Thiopyridazine-Based Copper Boratrane Complexes Demonstrating

Article pubs.acs.org/IC

Thiopyridazine-Based Copper Boratrane Complexes Demonstrating the Z‑type Nature of the Ligand Stefan Holler,† Michael Tüchler,† Ferdinand Belaj,† Luis F. Veiros,‡ Karl Kirchner,§ and Nadia C. Mösch-Zanetti*,† †

Institute of Chemistry, University of Graz, Schubertstrasse 1, 8010 Graz, Austria Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais No. 1, 1049-001 Lisboa, Portugal § Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9, 1060 Vienna, Austria ‡

S Supporting Information *

ABSTRACT: In Z-type ligands the electrons for the coordination bond are formally provided by the metal. They represent an important addition to the much more extensively used L- and X-type σ-donor ligands for the development of transition metal complexes with new reactivities. We report here a new boron Z-type ligand with three tethering thiopyridazinyl donors forming exclusively complexes that feature a metal boron bond. Rational substitution pattern in the backbone of the pyridazinyl heterocycle led to a well-behaved ligand system that allowed preparation of a series of copper boratrane complexes in high yields. They are found to be more soluble in common organic solvents allowing reactivity studies in contrast to previous complexes with this type of ligand. Thus, copper complexes [Cu{B(PnMe,tBu)3}X] with X = Cl, OTf, N3, and κN-NCS are reported. Solution behavior was explored, and the molecular structures were determined by single-crystal X-ray diffraction analyses. The thiocyanate ligand is found to coordinate via its nitrogen atom pointing to a high oxidation state of the copper. Density functional theory calculations indicate a high positive charge on copper and a strong copper−boron interaction. Thus, here reported complexes deliver synthetic evidence for the Z-type nature of the ligand. These findings are important for further dissemination of these types of ligands in coordination chemistry.

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INTRODUCTION According to the Covalent Bond Classification introduced by Green in 1995 ligands may be divided into three basic types.1 While the vast majority of metal compounds involve electron donation from main-group atoms to metals (X-type and L-type ligands), in the significantly rarer compounds with Z-type ligands the metal formally provides the electrons for the coordination bond.2,3 Complexes where boron is directly ligated to a metal belong to this latter group.4 Among them a class of molecules with a cagelike structure that feature M−B bonds, so-called boratranes and related complexes, have raised significant interest.5−23 A wide variety of borane ligands exhibiting various structural motifs were developed since Hill’s first discovered boratrane compound.24 Tethering groups may contain nitrogen, sulfur, or phosphorus donors as displayed in Figure 1.25−29,8 Since the discovery of boratranes, the way to account the valence electrons in boratranes and therefore the nature of the metal-to-boron bond is subject of intensive debate.30,31 While typically the dn configuration corresponds to the number of valence electrons for the neutral transition metal minus its oxidation number, in boratranes this method is not adequate. Because of the similar Pauling electronegativities of late transition elements (1.8−2.2) and boron (2.0) the formal © XXXX American Chemical Society

oxidation state of the central atom is depending on the electron distribution over the metal−boron bond. Therefore, in boratrane complexes the dn configuration is not in line with the accepted norms (dn vs dn−2), which has been discussed for Rh and Ir complexes by Parkin.31 Such compounds have long passed the step of mere curiosity but have found entry into highly interesting applied chemistry. For example, boratrane complexes are capable of dinitrogen fixation and its catalytic conversion to ammonia.32−34,18 Reversible hydride migration as well as oxidative addition of small molecules makes them also interesting for hydrogenation and related reactions.35−38 We have recently developed thiopyridazine-based borate ligands Na[HB(Pn)3] (NaTn) and found them showing high selectivity for the formation of boratranes [M{B(Pn)3}X].39,29 Thus, we were able to prepare several first-row transition metal complexes including the first copper boratrane complex with sulfur-based ligands [Cu{B(PntBu)3}Cl]. The only other copper complex of this type described by Bourissou and co-workers contains the borane ligand tris[2-(diisopropylphosphino)phenyl]borane (TPB) with B−C instead of B−N bonds and Received: March 1, 2016

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DOI: 10.1021/acs.inorgchem.6b00464 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Boratrane complexes with three tethering groups showing different N-, S-, or P-based donor moieties.

Scheme 1. Preparationa of KTnMe,tBu

(a) (i) Lithium diisopropylamide, THF, −78 °C, 1 h; (ii) methyl pyruvate, −78 °C, 2 h; (iii) NH4Cl, −78 °C to rt, 92%; (b) N2H4·H2O, pTsOH, nBuOH, 118 °C, 2 d, 63%; (c) Lawesson’s reagent, toluene, 110 °C, 1 h, 92%; (d) K[BH]4, diphenylmethane, 190 °C, 4 h 30 min, 63%. a

was prepared via a three-step procedure as shown in Scheme 1. A crossed aldol reaction between pinacolone and methyl pyruvate led to the α-hydroxy-γ-keto ester. Cyclization to the oxopyridazine heterocycle occurred smoothly with hydrazine monohydrate in n-butanol, and subsequent reaction with Lawesson’s reagent led to the desired 4-methyl-6-tert-butyl-3thiopyridazine (HPnMe,tBu) in good overall yield (53%) and high purity after vacuum sublimation. This procedure allowed the preparation in multiple gram scale. The potassium salt of the scorpionate ligand tris(4-methyl-6tert-butyl-3-thiopyridazinyl)borate (KTnMe,tBu) was obtained in good yield (63%) by reaction of a slight excess (3.3 equiv) of HPnMe,tBu with K[BH4] in diphenylmethane up to 190 °C analogous to KTntBu.29 The reaction was performed under exclusion of light, due to the above-described recently demonstrated photochemical sensitivity of KTntBu.40 In the latter, upon irradiation, reduction of the heterocycle to H2PntBu was observed. Similar behavior was also found in KTnMe,tBu, as it decomposes slowly in deuterated solvents when exposed to daylight. However, in anhydrous solvents such as chloroform and under exclusion of light the ligand can be stored indefinitely. The introduction of an ortho-substituent increased its solubility compared to KTntBu. While both are soluble in polar organic solvents and insoluble in aliphatic hydrocarbons such as pentane, KTnMe,tBu is additionally soluble in diethyl ether and benzene.29 This is particularly interesting for its general use in coordination chemistry. KTnMe,tBu was characterized by 1H and 13C NMR spectroscopy in deuterated dimethyl sulfoxide (DMSO-d6) and CDCl3 revealing a symmetric species in solution evident by only one set of resonances for the heterocycles. Formation of the scorpionate is apparent since all resonances are significantly shifted compared to free pyridazinethione. For example, in the 1 H NMR spectrum of KTnMe,tBu in DMSO-d6, the resonance at 0.88 ppm assignable to the t-butyl groups is high-field shifted by 0.37 ppm compared to HPnMe,tBu. Similar to KTntBu no absorption band for the B−H stretch, typically found around 2400 cm−1, could be detected by IR spectroscopy.29 The absorption at 1164 cm−1 is typical for a strong C−S double bond and similar to the absorption of the

three tethering phosphine ligands rendering the electronic situation significantly different.12 While phosphines are strong donors the sulfur of the thiopyridazine moiety is a much weaker donor due to the electron-deficient nature of the nitrogen heterocycle. However, the copper boron distance in [Cu{B(PntBu)3}Cl] is with 2.060(3) Å much shorter than in the phosphine compound (2.508(2) Å). In phosphine boratrane complexes the bond lengths are also significantly influenced by the oxidation state of the central copper atom, which was shown recently via one-electron reductions from {(TPB)Cu}+ (2.495 Å) to (TPB)Cu (2.289 Å) and {(TPB)Cu}¯ (2.198 Å) by Peters and co-workers.14 Thus, the copper−boron distance in the thiopyridazine-based compound is even shorter than in the doubly reduced phosphine complex indicating a strong metal−boron interaction in the former. The synthetic procedures for the preparation of complexes with the Tn ligands were hampered by relatively low yields. Recently, they could significantly be improved as we discovered the NaTn ligand to be photoreactive.40 Decomposition to thiopyridazine and dihydrothiopyridazine can be prevented by working under exclusion of light. However, exploration of further reactivity of the unusual complexes was challenged by exceedingly low solubilities in suitable, preferably nonpolar, organic solvents. For this reason, we wish to introduce further substituents at the heterocycle. By doing so, not only the solubility is influenced but also the electronic properties. The steric influence is probably similarly small as in methimazolebased scorpionate complexes.41 Here the synthetic procedure of a new thio-pyridazine heterocycle HPnMe,tBu substituted by a methyl group in 4position (ortho to sulfur) and a tert-butyl group in 6-position (para to sulfur) is presented. Thermic reaction with K[BH4] allowed the preparation of the corresponding tris(thiopyridazinyl)borate ligand K[HB(PnMe,tBu)3] (KTnMe,tBu). Its coordination behavior toward copper was explored and is described here.

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RESULTS AND DISCUSSION Ligand Synthesis. The ligand with two substituents at the heterocycle 4-methyl-6-tert-butyl-3-thio-pyridazine (HPnMe,tBu) B


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Inorganic Chemistry thiocarbonyl moiety in HPnMe,tBu, indicating a rather weak interaction between the thiocarbonyl sulfur atoms and the potassium ion. The molecular structure of KTnMe,tBu was determined by single-crystal X-ray diffraction analysis. Suitable single crystals were obtained via slow evaporation of an aqueous acetonitrile solution. A molecular view is shown in Figure 2; selected bond lengths and angles are given in Table 1, and crystallographic data are in Table 3.

3.6009(7) Å), two aqua ligands (K1−O1 2.9303(16) Å, K1− O2 2.9192(16) Å), an acetonitrile ligand (K1−N1 2.811(2) Å), and two hydridic H atoms (K1−H1 2.79(2) Å, K1−H2 2.89(2) Å; B1−H1−K1 150.7(14)°, B2−H2−K1 154.8(14)°). In contrast, K2 shows a distorted octahedral surrounding with distinctly shorter distances of the pyridazine-3(2H)-thione ligands (K2−S 3.2530(7)−3.3690(6) Å) and of the aqua ligands (K2−O1′ 2.6949(15) Å, K2−O2′ 2.7200(15) Å) but about the same distances to the trans acetonitrile ligands (K2− N3 2.8201(19) Å, K2−N5 2.8173(19) Å). In addition, the structure is held together by four hydrogen bonds from the aqua ligands to the nonbridging S atoms (O···S 3.2581(15)− 3.2956(14) Å, O−H···S 157.0(14)−174.4(8)°). The presence of the aqua molecules explains why in anhydrous acetonitrile no single crystals could be obtained. In contrast, KTntBu crystallizes without water as a one-dimensional coordination polymer bridged solely by sulfur atoms,29 pointing to a lower steric demand due to the absence of the ortho-methyl group in the latter. Complex Synthesis. The reaction of KTnMe,tBu with CuCl2· 2H2O in a mixture of CH2Cl2/MeOH led to clean formation of the diamagnetic, red Cu(I) boratrane compound [Cu{B(PnMe,tBu)3}Cl] (1) as shown in Scheme 2. In the course of the reaction formal reduction from Cu(II) to Cu(I) occurs with most likely concomitant oxidation of the hydride to dihydrogen.39 The compound was obtained in very good yield of 90%, which hardly increased by exclusion of light (93%). We found the product boratrane not only to be insensitive toward light but also stable toward ambient atmosphere as demonstrated by storing solutions in various solvents over time. While the solubility of KTnMe,tBu was significantly higher in organic solvents compared to KTntBu, the corresponding copper−boratrane compound is only slightly more soluble in acetone, chloroform, and methylene chloride in which the solubility is in the range from 5 to 10 mg/mL. Nevertheless, it facilitates its use in coordination chemistry, which was hampered with our previous [Cu{B(TntBu)3}Cl] by its pronounced low solubility in most common solvents. The diamagnetic copper species 1 exhibits in CDCl3 or in CD2Cl2 a 1H NMR spectrum with only three resonances for one tert-butyl, one methyl group, and one aromatic proton consistent with a symmetric species in solution. All resonances are shifted downfield compared to KTnMe,tBu pointing to the more electron-deficient nature of the boratrane. For example, in the 1H NMR spectrum of 1 in CDCl3, the resonance at 7.21 ppm assignable to the aromatic hydrogen is downfield shifted by 0.14 ppm. Attempted cyclic voltammetry of 1 in acetonitrile revealed irreversible waves pointing to ligand-based redox processes. In fact, in all our thiopyridazine complexes we are unable to detect reversibility. This is in contrast to the TPB copper boratrane complexes, where a quasi-reversible wave of −1.6 V vs Fc/Fc+ (ferrocene) for the (TPB)Cu0/+ couple was reported.14 Single crystals suitable for X-ray diffraction analysis could be obtained by slow evaporation of a CHCl 3 solution unambiguously confirming the formation of the boratrane. A molecular view is shown in Figure 3; selected bond lengths and angles are given in Table 2, and crystallographic data are given in Table 3. The copper center has a slightly distorted trigonal bipyramidal environment (B1−Cu1−Cl1 177.94(7)°; Cl1− Cu1−S 97.37(2)°−100.30(2)°). The copper boron distance is with 2.065(2) Å very similar to the one in [Cu{B(PntBu)3}Cl]

Figure 2. Molecular view of a part of the polymeric structure of KTnMe,tBu. Hydrogen bonds to carbon, t-butyl groups, and solvent molecules were omitted for clarity.

Table 1. Selected Bond Lengths (Å) and Angles (deg) of KTnMe,tBu K1−S1 K1−S2 K1−S4 K1−S5 K1−H1 K1−H2 K1−N1 K1−O1 K1−O2 K2−S1 K2−S4 K2−O1

3.4362(7) 3.5503(7) 3.6009(7) 3.5633(8) 2.79(2) 2.89(2) 2.811(2) 2.9303(2) 2.9192(2) 3.3690(6) 3.2530(7) 2.6949(2)

K2−O2 K2−N3 K2−N5 B1−N12 B1−H1 S1−C13 S2−C23 B2−H2−K1 O1−K1−O2 O2−K1−S1 H1−K1−H2 K2−O1−K1

2.8201(2) 2.7200(2) 2.8173(2) 1.567(3) 1.11(2) 1.6996(2) 1.700(2) 154.8(1) 73.96(4) 162.82(3) 165.1(6) 102.65(5)

The scorpionate was found to crystallize as a onedimensional coordination polymer with alternating K atoms K1 and K2, either linked by two aqua ligands (K1····K2′ 4.3940(7) Å) or by two pyridazine-3(2H)-thione ligands (K1····K2 5.8367(8) Å). The coordination of K1 consists of four pyridazine-3(2H)-thione ligands (K1−S 3.4362(7)− C


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Inorganic Chemistry Scheme 2. Synthesis of Copper(I) Boratrane [Cu{B(PnMe,tBu)3}Cl] (1)

a significant steric impact at the metal center. On the other hand, the methyl group shields the sulfur atom to avoid its reaction with electrophiles, which is known for related methimazolyl borate systems.44 Substitution Chemistry. The higher solubility of [Cu{B(PnMe,tBu)3}Cl] (1) compared to [Cu{B(PntBu)3}Cl] allowed investigating substitution chemistry. In coordinating solvents an interesting solution behavior was observed. The 1H NMR spectrum of 1 in a 1:10 mixture of CD3CN and CD2Cl2 shows two sets of resonances for pyridazine rings in the ratio of ∼7:1, with the larger upfield signal set being assignable to 1. As shown in Figure 4b in the aromatic region next to the broad quartet at 7.35 ppm (splitting caused by a long-range coupling to the adjacent methyl group) for 1 an additional broad resonance at 7.47 ppm appears. The latter may be assigned to a cationic species formed upon displacement of the labile chlorine atom. This is supported by the reactivity of 1 toward addition of 1 equiv of silver triflate, as the respective 1H NMR spectrum in CD2Cl2 shows only resonances for the cation at the expense of the chloro compound (Figure 4c). It seems likely that an acetonitrile coordinated species of the type [Cu{B(PnMe,tBu)3}(CH3CN)]Cl (2) is formed in solution as shown in eq 1.

Figure 3. Molecular view of [Cu{B(PnMe,tBu)3}Cl] (1). Hydrogens and solvent molecules were omitted for clarity.

(2.068(3) Å)39 but significantly shorter than in triphosphine boratranes.14,12 The S−Cu−B−N torsion angles are in the range of 35.18(13)°−38.47(13)° aligning the heterocycles in a paddlewheel-like twisted fashion around the copper. The copper chlorine bond distance is with 2.2871(6) Å in the typical range for similar complexes.42,43 On the one hand, the crystal structure reveals the methyl groups pointing away from the chlorine atom, so larger substituents would be necessary for

[Cu{B(Pn Me,tBu)3 }Cl] 1 +CD3CN

HooooooooI [Cu{B(Pn Me,tBu)3 }(NCCD3)]Cl −CD3CN

(1)

2

The triflate compound formed upon treatment of 1 with AgOTf in acetonitrile can be isolated in bulk. After workup

Table 2. Selected Bond Lengths (Å) and Angles (deg) of Complexes 1, 3, 4, 5, 6, and 7

Cu−B Cu−X Cu−S

B−N

N−B−N

N−B−Cu

B−Cu−S

B−Cu-X

1

3

4

5

6

7

2.065(2) 2.2871(6) 2.2815(6) 2.3205(6) 2.3169(6) 1.531(3) 1.533(3) 1.542(3) 109.99(17) 111.20(17) 109.52(17) 109.30(14) 108.33(14) 108.44(14) 80.94(7) 80.32(7) 81.60(7) 177.94(7)

2.0432(14) 2.0210(10) 2.3266(4) 2.3310(4) 2.3048(4) 1.5292(17) 1.5316(17) 1.5329(17) 111.33(10) 109.51(10) 110.29(10) 109.19(8) 108.22(8) 108.22(8) 81.23(4) 82.42(4) 80.91(4) 177.08(5)

2.0654(17) 2.0155(14) 2.3056(5) 2.3064(5) 2.3386(5) 1.528(2) 1.530(2) 1.532(2) 111.85(13) 110.61(13) 109.78(13) 107.88(11) 108.44(10) 108.17(11) 82.10(5) 80.28(5) 80.80(5) 172.71(7)

2.0667(13) 1.9703(12) 2.2939(4) 2.3030(4) 2.3674(3) 1.5333(16) 1.5344(17) 1.5344(17) 109.61(10) 110.74(10) 110.33(10) 108.29(8) 110.32(8) 107.51(8) 79.86(4) 81.54(4) 79.36(4) 173.45(5)

2.068(4) 2.039(3) 2.3029(11) 2.2887(12) 2.3247(11) 1.533(6) 1.535(5) 1.544(5) 111.4(3) 109.7(3) 110.7(3) 108.5(3) 108.8(3) 107.5(3) 81.38(13) 81.74(13) 81.36(12) 175.48(15)

2.059(4) 2.035(3) 2.311(4) 2.353(2) 2.298(2) 1.529(3) 1.535(3) 1.535(3) 109.3(2) 111.8(2) 109.61(19) 106.1(3) 114.6(3) 105.3(3) 81.9(3) 75.6(4) 83.8(4) 176.6(2)

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Inorganic Chemistry

1). When a solution of [Cu{B(PnMe,tBu)3}Cl] (1) in CD2Cl2/ CD3CN was cooled to −50 °C, the ratio changes toward a slight increase of 1. This is consistent with the higher entropy of the cationic species being favored at high temperature (Supporting Information, Figure S1 and Table S1). The complex should again be compared to the TPB Cu system, where also a cationic complex was obtained of the type [(TPB)Cu][BArF4] (BArF4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate).14 The BArF4 anion is noncoordinating leading to a cationic copper ligated by the TPB ligand only. Interestingly, our previously reported copper boratrane with the less-substituted pyridazine ring [Cu{B(PntBu)3}Cl] seems to be less capable for the dissociation of the chloro ligand since the addition of CD3CN to a CD2Cl2 solution does not change the signals in the 1H NMR spectrum, and addition of AgOTf leads again to a more electron-deficient downfield set of signals (Supporting Information, Figure S2). This is consistent with the higher electron-donating capability of the three methyl groups in 1 stabilizing a cationic intermediate. Unambiguous evidence for the solid-state structure of the triflate compound was provided by single-crystal X-ray diffraction analysis. Suitable crystals of 3 were obtained by layering a CH2Cl2 solution with pentane. The structure is discussed together with the cationic, acetonitrile-coordinated complex [Cu{B(PnMe,tBu)3}(NCMe)](OTf) (4) mentioned above. Molecular views are shown in Figure 5; selected bond lengths and angles are given in Table 2, and crystallographic data are in Table 3. In both compounds the copper centers are again found in a slightly distorted trigonal bipyramidal environment (B1−Cu1− O41 177.08(5)°; O41−Cu1−S in the range of 96.10(3)°− 101.68(3)° in 3 and B1−Cu1−N1 172.71(7)°; N1−Cu1−S in the range of 93.64(4)°−106.38(5)° in 4). The copper−boron distance in 3 is with 2.0432(14) Å shorter compared to 4 (2.0654(17) Å) and 1 (2.065(2) Å). This is likely due to the smaller trans influence of triflate compared to chloride in 1 and acetonitrile in 4. These differences in the B−Cu distance solely depending on the trans ligand corroborate a bonding situation between the boron and copper atoms. The Cu−O bond length in 3 is with 2.0210(10) Å very short compared to other monomeric copper triflate complexes with sulfur donors, which are usually found between 2.0 and 2.8 Å.45−48 This finding supports the electron-withdrawing effect of the boron atom leading to strong donation of the triflate.

Figure 4. Aromatic region of the 1H NMR spectrum of [Cu{B(PnMe,tBu)3}Cl] (1) (a) in 500 μL CD2Cl2, (b) in 500 μL of CD2Cl2 and 50 μL of CD3CN, (c) after addition of 1 equiv of AgOTf in 500 μL of CD2Cl2 and 50 μL of CD3CN.

[Cu{B(PnMe,tBu)3}(OTf)] (3) was obtained in 85% yield as an air- and moisture-stable orange solid (Scheme 3). NMR spectroscopic data in acetonitrile containing solutions are consistent to those of the cationic species in the abovedescribed NMR experiment. NMR spectroscopy in CD2Cl2 shows no acetonitrile ligand pointing to a triflate coordinated species in absence of acetonitrile. The 19F NMR spectrum of 3 in CD2Cl2 shows a singlet at −78.7 ppm also indicating the formation of one single triflate-containing product. Furthermore, in one of the reaction solutions of [Cu{B(PnMe,tBu)3}(OTf)] left on the laboratory bench we observed the formation of single crystals. The determination of the molecular structure by X-ray diffraction analysis revealed it to be [Cu{B(PnMe,tBu)3}(CH3CN)](OTf) (4). A molecular view is given in Figure 5. When these crystals were dried in vacuo, the triflate compound 3 was reformed. Unfortunately, we were unable to reproduce 4. Nevertheless, these data support the occurrence of an equilibrium between 3 and 4 in solution in the presence of acetonitrile. In absence of a coordinating solvent (CH2Cl2) a 4-coordinate cation [Cu{B(PnMe,tBu)}]+ is formed, with the triflate being the anion or loosely bound. These findings also point to an equilibrium between 1 and 2 in solution further corroborated by its temperature behavior (eq Scheme 3. Substitution Chemistry of [Cu{B(PnMe,tBu)3}Cl] (1)

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Figure 5. Molecular views of [Cu{B(PnMe,tBu)3}(OTf)] (3, left) and [Cu{B(PnMe,tBu)3}(CH3CN)](OTf) (4, right). Hydrogens and solvent molecules were omitted for clarity.

toluene. A molecular view of the compound crystallized from toluene is shown in Figure 6; selected bond lengths and angles

After demonstrating the labile coordination of the chloride ligand in [Cu{B(PnMe,tBu)3}Cl], we were interested in the nature of the cationic species. Because of the formal low oxidation state of +I, on the one hand, and the electronwithdrawing effect of the Z-type boratrane ligand, on the other hand, the copper center can potentially act as both a hard and a soft Lewis acidic center. To investigate this situation we decided to substitute the chloride with thiocyanate as an ambivalent nucleophile. Thiocyanate shows a variety of coordination modes to copper including monodentate coordination via sulfur or nitrogen as well as polydentate bridging coordination modes with Cu−NCS and Cu−NCS− Cu coordination modes being the most common.49 Different factors like steric property, polarizability of the central atom, or the solvent can change its bonding mode. Nevertheless, according to the HSAB principle a low effective positive charge at copper increases the tendency for S-coordination of the thiocyanate ligand and vice versa. By stirring [Cu{B(PnMe,tBu)3}Cl] with NaSCN in acetone for 24 h under refluxing conditions [Cu{B(PnMe,tBu)3}(κN-NCS)] (5) could be isolated in 67% yield as an orange air- and moisture-stable solid. In acetone NaCl precipitates allowing easy isolation of the product. The compound was found to be only moderately soluble in common organic solvents except dichloromethane, chloroform, and acetone, where it is slightly more soluble. The 1H and 13C NMR signals for the pyridazine rings show only slight deviations to those of the chloro compound, which indicates a similar electronic environment. The formation of the thiocyanate compound was demonstrated by strong absorptions in the IR spectrum at 2098 cm−1 for the C−N and at 812 cm−1 for the C−S stretching frequency. Both wavenumbers are typical for κN coordination of a thiocyanate ligand.50 Very recently a structurally related cationic Cu(III) complex with a tetradendate S3P donor ligand has been reported.51 Albeit a sluggish reaction with thiocyanate and the lack of crystallographic characterization the authors interpreted their data also as a κN-SCN coordinated compound based on IR spectroscopic data. Suitable crystals of 5 for X-ray diffraction analysis were obtained by recrystallization in CH2Cl2/heptane as well as in

Figure 6. Molecular view of [Cu{B(PnMe,tBu)3}(κN-NCS)] (5). Hydrogen atoms and solvent molecules were omitted for clarity.

are given in Table 2, and crystallographic data are given in Table 3. A view of the structure crystallized from CH2Cl2/ heptane can be found in the Supporting Information (Figure S8 and Table S4). Similar to the structure of the chloro and triflate compound the copper center is again five-coordinate in a slightly distorted trigonal bipyramidal environment (B1−Cu1−N4 173.45(5)°; N4−Cu1−S in the range of 95.33(4)°−106.88(4)°) with a Cu−B bond length of 2.0667(13) Å. The most interesting fact in the structure of 5 is that the thiocyanato ligand binds via its nitrogen atom and not via the softer sulfur pointing to a hard F


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Inorganic Chemistry

Figure 7. Molecular views of [Cu{B(PnR,tBu)3}(N3)] (R = Me 6; R = H 7). Hydrogens and solvent molecules were omitted for clarity.

complexes have a slightly distorted trigonal bipyramidal environment with similar Cu−B bond lengths. Selected bond lengths and angles are given in Table 2, and crystallographic data are presented in Table 3. Boratrane azide complexes are extremely rare. Only one example with nickel has been reported as yet.11 Density Functional Theory Calculations. As indicated above, the fact that a thiocyanate ligand coordinates in a κN fashion leads to the suggestion of a hard Cu(III) d8 metal center in our synthesized boratrane complexes. To get further insight in the Cu−B bond the complexes [Cu{B(PnMe,tBu)3}(κN-NCS)] (5) and [Cu{B(PnMe,tBu)3}(κS-SCN)] (8) featuring a N- and S-bound SCN ligand, respectively, were investigated by means of density functional theory (DFT)/ PBE0 calculations, and the optimized structures are shown in Figure 8.

electrophilic metal center. To get further insight into the bonding situation theoretical calculations were performed (vide infra). The preferred κN coordination of the thiocyanate ion prompted us to investigate substitution reactivity toward azide, a Lewis base with similar hardness. This is interesting, as, while copper(II) chemistry is well-established, mononuclear copper azide complexes in the oxidation state +1 or +3 are significantly less investigated.52,53 Therefore, copper chloride complex 3 was treated with excess sodium azide in acetone at room temperature (rt) for 24 h leading to the orange azide complex [Cu{B(PnMe,tBu)3}(N3)] (6) in 83% yield. Increasing the temperature (refluxing acetone) did only marginally increase the yield (87%). Characterization in solution by 1H NMR spectroscopy revealed again a symmetric species with only one set of resonances for the pyridazine rings. It is noteworthy that again the chemical shifts are hardly influenced compared to the chlorine compound indicating similar electronic properties. The existence of the azide ligand is clearly evidenced via IR spectroscopy where a strong absorption at 2034 cm−1 is apparent typical for the asymmetric stretching frequency of copper azides.53 For comparative reasons the respective reaction employing the previously reported copper boratrane [Cu{B(PntBu)3}Cl] was also reacted with excess sodium azide in acetone at room temperature. Usual workup also allowed the isolation of the azide compound [Cu{B(PntBu)3}(N3)] (7), albeit in lower yield (63%). Furthermore, upon increasing the temperature to refluxing acetone condition complete decomposition to unidentified products is observed. In solid state both compounds 6 and 7 were found to be air- and moisture-stable. Thus, the additional methyl groups in 6 not only lead to higher solubility but also to an increase of the stability in solution. The solid-state molecular structures of 6 and 7 determined by X-ray diffraction analysis (Figure 7) reveal end-on coordination of the azide (Cu1−N11−N2 117.4(3)°, Cu1− N1 2.039(3) Å for 6 and Cu1−N13−N14 116.1(3)°, Cu1− N13 2.035(3) Å for 7). The Cu−N bond distances are in the expected range compared to similar mononuclear copper azide complexes.54,55 As the chloro derivative 1, both azido

Figure 8. DFT-calculated structures of [Cu{B(PnMe,tBu)3}(κN-NCS)] (5, left) and [Cu{B(PnMe,tBu)3}(κS-SCN)] (8, right).

In agreement with the experimental data, isomer 5 is thermodynamically more stable by ΔG = 4.9 kcal/mol than the isomer 8, which was not observed. The electronic structures of both complexes were investigated by means of a natural population analysis (NPA), and the major difference found is the charge of the Cu-atom, being more positive (0.93) in the complex with N-coordinated NCS than in the other one (0.84). The relatively high metal charges (close to unity) are indicative of a high oxidation state. The relevant Wiberg indices (WI) for G


Article

Inorganic Chemistry

Figure 9. Relevant molecular orbitals (d-splitting) for 5 (left) and for 8 (right). Energy values in atomic units.

Table 3. Crystallographic Data and Structure Refinement for KTnMe,tBu and 1, 3−7 crystallized from crystal system space group a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] data/parameters final R1 [I > 2σ(I)] CCDC No.

KTnMe,tBu

1

3

4

5

6

7

NCMe triclinic P1̅ 10.1685(5) 19.1118(11) 21.4271(11) 73.3783(16) 79.7478(14) 74.8793(17) 15 041/893 0.0389 1454473

CHCl3 monoclinic P21/c 9.7437(4) 22.9874(9) 18.5472(7) 90 99.4121(13) 90 11 920/450 0.0470 1454474

CH2Cl2/pent triclinic P1̅ 9.7806(4) 12.2287(5) 15.9639(7) 108.4873(18) 96.4871(19) 98.3205(18) 10 313/486 0.0291 1454475

NCMe triclinic P1̅ 10.0404(5) 13.6851(7) 14.7632(7) 78.4903(13) 81.8808(14) 79.6641(15) 9372/469 0.0323 1454476

toluene triclinic P1̅ 9.6744(3) 14.0734(5) 14.2630(4) 106.0926(14) 93.2862(12) 102.7474(13) 10 519/463 0.0287 1454477

CH2Cl2/hept monoclinic P21/c 9.8606(4) 19.8005(10) 18.3543(8) 90 92.2761(14) 90 6278/420 0.0566 1454478

NCMe triclinic P1̅ 9.5053(4) 12.4791(6) 13.8509(6) 105.283(2) 99.527(2) 95.101(2) 6053/447 0.0370 1454479

cationic species and the chloride counterion is observed. Thus, chloride was easily substituted by triflate, azide, and thiocyanate leading to the corresponding boratrane complexes in good yields. The latter is particularly interesting as the determination of the molecular structure by X-ray diffraction analysis revealed the thiocyanate ligand to be coordinated by its nitrogen atom. This points to the electrophilic nature of the copper atom corroborating transfer of electron density from the metal to boron, as a Z-type bond would suggest. Further evidence for a high oxidation state copper delivered DFT calculations. They confirm the higher thermodynamic stability of the Ncoordinated compound by 4.9 kcal/mol over the S-coordinated derivative. Moreover, the results from an NPA revealed strong B−Cu bonds and high positive charges on the copper atoms, suggesting a Cu(III) oxidation state.

the complexes with a N- and S-coordinated SCN-ligand indicate strong Cu−B bonds with WI = 0.42 and 0.36, respectively. Accordingly, the Cu−B bond is stronger in the complex with the NCS coordinated by the N atom. The relevant molecular orbitals of these complexes (d-splitting) are depicted in Figure 9, representing a trigonal bipyramidal complex of a d8 metal, that is, Cu(III).56 In both cases, the highest occupied molecular orbital is centered on the NCS ligand, and the lowest unoccupied molecular orbital is a Cu−B σ* orbital with participation of metal z2, and the axial positions in the trigonal bipyramidal geometry are occupied by the B atom and the NCS ligand.

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CONCLUSIONS In the present study, we report the synthesis of a thiopyridazine-based scorpionate ligand with additional methyl groups in ortho-position to sulfur. This allowed the selective preparation of the copper boratrane complex [Cu{B(PnMe,tBu)3}Cl] (1), which was found to be both more soluble in common organic solvents as well as more stable compared to the related nonmethylated compound. The high lability of the chlorine ligand in 1 is demonstrated by its behavior in coordinating solvents, where an equilibrium between 1 and a

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EXPERIMENTAL SECTION

General Information. If not otherwise noted, reactions were performed under N2 atmosphere, using standard Schlenk technique. Diisopropylamine was dried via refluxing over CaH2 and distillation under N2 atmosphere. Acetone was dried via refluxing over CaCl2 and distillation under N2 atmosphere. Lawesson’s reagent, KTntBu and [Cu{B(PntBu )3 }Cl], were synthesized according to published H


Article

Inorganic Chemistry procedures.57,39,29 All other chemicals were purchased from commercial sources and used without purification. Acetonitrile, tetrahydrofuran (THF), and toluene were purified via a Pure-Solv MD-4-EN solvent purification system from Innovative Technology, Inc. The NMR spectra were measured on a Bruker Avance III 300 MHz spectrometer at 25 °C if not otherwise stated. Chemical shifts are given in parts per million and are referenced to residual protons in the solvent. Electron impact mass spectra were recorded on an Agilent 5973 MSD using EI ionization technique. Electrospray mass spectra were recorded using a Thermo Scientific Q-Exactive mass spectrometer. Spectra were obtained using flow injection of ∼1 μM solutions of compounds in acetonitrile/acetone mixtures. Both positive and negative modes were employed. Infrared spectra were recorded on a Bruker Alpha Platinum ATR spectrometer. Elemental analyses were performed using a Heraeus Vario Elementar automatic analyzer at the Technical University of Graz. X-ray Structure Determination. X-ray data collection was performed with a Bruker AXS SMART APEX 2 CCD diffractometer by using graphite-monochromated Mo Kα radiation (0.710 73 Å) from an Incoatec microfocus sealed tube at 100(2) K. SHELXS-9758 was used as the structure solution, and structure refinement program SHELXL-2014/6 was also used. Full-matrix least-squares cycles on F2 were employed as the refinement method. Further details on the solution of the structures can be found in Table 3 and in the Supporting Information. Theoretical Details. Calculations were performed using the Gaussian09 software package59 and the PBE0 functional without symmetry constraints. That functional uses a hybrid generalized gradient approximation, including 25% mixture of Hartree−Fock60 exchange with DFT61 exchange-correlation, given by Perdew, Burke, and Ernzerhof (PBE) functional.62,63 The optimized geometries were obtained with the Stuttgart/Dresden ECP (SDD) basis set64−66 to describe the electrons of the metal atom, and for all other atoms a standard 6-31G** basis set was employed.67−72 Frequency calculations yielded no imaginary frequencies confirming the stationary points as minima. The free energy values reported were calculated at 298.15 K and 1 atm by using zero point energy and thermal energy corrections based on structural and vibration frequency data calculated at the same level. An NPA73−80 and the resulting Wiberg indices81 were used to study the electronic structure and bonding of the optimized species. Three-dimensional representations of the orbitals were obtained with Molekel.82 Methyl-2-hydroxy-2,5,5-trimethyl-4-oxohexanoate. To a stirred solution of dry diisopropylamine (13.8 mL, 98.0 mmol) in dry THF (400 mL) at −78 °C n-BuLi (39.0 mL, 2.5 M in hexanes, 97.5 mmol) was added over a period of 10 min. The resulting colorless solution was allowed to warm to −40 °C and cooled again to −78 °C, before pinacolone (11.2 mL, 89.1 mmol) was added over a period of 10 min. The resulting suspension was stirred at −78 °C for 1 h, before methyl pyruvate (8.9 mL, 98.0 mmol) was added over a period of 10 min. After 2 h the reaction was quenched with saturated NH4Cl (50 mL) at −78 °C and allowed to warm to room temperature. After the addition of H2O (100 mL) and Et2O (100 mL) the layers were separated. The aqueous layer was extracted with Et2O (3 × 100 mL), dried over MgSO4, and concentrated to dryness to obtain the product as a white solid (16.6 g, 92%). 1H NMR (300 MHz, CDCl3): δ 3.91 (s, 1H, OH), 3.73 (s, 3H, OMe), 3.16 (d, J = 17.8 Hz, 1H, diastereotopic C−H), 2.82 (d, J = 17.8 Hz, 1H, diastereotopic C−H), 1.38 (s, 3H, Me), 1.11 (s, 9H, tBu) ppm; 13C NMR (75 MHz, CDCl3): δ 215.8 (ketone-C), 176.5 (ester-C), 72.8 (quart. C−OH), 52.8 (OMe), 46.4 (methylene-C), 44.2 (tBu quart. C), 26.4 (Me-C), 26.2 (tBu-Me C) ppm; IR (ATR, cm−1): 3520, 2975, 1733, 1698, 1368, 1216, 1191, 1117, 1067, 786; Rf = 0.54 (2:1 EtOAc/cyclohexane); EI-MS (70 eV) m/z (%): 145 (22) [M+-t-Bu], 143 (28) [M+-CO2Et], 117 (70) [M+piv], 85 (69) [piv+], 57 (100) [t-Bu+]. 6-(tert-Butyl)-4-methylpyridazin-3(2H)-one. This reaction was performed under ambient atmosphere. To a stirred solution of methyl 2-hydroxy-2,5,5-trimethyl-4-oxohexanoate (23.8 g, 118 mmol) in nbutanol (400 mL), hydrazine hydrate (16.6 g, 50% in H2O, 259 mmol) and p-TsOH (20 mg, 0.12 mmol) were added. The resulting solution

was refluxed for 2 d and concentrated in vacuo. The crude product was washed with 50 mL of cyclohexane to obtain the title compound as a pale yellow crystalline solid (12.4 g, 63%). 1H NMR (300 MHz, CDCl3) δ 12.02 (bs, 1H, NH), 7.24 (d, J = 1.2 Hz, 1H, Ar−H), 2.22 (d, J = 1.2 Hz, 3H, Me), 1.26 (s, 9H, tBu) ppm; 13C NMR (75 MHz, CDCl3) δ 162.7 (CO), 155.8 (CN), 139.9 (arom. C−Me), 129.0 (arom. C−H), 36.1 (tBu quart. C), 29.2 (tBu-Me C), 16.7 (Me) ppm; IR (ATR, cm−1): 2957, 1645, 1605, 1455, 1373, 1261, 1117, 996, 944, 888, 676; Rf = 0.20 (2:1 EtOAc/cyclohexane); EI-MS (70 eV) m/z (%): 166 (27) [M+], 151 (100) [M+−Me], 124 (51). 6-(tert-Butyl)-4-methylpyridazine-3(2H)-thione. 6-(tert-Butyl)4-methylpyridazin-3(2H)-one (11.9 g, 71.6 mmol) and Lawesson’s reagent (20.3 g, 50.1 mmol) were suspended in 150 mL of dry toluene and heated to reflux, whereupon the solids got dissolved. After 1 h the reaction was allowed to cool to room temperature and concentrated in vacuo. The resulting orange solid was purified via sublimation (0.6 mbar, 100 °C) to obtain the product as a pale yellow crystalline solid (12.1 g, 92%). 1H NMR (300 MHz, CDCl3) δ 12.85 (bs, 1H, NH), 7.23 (s, 1H, Ar−H), 2.45 (s, 3H, Me), 1.30 (s, 9H, tBu) ppm; 1H NMR (300 MHz, DMSO-d6) δ 14.50 (bs, 1H, NH), 7.63 (d, J = 1.1 Hz, 1H, Ar−H), 2.28 (d, J = 1.1 Hz, 3H, Me), 1.25 (s, 9H, tBu) ppm; 13 C NMR (75 MHz, CDCl3) δ 179.5 (CS), 161.2 (CN), 149.4 (arom. C-Me), 124.0 (arom. C−H), 36.2 (tBu quart. C), 29.1 (tBu-Me C), 21.5 (Me) ppm; IR (ATR, cm−1): 2956, 1604, 1558, 1277, 1163 (CS), 1059, 997, 934, 893, 820, 710, 628; Rf = 0.55 (2:1 EtOAc/ cyclohexane); EI-MS (70 eV) m/z (%): 182 (74) [M+], 167 (100) [M+−Me], 140 (85). Potassium Tris(6-(tert-butyl)-4-methyl-3-thioxopyridazin1(6H)-yl)hydroborate (KTnMe,tBu). 6-(tert-Butyl)-4-methylpyridazine-3(2H)-thione (2.00 g, 11.0 mmol) and potassium borohydride (0.179 g, 3.3 mmol) were suspended in diphenylmethane (3 mL). The reaction mixture was protected from daylight via covering the flask with aluminum foil and was gradually heated to 190 °C over a period of 3 h. The reaction progress was monitored via evolution of H2. After 90 min at 190 °C gas evolution ceased, and the reaction was allowed to cool to room temperature. The resulting yellow slurry was poured into 250 mL of pentane, whereupon a yellow precipitate was formed. The solid was filtered, and the remaining organic impurities were removed via Soxhlet extraction with pentane to obtain the ligand as yellow powder (1.25 g, 63%). Crystals suitable for X-ray diffraction analysis were obtained by solvent evaporation of an acetonitrile solution. 1H NMR (300 MHz, DMSO-d6) δ 7.16 (s, 3H, Ar−H), 5.91 (br s, 1H, BH), 2.23 (s, 9H, Me), 0.88 (s, 27H, tBu) ppm; 1H NMR (300 MHz, CDCl3), 7.07 (s, 3H, Ar−H), 2.39 (s, 9H, Me), 0.95 (s, 27H, tBu); 13C NMR (75 MHz, DMSO) δ 181.0 (CS), 154.9 (CN), 146.7 (arom. C−Me), 119.2 (arom. C−H), 35.3 (tBu quart. C), 28.9 (tBuMe C), 22.9 (Me) ppm; IR (ATR, cm−1): 2963, 1605, 1477, 1371, 1323, 1267, 1250, 1198, 1164 (CS), 1017, 951, 886, 810, 615, 551; Anal. calcd for C27H40BKN6S32·2H2O: C, 51.12; H, 7.05; N, 13.25; S, 15.16; Found: C, 51.41; H, 6.70; N, 12.87; S, 15.30%; HRMS-ESI (m/ z): [TnMe,tBu]− calcd for C27H40BN6S3: 555.2570, found: 555.2554. [Cu{B(PnMe,tBu)3}Cl] (1). Solutions of CuCl2·2H2O (143 mg, 0.841 mmol) in 40 mL of a CH2Cl2 MeOH mixture (3:1, v/v) and KTnMe,tBu (500 mg, 0.841 mmol) in 20 mL of CH2Cl2 were prepared and covered with aluminum foil to protect the reactants from daylight. The yellow solution of the scorpionate ligand was added to the copper solution in one portion, whereupon a color change to intense red occurred. After 1 h, all volatiles were removed in vacuo. The obtained orange powder was dissolved in 5 mL of a CH2Cl2 MeOH mixture (20/1, v/v) and eluted over silica gel (5 g) with the same solvent mixture. The product was precipitated with pentane and filtered to obtain an air- and moisture-stable orange solid (510 mg, 93%). Single crystals suitable for X-ray diffraction analysis were obtained by slow evaporation of a chloroform solution. 1H NMR (300 MHz, CD2Cl2) δ 7.29 (s, 3H, Ar−H), 2.45 (s, 9H, Me), 1.10 (s, 27H, tBu) ppm. 1H NMR (300 MHz, CDCl3) δ 7.21 (s, 3H, Ar−H), 2.44 (s, 9H, Me), 1.10 (s, 27H, tBu) ppm; 13C NMR (75 MHz, CD2Cl2) δ 179.0 (C S), 163.6 (CN), 146.5 (arom. C−Me), 126.2 (arom. C−H), 36.8 (tBu quart. C), 29.3 (tBu-Me C), 20.1 (Me) ppm; IR (ATR, cm−1): 2965, 1598, 1520, 1479, 1367, 1319, 1204, 1178, 1168 (CS), 933, I


Article

Inorganic Chemistry 814, 695, 630; HRMS-ESI (m/z): [Cu{B(PnMe,tBu)3}]+ calcd for C27H39BCuN6S3: 617.1787, found: 617.1769. [Cu{B(PnMe,tBu)3}(OTf)] (3). AgOTf (29.2 mg, 0.11 mmol) and 1 (67.8 mg, 0.10 mmol) were suspended in anhydrous acetonitrile (4 mL). After it was stirred for 90 min at room temperature, the resulting suspension was centrifuged and filtered. The solvent was removed in vacuo to obtain an orange solid. This solid was dissolved in CH2Cl2 (5 mL), filtered, and concentrated to dryness to obtain 3 as an orange powder (68.1 mg, 85%). Single crystals suitable for X-ray diffraction analysis were obtained by layering a CH2Cl2 solution with pentane. 1H NMR (300 MHz, CD2Cl2) δ 7.39 (s, 3H, ArH), 2.49 (s, 9H, Me), 1.11 (s, 27H, tBu) ppm; 13C NMR (75 MHz, CD2Cl2) δ 177.9 (CS), 164.9 (CN), 146.8 (arom. C−Me), 127.4 (arom. C−H), 37.0 (tBu quart. C), 29.1 (tBu-Me C), 19.9 (Me) ppm; 19F NMR (282 MHz, CD2Cl2) δ −78.7; IR (ATR, cm−1): 2968, 1595, 1477, 1446, 1379, 1312, 1202, 1169 (CS), 1018, 933, 813, 695, 634, 516; HRMS-ESI (m/z): [Cu{B(PnMe,tBu)3}]+ calcd for C27H39BCuN6S3: 617.1787, found: 617.1767. [Cu{B(PnMe,tBu)3}(N3)] (6). NaN3 (24.9 mg, 0.382 mmol) and 1 (50 mg, 0.076 mmol) were suspended in anhydrous acetone (3 mL). The reaction mixture was heated to reflux for 24 h, and all volatiles were removed in vacuo. The resulting orange solid was dissolved in CH2Cl2 and eluted over a short pad of Celite. To the red solution n-heptane was added, and CH2Cl2 was slowly evaporated overnight, giving red crystals. They were filtered and washed with pentane (20 mL) to obtain 6 (42.3 mg, 87%). Single crystals suitable for X-ray diffraction analysis were taken from the CH2Cl2/n-heptane crystallized product. 1 H NMR (300 MHz, CD2Cl2) δ 7.31 (q, J = 1.0 Hz, 3H, ArH), 2.46 (d, J = 0.8 Hz, 9H, Me), 1.10 (s, 27H, tBu) ppm; 13C NMR (75 MHz, CD2Cl2) δ 178.7 (CS), 163.9 (CN), 146.6 (arom. C−Me), 126.4 (arom. C−H), 36.8 (tBu quart. C), 29.3 (tBu-Me C), 20.1 (Me) ppm; IR (ATR, cm−1): 2965, 2034 (N3), 1599, 1378, 1317, 1200, 1167 (CS), 931, 814, 695, 629; HRMS-ESI (m/z): [Cu{B(PnMe,tBu)3}]+ calcd for C27H39BCuN6S3: 617.1787, found: 617.1767. Alternative Synthesis of [Cu{B(PnMe,tBu)3}(N3)] (6). NaN3 (30.5 mg, 0.469 mmol) and 1 (53.9 mg, 0.082 mmol) were suspended in anhydrous acetone (5 mL). The reaction mixture was stirred at room temperature for 24 h, and all volatiles were removed in vacuo. The resulting orange solid was dissolved in CH2Cl2 and eluted over a short pad of Celite. To the obtained red solution n-pentane was added, and the orange precipitate was filtered and washed with pentane (20 mL) to obtain 6 (45 mg, 83%). [Cu{B(PntBu)3}(N3)] (7). NaN3 (27.0 mg, 0.408 mmol) and [Cu{B(PntBu)3}(Cl)] (50.0 mg, 0.082 mmol) were suspended in anhydrous acetone (5 mL). The reaction mixture was stirred at room temperature for 24 h, and all volatiles were removed in vacuo. The resulting orange solid was dissolved in CH2Cl2 and eluted over a short pad of Celite. To the obtained red solution n-pentane was added, and the orange precipitate was filtered and washed with pentane (20 mL) to obtain 7 (32 mg, 63%). Single crystals suitable for X-ray diffraction analysis were taken from the with NCMe recrystallized product. 1H NMR (300 MHz, CD2Cl2) δ 7.82 (d, J = 9.2 Hz, 3H), 7.43 (d, J = 9.2 Hz, 3H), 1.12 (s, 27H) ppm; 13C NMR (75 MHz, CD2Cl2) δ 178.0 (CS), 163.4 (CN), 137.0 (arom. C−Me), 128.0 (arom. C−H), 37.0 tBu quart. C), 29.1 (tBu-Me C), ppm; IR (ATR, cm−1): 2966, 2039, 1591, 1477, 1429, 1256, 1212, 1135; HRMS-ESI (m/z): [Cu{B(PntBu)3}]+ calcd for C24H33BCuN6S3: 575.1317, found: 575.1297. [Cu{B(PnMe,tBu)3}(κN-NCS)] (5). NaSCN (12.5 mg, 0.154 mmol) and 1 (50.3 mg, 0.077 mmol) were suspended in acetone (5 mL). The reaction mixture was stirred under reflux for 24 h, before all volatiles were removed in vacuo to obtain an orange solid. This solid was dissolved in CH2Cl2 (5 mL), filtered, and concentrated to dryness to obtain 5 as an orange powder (35 mg, 67%). Single crystals suitable for X-ray diffraction analysis were obtained from both a CH2Cl2/nheptane and a toluene solution. 1H NMR (300 MHz, CD2Cl2) δ 7.32 (s, 3H, ArH), 2.46 (s, 9H, Me), 1.09 (s, 27H, tBu) ppm; 13C NMR (75 MHz, CD2Cl2) δ 178.6 (CS), 164.0 (CN), 146.5 (arom. C−Me), 126.5 (arom. C−H), 36.9 (tBu quart. C), 29.3 (tBu-Me C), 20.1 (Me) ppm; IR (ATR, cm−1): 2970, 2098 (C−N stretching), 1595, 1378,

1203, 1172 (CS), 932, 812 (C−S stretching), 697; HRMS-ESI (m/ z): [Cu{B(PnMe,tBu)3}]+ calcd for C27H39BCuN6S3: 617.1787, found: 617.1763.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00464. CCDC Nos. 1454473−1454480 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif. Further details on low-temperature 1H NMR experiment of 1, X-ray structure determinations of KTnMe,tBu, 1, 3, 4, 5, 6, 7, and atomic coordinates of the optimized molecules (PBE0). (PDF) X-ray crystallographic information. (CIF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +43 (0)316 380−5286. Fax: +43 (0)316 380−9835. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from NAWI Graz is gratefully acknowledged. REFERENCES

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