.; Decken, A.; Kerton, F. M.; Kozak, C. M. Dalton Trans. 2010, 39, 5462−5477. (37) τ4 = [360 − (α + β)]/141, where α and β are the two largest angles. τδ = τ4(β/α), where α > β. An idealized tetrahedron is characterized by a value of 1.00, while an idealized trigonal monopyramid is characterized by a value of 0.85. See: (a) Yang, L.; Powell, D. R.; Houser, R. P. Dalton Trans. 2007, 955−964. (b) Reineke, M. H.; Sampson, M. D.; Rheingold, A. L.; Kubiak, C. P. Inorg. Chem. 2015, 54, 3211−3217. (38) It should be noted that the trigonal-monopyramidal geometry, with bond angles of 120 and 90°, is referred to as “trigonal pyramidal” in ref 37, although the latter term is more appropriately used for threecoordinate geometries: e.g., NH3. (39) τ′ = ({β + 90} − α)/60, where β is the largest angle between a meridional donor and the apical donor and α is the largest angle between two meridional donors. An idealized tetrahedron is characterized by a value of 1.50, while an idealized trigonal monopyramid is characterized by a value of 1.00. See ref 36. (40) For some examples of zinc complexes with trigonal-monopyramidal geometries, see: (a) Ray, M.; Hammes, B. S.; Yap, G. P. A.; Rheingold, A. L.; Borovik, A. S. Inorg. Chem. 1998, 37, 1527−1532. (b) Blacquiere, J. M.; Pegis, M. L.; Raugei, S.; Kaminsky, W.; Forget, A.; Cook, S. A.; Taguchi, T.; Mayer, J. M. Inorg. Chem. 2014, 53, 9242− 9253. (41) Searches of the Cambridge Structural Database were performed with version 5.38. See: Groom, C. R.; Bruno, I. J.; Lightfoot, M. P.; Ward, S. C. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2016, 72, 171−179. (42) Weinhold, F.; Landis, C. R. Valency and Bonding: a natural bond orbital donor-acceptor perspective; Cambridge University Press: New York, 2005. (43) Green, M. L. H.; Parkin, G. Dalton Trans. 2016, 45, 18784− 18795. (44) Chemistry of Hypervalent Compounds; Akiba, K., Ed.; Wiley-VCH: New York, 1999. (45) (a) Matsumoto, N.; Akui, T.; Ohyoshi, A.; Okawa, H. Bull. Chem. Soc. Jpn. 1988, 61, 2250−2252. (b) Wu, H. L.; Ma, X. K. Z. Krist. New Cryst. Struct. 2007, 222, 39−42. (46) For some other structurally characterized tris(2-benzimidazolylmethyl)amine zinc compounds, see: (a) Wu, H. L.; Ying, W.; Pen, L.; Gao, Y. C.; Yu, K. B. Synth. React. Inorg. Met.-Org. Chem. 2004, 34, 1019−1030. (b) Wu, H. L.; Ying, W.; Fan, X. Y.; Sun, Q. Y.; Qi, B. L.; Xu, Y. H. Synth. React. Inorg. Met.-Org. Nano-Metal Chem. 2009, 39, 250− 255. (c) Wu, H. L.; Gao, Y. C. Transition Met. Chem. 2004, 29, 175−179. (d) Nakata, K.; Uddin, M. K.; Ogawa, K.; Ichikawa, K. Chem. Lett. 1997,

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(55) It is worth noting that the S−C−S bond angles for [TitmPr Benz]ZnOAr (108.8−111.5°) are larger than those for [TitmMe]ZnOAr (112.6−113.4°), which is consistent with a flattening of the [CS3] moiety as the Zn−C distance increases. (56) (a) McNally, J. P.; Leong, V. S.; Cooper, N. J. Cannula techniques for the manipulation of air-sensitive materials. In Experimental Organometallic Chemistry; Wayda, A. L., Darensbourg, M. Y., Eds.; American Chemical Society: Washington, DC, 1987; Chapter 2, pp 6− 23. (b) Burger, B. J.; Bercaw, J. E. Vacuum line techniques for handling air-sensitive organometallic compounds in Experimental Organometallic Chemistry; Wayda, A. L., Darensbourg, M. Y., Eds.; American Chemical Society: Washington, DC, 1987; Chapter 4, pp 79−98. (c) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-Sensitive Compounds, 2nd ed.; Wiley-Interscience: New York, 1986. (57) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. Organometallics 2010, 29, 2176−2179. (58) (a) Harris, R. K.; Becker, E. D.; De Menezes, S. M. C.; Goodfellow, R.; Granger, P. Pure Appl. Chem. 2001, 73, 1795−1818. (b) Harris, R. K.; Becker, E. D.; De Menezes, S. M. C.; Granger, P.; Hoffman, R. E.; Zilm, K. W. Pure Appl. Chem. 2008, 80, 59−84. (59) Starikova, O. V.; Dolgushin, G. V.; Larina, L. I.; Ushakov, P. E.; Komarova, T. N.; Lopyrev, V. A. Russ. J. Org. Chem. 2003, 39, 1467− 1470. (60) Crane, F. E. Anal. Chem. 1956, 28, 1794−1797. (61) Budzelaar, P. H. M. gNMR Version 5.0.1.0; Ivory Soft, 2002. (62) (a) Sheldrick, G. M. SHELXTL, An Integrated System for Solving, Refining, and Displaying Crystal Structures from Diffraction Data; University of Göttingen, Göttingen, Federal Republic of Germany, 1981 (b) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (c) Sheldrick, G. M. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (63) Jaguar, version 8.9; Schrodinger, Inc., New York, NY, 2015. (b) Bochevarov, A. D.; Harder, E.; Hughes, T. F.; Greenwood, J. R.; Braden, D. A.; Philipp, D. M.; Rinaldo, D.; Halls, M. D.; Zhang, J.; Friesner, R. A. Int. J. Quantum Chem. 2013, 113, 2110−2142. (64) (a) Hall, M. B.; Fenske, R. F. Inorg. Chem. 1972, 11, 768−775. (b) Bursten, B. E.; Jensen, J. R.; Fenske, R. F. J. Chem. Phys. 1978, 68, 3320. Manson, J.; Webster, C. E.; Pérez, L. M.; Hall, M. B. http://www. chem.tamu.edu/jimp2/index.html. (65) Lichtenberger, D. L. Department of Chemistry, University of Arizona, Tuscon, AZ 85721, 1993.

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