Reactions of α-Diimine-Stabilized Zn–Zn-Bonded Compounds with

Article pubs.acs.org/Organometallics

Reactions of α-Diimine-Stabilized Zn−Zn-Bonded Compounds with Phenylacetylene Jing Gao,†,§ Shaoguang Li,†,§ Yanxia Zhao,†,§ Biao Wu,†,‡ and Xiao-Juan Yang*,† †

State Key Laboratory for Oxo Synthesis & Selective Oxidation, Lanzhou Institute of Chemical Physics, CAS, Lanzhou 730000, People's Republic of China ‡ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, CAS, Fuzhou 350002, People's Republic of China § Graduate University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China S Supporting Information *

ABSTRACT: Treatment of the Zn−Zn-bonded compounds [L2−Zn−ZnL2−]·[M(THF)2]2 (1a, M = Na; 1b, M = K; L = [(2,6-iPr2C6H3)NC(Me)]2), which contain doubly reduced αdiimine ligands, with 15-crown-5 and 18-crown-6 led to the ionseparated compounds [L2−Zn−ZnL2−]·[Na(15-crown-5)(THF)2]2 (2a), [L 2− Zn−ZnL 2− ]·[K(15-crown-5) 2 ] 2 ·4THF (2b), and [L2−Zn−ZnL2−]·[K(18-crown-6)(THF)2]2·2THF (2c). In the products, the alkali metal ions originally bound by the ligands have been captured by the crown ethers. The Zn−Zn bond distances in 2a, 2b, and 2c are longer than those in the corresponding parent compounds 1a and 1b and in an analogous compound, [L−Zn−ZnL−] (3), bearing the monoanionic α-diimine ligands. Theoretical computations suggested that the Zn−Zn bonds in 2a−c are less stable than those in 1a and 1b. Reactions of [L−Zn−ZnL−] (3) with different amounts of PhCCH afforded the dimeric product [L−Zn(μ-CCPh)]2 (4) and the monomeric [L0Zn(CCPh)2]·2THF (5), respectively, while the reaction of the crown ether-containing compound 2b with PhCCH gave a homoleptic zinc alkynide, [Zn(CCPh)4]·[K(15-crown5)2]2·THF (6).

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target in most studies. The reactions of [Zn2(η5-C5Me5)2] with CNXyl (Xyl = 2,6-Me2C6H3), H2O, and tBuOH caused disproportionation of the Zn22+ unit into Zn0 and the corresponding ZnII compounds.1 Reactions with ZnR2 (R = Me, Mes) yielded half-sandwich compounds (C5Me5)ZnR and Zn metal, whereas reaction with I2 led to formation of Zn(C5Me5)2 and ZnI2.2 The treatment with the Lewis base dmap (4-dimethylaminopyridine) gave [(η5-C5Me5)Zn−Zn(dmap)2(η5-C5Me5)], the first Lewis acid−base adduct of dizincocene,10a which further reacted with [H(OEt2)2][Al{OC(CF3)3}4] to yield the base-stabilized Zn22+ compound [Zn2(dmap)6][Al{OC(CF3)3}4]2.10b Reactions with sterically demanding O-donor ligands ArMesOH (2,6-(2,4,6-Me3C6H2)C6H3OH) and C5Me5OH led to the formation of the corresponding (base-stabilized) ZnI−ZnI aryloxides. The reactions were carried out in the presence of strong Ncontaining Lewis bases, pyr-py (4-pyrrolidinopyridine) or DBU (diaza-1,3-bicyclo[5.4.0]undecane), which are needed to prevent disproportionation of the Zn22+ unit.11 Moreover, [Zn2(η5-C5Me5)2] can also behave as a promising starting reagent for the synthesis of novel low-valent organozinc

INTRODUCTION In 2004, the first structurally characterized stable Zn−Znbonded compound, decamethyldizincocene [Zn2(η5-C5Me5)2], was reported by Carmona et al.1a This landmark work initiated the synthesis of a large number of dizinc(I) complexes with different types of sterically demanding ligands.2−8 The commonly utilized ligands include bulky, substituted cyclopentadienyl units,1 m-terphenyl groups,3 and a variety of chelating N-donor ligands, such as the β-diketiminates {(2,6iPr 2 C 6 H 3 )N(Me)C} 2 CH (Dipp-nacnac) 4 and {(2,4,6Me3C6H2)N(Me)C}2CH (Mes-nacnac),5a bis(iminophosphorano)methanes [CH(Ph2PNR)2],5b α-diimines 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene6 and [(2,6-iPr2C6H3)NC(Me)]2,7 tris(3,5-dimethylpyrazolyl) hydridoborate (TpMe2),8a aminotroponimines,8b and Me2Si[N(2,6-iPr2C6H3)]2.8c Experimental and theoretical studies of these compounds have demonstrated that the Zn22+ unit is kinetically stabilized by sterically encumbered ligands, and the electronic and steric properties of these ligands have great impacts on the nature of the metal−metal bond, e.g., bond length, bond order, and orbital composition.2−8 Besides the development of new Zn−Zn-bonded compounds, efforts have also been devoted to the exploration of their reactivity. For this purpose, the dizincocene has been the © 2012 American Chemical Society

Received: September 15, 2011 Published: March 21, 2012 2978

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complexes.5,8a,b,12 Very recently, [Zn2(η5-C5Me5)2] was applied as a homogeneous catalyst for intra- and intermolecular hydroamination reactions, which is the first report of the catalytic applications of Zn−Zn-bonded species.13 Despite these results based on [Zn2(η5-C5Me5)2], the reactivity of other Zn−Zn-bonded compounds has been much less studied. Recently the reaction of another dizinc(I) compound, [Zn2(dpp-BIAN)2] (dpp-BIAN = 1,2-bis[(2,6diisopropylphenyl)imino]acenaphthene), with phenylacetylene was reported, which occurred with H2 elimination and formation of a binuclear, acetylene-bridged Zn2+ complex.14 α-Diimine species have been successful in the stabilization of metal−metal bonds. We (and others) have used such ligands to synthesize a series of metal−metal-bonded compounds, such as [L 2− Mg−MgL 2− ]·[K(THF) 3 ] 2 , 9 a [L 2− Zn−ZnL 2 − ]·[M(THF)2]2 (1a, M = Na; 1b, M = K; L = [(2,6-iPr2C6H3)NC(Me)]2),7a,b and [L−Zn−ZnL−] (3),7c by the alkali metal reduction method. In these compounds, the metal ions (Zn2+ or Mg2+) are reduced to the formal oxidation state of +1 (Zn+ or Mg+), while the ligands exist as either dianion (1a and 1b) or monoanion (3). In the compounds containing the dianionic ligands, a (solvated) Na+ or K+ ion is η4-bonded to the enediamido NCCN moiety of the ligand. These alkali metal ions could have significant influence on the ligand and subsequently on the metal−metal bonds. In order to get more insight into the effects of the steric and electronic properties of the ligands on the nature of the metal−metal bond, we studied the Zn−Zn bonding after removing the alkali metal ions from the ligand backbone in 1a and 1b. It is well known that crown ethers have specific ability in the selective binding of alkali metal cations of different sizes. In this work, we used crown ethers to trap the Na+ or K+ ions in 1a and 1b and obtained three compounds, [L2−Zn−ZnL2−]·[Na(15crown-5)(THF) 2 ] 2 (2a), [L 2− Zn−ZnL 2− ]·[K(15-crown5)2]2·4THF (2b), and [L2−Zn−ZnL2−]·[K(18-crown-6)(THF)2]2·2THF (2c), in which the alkali metal cations are encapsulated by crown ethers and the Zn−Zn bonds are persistent. Moreover, we have studied the reactivity of the αdiimine-stabilized dizinc(I) compounds 1−3. Herein we report the synthesis and structures of the crown ether-containing dizinc(I) compounds 2a−c and the reaction of Zn−Zn-bonded compounds 2 and 3 with phenylacetylene, which afforded the dimeric [L−Zn(μ-CCPh)]2 (4) and the monomeric products [L0Zn(CCPh)2]·2THF (5) and [Zn(CCPh)4]·[K(15crown-5)2]2·THF (6).

Scheme 1. Synthesis of Compounds 2a−c

spectively (Scheme 2). The reactions proceeded with elimination of H2 and oxidation of ZnI to ZnII for both Scheme 2. Reactions of Zn−Zn-Bonded Compounds with PhCCH and Structures of the Products 4−6

compounds and also oxidation of L− to L0 for 5. The oxidation of the ligand in the latter case was reflected by the changes of the C−N and C−C bond lengths as indicated in the crystal structures (see below). In compounds 2a−c the ligand exists as a dianion, while it is radical monoanionic in 4 and neutral in 5. Under similar conditions, the parallel reactions of 1a, 1b, and 2a−c with PhCCH were also carried out. A color change of the reaction solution from deep red to orange was observed in all cases. However, only the product [Zn(CCPh)4]·[K(15crown-5)2]2·THF (6) was isolated from the starting material 2b. To evaluate the general applicability of terminal alkynes, ptolyacetylene and 4-methoxyphenylacetylene were employed to react with compounds 1−3. A similar color change of the solution was also observed; unfortunately, the products could not be isolated. Compounds 2a−c and 4−6 are highly sensitive to air and moisture, but are thermally quite stable under argon at room temperature. Single crystals suitable for X-ray diffraction studies were obtained by slow evaporation of their THF solutions. X-ray Crystal Structures. [L 2− Zn−ZnL2−]·[Na(15crown-5)(THF) 2 ] 2 (2a), [L 2− Zn−ZnL 2− ]·[K(15-crown5) 2 ] 2 ·4THF (2b), and [L 2− Zn−ZnL 2− ]·[K(18-crown-6)(THF)2]2·2THF (2c). Compound 2a contains a dianionic dizinc moiety [LZn−ZnL]2− and two [Na(15-crown-5)(THF)2]+ countercations (Figure 1a). As expected, the Na+ ions initially bound by the NCCN moieties of the ligand L have transferred to the crown ether molecules. In the cationic part, each Na+ ion

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RESULTS AND DISCUSSION Synthesis. According to the diameter of the crown cavity relative to that of the alkali metal ion, 15-crown-5 can bind sodium and the larger 18-crown-6 prefers the potassium ion. In addition, the K+ ion can be coordinated by two 15-crown-5 macrocycles in sandwich structures. These two macrocycles were utilized to capture the alkali metal ions in the Zn−Znbonded compounds 1a and 1b, and the compounds 2a, 2b, and 2c were yielded, in which three crown ether−M interaction types were observed (Scheme 1). To test the reactivity of 1−3, comparative experiments were conducted with various unsaturated organic molecules. In the case of alkynes, the reaction of [L−Zn−ZnL−] (3) with different amounts of phenylacetylene (PhCCH) led to cleavage of the Zn−Zn bond and formation of the acetylenebridged binuclear compound [L−Zn(μ-CCPh)]2 (4) and the mononuclear compound [L0Zn(CCPh)2]·2THF (5), re2979

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longer than that in 1a. In contrast to the deviation of the Zn atom from the NCCN plane in 1a (vertical distance: 0.402 Å), the Zn atom in 2a is almost coplanar with the NCCN plane (0.029 Å). Moreover, the NCCN planes of the two ligands within a molecule of 2a are parallel and much closer than in 1a (vertical distances of 0.384 and 1.29 Å for 2a and 1a, respectively). Since the sodium ions are isolated from the ligand backbone, the Zn···Na separation in 2a is evidently longer than that in 1a (7.157 and 3.165 Å, respectively). The structures of the analogous products 2b and 2c are similar to 2a, and the K+ ions are trapped by the crown ethers. For 2b, although it was crystallized from THF, the complex is not solvated (Figure 2). Each K+ ion is sandwiched by two 15-

Figure 1. (a) Molecular structure of 2a (thermal ellipsoids are at the 10% probability level; H atoms are omitted for clarity; C atoms of THF are drawn as smaller spheres). (b) Space-filling diagram of the [LZn−ZnL]2− unit. (c) Side view of the whole molecule with the [Na(15-crown-5)(THF)2]+ units.

Figure 2. Molecular structure of 2b (thermal ellipsoids are at the 10% probability level; H atoms are omitted for clarity).

is coordinated by a 15-crown-5 macrocycle and two THF molecules located on the same side of the 15-crown-5 molecule, with an O−Na−O angle of 82.89(11)° (Table 1). The Na−

crown-5 molecules with a coordination number of 10. The two crown ethers are oriented in a way to avoid mutual contacts; that is, they adopt a staggered conformation. The K−O bond lengths vary over the range 2.805(4) to 3.056(6) Å, which are in agreement with previously reported examples of 15-crown-5sandwiched potassium ions.15 The Zn−Zn bond length in 2b (2.4447(12) Å) is longer than that in 2a and in the parent complex 1b. The Zn atoms are also nearly coplanar with the NCCN planes (deviation of 0.038 Å compared to 0.262 Å in 1b), and the NCCN planes of the two ligands within a complex molecule are parallel and have a much smaller vertical distance (0.182 Å) than that in 1b (0.70 Å). The Zn···K separation is 7.608 Å. Crystals of compound 2c contain the anionic [LZn−ZnL]2− part and two crown ether-potassium countercations (Figure 3 and Table 1). In the cations the potassium ion resides in the center of the 18-crown-6 molecule and is additionally coordinated by two THF molecules at the axial positions in a [KO6+2] hexagonal bipyramidal geometry with a crystallo-

Table 1. Selected Bond Distances (Å) and Bond Angles (deg) for 2a−c Zn(1)−Zn(1A) Zn(1)−N(1) Zn(1)−N(2) Zn(1)···M(1) C(1)−C(2) C(1)−N(1) C(2)−N(2) M(1)−O(1) M(1)−O(2) N(1)−Zn(1)−N(2) N(1)−Zn(1)−Zn(1A) N(2)−Zn(1)−Zn(1A) O(1)−M(1)−O(2)

2a (M = Na)

2b (M = K)

2c (M = K)

2.4247(8) 2.012(3) 1.994(3) 7.158 1.365(5) 1.405(4) 1.405(5) 2.347(3) 2.642(3) 82.49(12) 137.57(9) 139.89(9) 82.89(11)

2.4447(12) 2.000(4) 2.025(4) 7.608 1.368(7) 1.414(6) 1.402(6)

2.4208(6) 2.015(2) 1.995(2) 7.431 1.366(4) 1.408(3) 1.407(4) 2.672(3) 2.717(3) 83.39(9) 139.79(7) 136.75(7) 165.75(13)

82.20(16) 138.18(12) 139.59(12)

O(THF) bond lengths are 2.347(3) and 2.642(3) Å, and the Na−O(15-crown-5) bond lengths span the range 2.458(3)− 2.577(4) Å. The entire structure of 2a closely resembles that of the parent compound 1a. First, as in 1a, the anionic part [LZn− ZnL]2− of 2a is also a centrosymmetric Zn−Zn-bonded dimer. The Zn centers are three-coordinate with a distorted trigonalplanar geometry, and the two NCCN planes adopt a parallel orientation. Second, the two separated [Na(15-crown-5)(THF)2]+ units are situated above and below the two fivemembered metallacycles, respectively, and also provide some additional shielding for the central Zn−Zn bond (Figure 1c). On the other hand, noticeable differences have been observed between 2a and 1a. For metal−metal dimers, the most intriguing metric is the metal−metal bond length. The Zn−Zn bond length in 2a (2.4247(8) Å) is about 0.025 Å

Figure 3. Molecular structure of 2c (thermal ellipsoids are at the 10% probability level; H atoms are omitted for clarity; C atoms of THF are drawn as smaller spheres). 2980

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explained by π-electron donation from the ethynyl bond into the vacant Zn orbitals, which is also reflected by the longer C C bond length (1.213(5) Å) compared to that in free phenylacetylene (1.183(2) Å).18 Similar elongation of the CC bond length was observed in [(dpp-BIAN)Zn(μ-C CPh)]2 (1.2099(17) Å),14 [L″Mn(μ-CCPh)]2 (1.226(3) Å),16 and [L″Ca(μ-CCPh)]2 (1.224(2) Å).17 The angles Zn(1)−C(29)−C(30) (169.7(3)o) and Zn(1)−C(29A)−C(30A) (106.2(3)o) correspond to a nearly linear and a nearly right-angled Zn−CCPh arrangement, respectively. The bond lengths of C(1)−C(2) (1.417(5) Å), C(1)−N(1) (1.345(4) Å), and C(2)−N(2) (1.343(4) Å) indicate that the ligands are the π-radical monoanions.7c The EPR spectrum of 4 recorded in toluene at 150 K confirmed the radical-anionic character of the α-diimine ligands, in which one electron is localized on each diimine ligand (g = 2.006). This is similar to the compound 3.7c

graphic center of inversion. The K−O(18-crown-6) bond lengths fall in the range 2.711(5)−2.787(6) Å. There are subvan der Waals contacts between the cations and anions in the lattice of compound 2c. The Zn−Zn bond length (2.4208(6) Å) is longer than that in the parent compound 1b (2.3934(8) Å). The Zn-heterocycle is virtually planar, with only slight deviation (0.014 Å) of Zn from the NCCN plane. Moreover, the vertical distance of the NCCN planes of the two ligands is 0.136 Å. The Zn···K distance in 2c (7.362 Å) is somewhat shorter than that in 2b (7.608 Å). A related K-cryptand-containing Zn−Zn-bonded compound, (K−C222)2[L′Zn−ZnL′] (L′ = η2-Me2Si(NDipp)2), and the analogous K2[L′Zn−ZnL′] have previously been reported.8c Compared to the latter structure, in which the potassium ions are embedded in the phenyl rings, the Zn−Zn bond distance in the former is only slightly shortened from 2.3685(17) Å to 2.3634(11) Å. However, a dramatically increased dihedral angle of the N(1)−Zn(1)−Zn(2) plane and the Zn(1)−Zn(2)− N(4) plane (from 10.8° to 50.6(4)°) was observed. In the present work, the separation of the alkali metal ions from the ligand backbone caused significant elongation of the Zn−Zn bond length (by 0.0253, 0.0513, and 0.0274 Å in 2a, 2b, and 2c, respectively) compared to the Na- or K-contacted structures 1a and 1b, and variations of the conformation also occurred. On the other hand, a comparison of the structures 2a−c (with dianionic ligands) with 3 (with monoanionic ligands), both of which lack the alkali metal ions bonding to the N−CC−N moiety, showed that the Zn−Zn distances in 2a−c are longer than that in 3 (2.3403 Å). Moreover, the two C2N2Zn planes in 3 are arranged in a twisted orientation with a dihedral angle of 46.4°, while they are nearly coplanar in compounds 2a−c. From these results, it can be seen that the encapsulation of the alkali metal cations by the crown ethers can considerably alter the structure and Zn−Zn bonding of these compounds, which has also been evaluated by theoretical studies (see below). [L−Zn(μ-CCPh)]2 (4) and [L0Zn(CCPh)2]·2THF (5). The reaction of the Zn−Zn-bonded compound 3 with two and four equivalents of PhCCH, respectively, yielded the products 4 and 5. In 4, the two tetrahedrally coordinated zinc atoms are bridged by two phenylethynyl ligands, and an inversion center is localized in the middle of the two zinc atoms. An examination of the literature revealed that the only other structurally characterized, acetylene-bridged binuclear zinc compound, [(dpp-BIAN)Zn(μ-CCPh)]2, was obtained by a similar redox reaction of the Zn−Zn-bonded compound [Zn2(dppBIAN)2] with PhCCH.14 Other related structures include the dimeric [L″Mn(μ-CCPh)]216 and [L″Ca(μ-CCPh)]217 (L″ = HC(CMeNAr)2, Ar = 2,6-iPr2C6H3), which were prepared by the metathesis reaction of L″Mn(μ-Cl)2Mn(THF)2(μ-Cl)2MnL″ with PhCCLi or by transamination of the calcium amide L″Ca{N(SiMe3)2}(THF) with PhCCH, respectively. In these acetylene-bridged binuclear compounds, the two NCCN (or NCCCN) planes of the ligands in each molecule are parallel to each other and are perpendicular to the central four-membered MCMC ring (M = Zn, Mn, or Ca) formed by the two metal atoms and two bridging carbon atoms. These structural features were also observed in the current work for 4. The Zn···Zn distance (2.9224(9) Å) in compound 4 is beyond a Zn−Zn bonding range. The Zn−C distances to the two bridging carbon atoms are quite different (1.970(4) and 2.382(3) Å), as was also observed in [(dpp-BIAN)Zn(μ-C CPh)]2 (1.9840 and 2.2569 Å). The longer Zn−C bond may be

Figure 4. Molecular structure of 4 (thermal ellipsoids are at the 20% probability level; H atoms are omitted for clarity).

The reaction of compound 3 with four equivalents of PhC CH yielded the product 5. In the mononuclear compound 5, the zinc atom sits in a distorted tetrahedral environment containing two nitrogen atoms of one α-diimine ligand and two

Figure 5. EPR spectrum of compound 4 (toluene, 150 K). 2981

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terminal carbon atoms of two PhCC− ligands. The Zn−C C angles (174.5(3)o and 172.2(3)o) are very close, and the dihedral angle between the N2Zn plane and C2Zn plane is 86.96°. These characteristics are also observed in the mononuclear compound [(TMEDA)·Zn(CCPh)2],19 which has two essentially equal Zn−N bonds (2.159(3) and 2.155(3) Å), two close Zn−C(alkynyl) bonds (1.942(4) and 1.960(4) Å), two close Zn−CC angles (169.5° and 171.4(10)o), and a dihedral angle of 87.0° between the N2Zn/C2Zn planes. The bond lengths of C−C (1.508(4) Å) and C−N (1.281(4) Å) of the central C2N2 moiety indicate that the ligand is neutral in the product 5.

C−M bond angle of 180°. The two remaining p orbitals are orthogonal to this sp hybrid orbital, and thus the π-bonds would have CC−M bond angles close to 90°. Therefore, in compound 4, one Zn−C(alkynyl) bond is a σ-bond and the other (the longer one) shows mainly π-character. In contrast, both the Zn−C(alkynyl) bonds in 5 are σ-bonds. Another difference of the two products is that the ligand is radical monoanionic in complex 4 but is neutral in 5, which resulted from the different amounts of PhCCH employed in the reactions. The reaction of 3 with two equivalents of PhCCH led to only oxidation of ZnI to ZnII, and the ligand remained radical-monoanionic. However, employing four equivalents of PhCCH, both the oxidations of ZnI to ZnII and L− to L0 occurred. The two redox reactions proceeded with deprotonation of PhCCH and elimination of H2. The results might imply that the ZnI ion has a stronger reducing ability than the monoanionic ligand L− so that, with an increasing amount of PhCCH, ZnI was first oxidized to ZnII and then L− to L0. [Zn(CCPh)4]·[K(15-crown-5)2]2·THF (6). Compound 6 was isolated from the reaction of the crown ether-containing compound 2b with four equivalents of PhCCH. The solidstate structure of 6 comprises a distorted tetrahedrally coordinated homoleptic tetraphenylethynylzinc [Zn(C CPh)4]2− anion and two [K(15-crown-5)2]+ cations. The Zn−C (av 2.047 Å) and CC (av 1.212 Å) bonds within the anionic [Zn(CCPh)4]2− complex are comparable to those in the lithium analogue [Li(tmen)]2[Zn(CCPh)4] (tmen = Me2NCH2CH2NMe2).20a The Zn−CC angles lie in the range 160.5(6)−173.1(6)°, reflecting the σ-character of the Zn−C(alkynyl) bonds. The redox reaction of 2b with PhC CH led to oxidation of L2− to L0 and disproportionation of the Zn22+ unit into the ZnII alkynide and Zn0, as confirmed by the observation of Zn metal in the reaction system (Scheme 2).1a,b The first zinc alkynide, K2[Zn(CCH)4], was prepared by reacting [Zn(SCN)2]·2NH3 or K2[Zn(CN)4] with KCCH in liquid ammonia, and the structure was determined from Xray powder diffraction data. Subsequently, single-crystal

Figure 6. Molecular structure of 5 (thermal ellipsoids are at the 20% probability level; H atoms are omitted for clarity).

An interesting difference between 4 and 5 is the Zn− C(alkynyl) bonding. The PhCC− ligand has the potential of forming both σ- and π-bonding with metals. A perfect σ-bond exhibiting sp-hybridization of the C atom would display a C Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for 4 and 5 4 Zn(1)−N(1) Zn(1)−N(2) Zn(1)−C(29) Zn(1)−C(37) Zn(1)−C(29A) Zn(1)···Zn(1A) C(1)−C(2) C(1)−N(1) C(2)−N(2) C(29)−C(30) C(37)−C(38) N(1)−Zn(1)−N(2) C(29)−Zn(1)−C(29A) C(29)−Zn(1)−C(37) N(1)−Zn(1)−C(29) N(1)−Zn(1)−C(29A) N(1)−Zn(1)−C(37) N(2)−Zn(1)−C(29) Zn(1)−C(29)−Zn(1A) Zn(1)−C(29A)−C(30A) Zn(1)−C(29)−C(30) Zn(1)−C(37)−C(38)

1.983(3) 1.993(3) 1.970(4) 2.382(3) 2.9224(9) 1.417(5) 1.345(4) 1.343(4) 1.213(5) 84.12(11) 96.19(13) 128.15(13) 110.95(12) 127.15(12) 83.81(13) 106.2(3) 169.7(3)

5 2.110(3) 2.117(3) 1.966(3) 1.953(3)

1.508(4) 1.281(4) 1.281(4) 1.200(4) 1.209(5) 76.60(9) 126.05(13) 110.47(11) 113.65(11) 111.54(11)

Figure 7. Crystal structure of the anionic [Zn(CCPh)4]2− unit of 6 (thermal ellipsoids are at the 50% probability level; H atoms are omitted for clarity).

174.5(3) 172.2(3) 2982

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Table 3. Selected Bond Distances (Å) and Bond Angles (deg) for 6 Zn(1)−C(1) Zn(1)−C(9) Zn(1)−C(17) Zn(1)−C(25) C(1)−Zn(1)−C(9) C(1)−Zn(1)−C(17) C(1)−Zn(1)−C(25) Zn(1)−C(1)−C(2) Zn(1)−C(9)−C(10)

2.041(8) 2.040(7) 2.058(7) 2.051(7) 115.66(18) 111.1(2) 109.7(3) 170.5(6) 173.1(6)

C(1)−C(2) C(9)−C(10) C(17)−C(18) C(25)−C(26) C(9)−Zn(1)−C(17) C(9)−Zn(1)−C(25) C(17)−Zn(1)−C(25) Zn(1)−C(17)−C(18) Zn(1)−C(25)−C(26)

structures of zinc alkynyls were reported, for example, the homoleptic tetraphenylethynyl zincate [Li(tmen)]2[Zn(C CPh)4]20a and [Na(12-crown-4)2]2[Zn(CCPh)3(THF)][Zn(CCPh)3]20b synthesized by nucleophilic substitution of the bis(trimethylsilyl)amido ligands in [Zn(N(SiMe3)2)2] by PhCCLi or reaction of the homoleptic amido complex [Na(12-crown-4)2][Zn{N(SiMe3)2}3] and PhCCH, respectively. Very recently, the cluster-like zinc alkynide compounds C[C(NiPr)2Zn(CCPh)]4 and C[C(NiPr)2Zn(CCH)]4 were synthesized by substitution reactions of the tetranuclear amidinato zinc hydride complex C[C(NiPr)2ZnH]4 with phenylacetylene and acetylene, respectively, with elimination of H2.20c In comparison, compound 6 in this work was obtained by the redox reaction of the subvalent Zn−Zn-bonded compound with PhCCH, and its structure shows some differences from the above analogues. In 6, the alkali metal ions are captured by the crown ether, but in the lithium analogue two [Li(tmen)]2 units are bonded by the tetrahedral [Zn(C CPh)4]2− anion via alkynyl−lithium π-interactions. In [Na(12crown-4)2]2[Zn(CCPh)3(THF)][Zn(CCPh)3], though the alkali metal ions are encapsulated by crown ethers, the zinc atoms are not tetrahedrally coordinated with the PhCC− ligand as in 6, but are in a trigonal-planar arrangement in the [Zn(CCPh)3]− group and form a flat trigonal pyramid with the THF in the apical position in the [Zn(CCPh)3(THF)]− moiety. In C[C(NiPr)2Zn(CCPh)]4 and C[C(NiPr)2Zn(CCH)]4, the Zn atoms are coordinated by three N atoms of amidinato groups and one acetylide moiety. It has been reported that these complexes may be the key element for the asymmetric carbon−carbon bond construction.21 Previous studies on the activation of PhCCH by metal−metal-bonded compounds such as Ga−Ga, Si−Si, and Cr−Cr revealed addition processes with or without metal−metal bond cleavage.22 In our work, the reactions occurred with Zn−Zn cleavage and formation of zinc alkynides (redox reaction). Density Functional Theory (DFT) Studies. The electronic structures and bonding properties of the Zn−Znbonded compounds 2a−c have been studied by DFT computations on the model complex [(CHNH)2Zn−Zn(NHCH)2]2− (2H) at the B3LYP/DZP level.23 In our previous work, the Zn−Zn bonding in 1a and 1b was evaluated by computations on similar molecules Na2[(CHNH)2Zn−Zn(NHCH)2] (1aH)7a and K2[(CHNH)2Zn−Zn(NHCH)2] (1bH).7b The optimized structure of 2H (Figure 8a) is in good agreement with the experimental results for 2a−c. The computed C−N (1.402 Å) and C−C (1.377 Å) distances correspond to a dianionic ligand, and the Zn−N bond lengths (2.006 Å) and N(1)−Zn(1)−N(2) angles (83.4°) are close to those in the compounds 2a, 2b, and 2c. The NCCN planes of the two ligands are coplanar, as in 2a−c. The theoretical Zn− Zn bond distance (2.484 Å) in 2H is longer than the

1.221(10) 1.202(9) 1.210(9) 1.214(9) 109.7(3) 111.4(2) 97.78(18) 160.5(6) 167.3(6)

Figure 8. (a) Optimized structure of 2H. (b) Zn−Zn bond orbital (HOMO−2) of 2H.

experimental values in 2a (2.4247(8) Å), 2b (2.4447(12) Å), and 2c (2.4208(6) Å) and those calculated for 1aH (2.373 Å), 1bH (2.396 Å), and [(CMeNAr)2Zn−Zn(NArCMe)2] (Ar = 2,6-C6H3Me2; 3′)7c (2.356 Å). Moreover, this distance is only slightly shorter than twice Pauling’s single-bond metallic radius (2.50 Å).24 The Wiberg bond index gave a smaller Zn−Zn bond order for 2H (0.323) than for the compounds 1aH (0.69), 1bH (0.64), and 3′ (0.75). The HOMO−2 involves the Zn−Zn σbond and some N→Zn coordinative bonding (Figure 8b). A natural bond orbital (NBO) analysis indicated that the Zn−Zn σ-bond in 2H has lower s character (90%) than that of 1aH, 1bH, and 3′ (ca. 95%), and the charge distribution on the Zn atom is +0.45. Notably, the negative bond dissociation energy E(Zn−Zn) of −24.2 kcal/mol demonstrates that the [(CHNH)2Zn−Zn(NHCH)2]2− moiety is unstable and easy to dissociate to two [(CHNH)2Zn]− moieties, which is significantly different from the dissociation energies in 1aH (57 kcal/mol) and 3′ (54.2 kcal/mol). Thus the presence of the Na +- or K+-crown ether moieties in 2a−c may have considerable contribution to the stabilization of the whole compound.

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CONCLUSIONS Three ion-separated Zn−Zn-bonded compounds (2a−c) with crown-ether encapsulated alkali metal cations have been 2983

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Article

148.4 (i-C 6 H 3 ), 158.1 ppm (N-CCH 3 ). Anal. Calcd for C96H160N4O16K2Zn2·2THF (1979.43): C, 63.10; H, 8.96; N, 2.83. Found: C, 63.47; H, 8.48; N, 2.74. IR (Nujol, ν/cm−1): 1640, 1586, 1262, 1125, 1095, 943, 858, 830, 760, 550. Synthesis of [L−Zn(μ-CCPh)]2 (4). Phenylacetylene (0.108 g, 1.06 mmol) was added to a THF solution of 37c (0.500 g, 0.530 mmol) at room temperature. In the course of stirring of the mixture for 24 h, the color changed from deep red to light red. After evaporation of the solvent under vacuum, red crystals of 4 (0.22 g, 32%) were obtained. Mp: 234 °C (dec). Anal. Calcd for C72H90N4Zn2 (1142.22): C, 75.70; H, 7.94; N, 4.90. Found: C, 75.81; H, 8.17; N, 4.79. IR (Nujol, ν/ cm−1): 2090, 1591, 1259, 1096, 1022, 795, 758, 690, 537. Synthesis of [L0Zn(CCPh)2]·2THF (5). Phenylacetylene (0.216 g, 2.12 mmol) was added to a THF solution of 3 (0.500 g, 0.530 mmol) at room temperature. Red crystals of 5 were isolated (0.23 g, 26%) as described above for the synthesis of 4. Mp: 187 °C (dec). 1H NMR (400 MHz, C6D6): δ 1.03 (d, 12H, CH(CH3)2), 1.24 (m, 12H, CH(CH3)2), 1.72 (s, 6H, N-CCH3), 3.14 (m, 4H, CH(CH3)2), 7.06− 7.12 ppm (m, 16H, C6H3, C6H5). 13C NMR (100.6 MHz, [D8]THF): δ 12.2 (N−CCH3), 26.4 (CH(CH3)2), 28.5 (CH(CH3)2), 99.5 (C CPh), 104.6 (CCPh), 118.5 (m-C6H3), 126.7 (p-C6H3), 136.5 (oC6H3), 144.6 (i-C6H3), 164.3 ppm (N-CCH3). Anal. Calcd for C44H50N2Zn·2THF (816.44): C, 76.49; H, 8.15; N, 3.43. Found: C, 76.25 ; H, 8.28 ; N, 3.62. IR (Nujol, ν/cm−1): 1966, 1646, 1594, 1462, 1380, 1204, 1064, 790, 757, 693, 531. Synthesis of [Zn(CCPh)4]·[K(15-crown-5)2]2·THF (6). Phenylacetylene (0.186 g, 1.828 mmol) was added to a THF solution of 2b (1.00 g, 0.456 mmol) at room temperature. Red crystals of 6 were obtained (0.08 g, 12%) as described above for the synthesis of 4. Mp: 193 °C (dec). 1H NMR (400 MHz, [D8]THF): δ 3.57 (m, 80H, CH2crown), 7.28−7.45 ppm (m, 20H, C6H5). 13C NMR (100.6 MHz, [D8]THF): δ 70.5 (CH2-crown), 80.2 (CCPh), 87.6 (CCPh), 123.7−133.4 ppm (C6H5). Anal. Calcd for C72H100O20K2Zn·THF (1501.19): C, 60.80; H, 7.25. Found: C, 60.67; H, 7.29. IR (Nujol, ν/ cm−1): 2090, 1670, 1594, 1475, 1362, 1289, 1094, 786, 759, 696, 534. X-ray Crystal Structure Determination. Diffraction data for compounds 2a−c and 4−6 were collected on a Bruker SMART APEX II diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). An empirical absorption correction using SADABS was applied for all data. The structures were solved by direct methods using the SHELXS program. All non-hydrogen atoms were refined anisotropically by full-matrix least-squares on F2 by the use of the SHELXL program. Hydrogen atoms bonded to carbon were included in idealized geometric positions with thermal parameters equivalent to 1.2 times those of the atom to which they were attached. In compounds 2a and 2c the carbon atoms of THF molecules are disordered and display unusual thermal parameters; thus restrained refinement was applied by using the command “ISOR”. For 2b, the main part, [L2−Zn−ZnL2−]2−, is clear and unambiguous with the command ISOR used on one carbon atoms of the isopropyl group. For compound 6, a racemic twin refinement and ISOR on one crown ether oxygen atom were applied. Crystallographic data are listed in Table S1. CCDC 838673−838678 contain the supplementary crystallographic data of compounds 2a−c and 4−6 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. Computational Methods. The structure optimization and NBO bond analyses for the model compound [(CHNH)2Zn−Zn(NHCH)2] (2H) were carried out at the B3LYP/DZP level of theory using the Gaussian 03 program. The N-2,6-diisopropylphenyl and the methyl groups on the central carbon atoms are replaced by hydrogen atoms. The Cartesian coordinates and structure of the optimized geometry, and selected frontier orbitals are given in the Supporting Information.

obtained. By trapping the alkali metal ions from the dianionic α-diimine ligands, the Zn−Zn bonds were elongated compared to those in the parent ion-contacted compounds (1a and 1b), and the vertical distances of the two ligands within a molecule were shortened. Computations on a simplified molecule (2H) indicate that the Zn−Zn bond is weakened in the crown etherinvolving compounds, as evidenced by the longer bond distance, smaller bond order, and negative bond dissociation energy. The Zn−Zn-bonded compounds can act as reductants and readily react with PhCCH to form various alkynylzinc derivatives. Compound 3, with the monoanionic α-diimine ligands, led to the heteroleptic products 4 and 5, while the crown ether-containing 2b showed disproportionation of the Zn22+ units and gave the homoleptic tetraphenylethynyl zincate 6.

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EXPERIMENTAL SECTION

General Procedures. All operations with air- and moisturesensitive compounds were carried out using standard Schlenk or drybox techniques under an atmosphere of nitrogen or argon. Tetrahydrofuran was dried by sodium/benzophenone and distilled under argon prior to use. [D8]THF used for the NMR measurements was dried with Na/K alloy at room temperature just prior to use and was condensed under vacuum into the NMR tube already containing the sample. NMR spectra were recorded on a Mercury Plus-400 spectrometer. Elemental analyses were performed with an Elementar VarioEL III instrument. The EPR spectrum was recorded on a Bruker EMX-10/12 spectrometer. Melting points were measured in sealed capillaries on an X-4 Digital Vision MP instrument. IR spectra were recorded using a Nicolet AVATAR 360 FT-IR spectrometer. Synthesis of [L2−Zn−ZnL2−]·[Na(15-crown-5)(THF)2]2 (2a). 15Crown-5 (0.173 g, 0.788 mmol) was added to a THF solution of 1a (0.500 g, 0.390 mmol) at room temperature. The mixture was stirred for 24 h, and the solution was concentrated to ca. 5 mL and kept at −20 °C for several days to give red crystals of 2a (0.53 g, 78%). Mp: 164 °C (dec). 1H NMR (400 MHz, [D8]THF): δ 1.13 (d, 24H, CH(CH3)2), 1.28 (m, 40H, CH(CH3)2, THF), 1.74 (s, 12H, NCCH3), 3.14 (m, 8H, CH(CH3)2), 3.54 (m, 56H, CH2-crown, THF), 7.12−7.20 ppm (m, 12H, C6H3). 13C NMR (100.6 MHz, [D8]THF): δ 14.1 (N-CCH3), 21.3 (CH(CH3)2), 22.6 (CH(CH3)2), 24.7 (THF), 27.5 (CH(CH3)2), 66.8 (THF), 68.8 (CH2-crown), 116.0 (m-C6H3), 121.0 (p-C6H3), 122.5 (o-C6H3), 143.6 (i-C6H3), 153.6 ppm (NCCH3). Anal. Calcd for C92H152N4O14Na2Zn2·2THF (1859.21): C, 66.34; H, 8.66; N, 2.86. Found: C, 66.72; H, 8.82; N, 2.65. IR (Nujol, ν/cm−1): 1645, 1583, 1255, 1119, 1089, 942, 850, 825, 765, 554. Synthesis of [L2−Zn−ZnL2−]·[K(15-crown-5)2]2·4THF (2b). In a similar manner to the synthesis of 2a described above, compound 2b was obtained from 15-crown-5 (0.337 g, 1.54 mmol) and 1b (0.50 g, 0.384 mmol) as red crystals (0.46 g, 69%). Mp: 172 °C (dec). 1H NMR (400 MHz, [D8]THF): δ 1.15 (d, 24H, CH(CH3)2), 1.25 (m, 24H, CH(CH3)2), 1.76 (s, 12H, N-CCH3), 3.12 (m, 8H, CH(CH3)2), 3.52 (s, 80H, CH2-crown), 7.08−7.20 ppm (m, 12H, C6H3). 13C NMR (100.6 MHz, [D8]THF): δ 16.1 (N-CCH3), 23.3 (CH(CH3)2), 24.6 (CH(CH3)2), 27.9 (CH(CH3)2), 69.6 (CH2-crown), 114.0 (m-C6H3), 122.4 (p-C6H3), 124.5 (o-C6H3), 146.6 (i-C6H3), 155.3 ppm (NCCH3). Anal. Calcd for C96H160N4O20K2Zn2 (1899.33): C, 60.71; H, 8.49; N, 2.95. Found: C, 60.52; H, 8.71; N, 2.75. IR (Nujol, ν/cm−1): 1642, 1581, 1258, 1122, 1092, 940, 853, 828, 768, 556. Synthesis of [L2−Zn−ZnL2−]·[K(18-crown-6)(THF)2]2·2THF (2c). In a similar manner to the synthesis of 2a, compound 2c was obtained from 18-crown-6 (0.203 g, 0.768 mmol) and 1b (0.500 g, 0.384 mmol) as red crystals (0.24 g, 34%). Mp: 175 °C (dec). 1H NMR (400 MHz, [D8]THF): δ 1.10 (d, 24H, CH(CH3)2), 1.23 (m, 40H, CH(CH3)2, THF), 1.75 (s, 12H, N-CCH3), 3.10 (m, 8H, CH(CH3)2), 3.55 (m, 64H, CH2-crown, THF), 7.10−7.21 ppm (m, 12H, C6H3). 13C NMR (100.6 MHz, [D8]THF): δ 16.5 (N-CCH3), 22.9 (CH(CH3)2), 24.2 (CH(CH3)2), 26.3 (THF), 27.4 (CH(CH3)2), 70.3 (THF), 72.6 (CH2-crown), 118.0 (m-C6H3), 124.4 (p-C6H3), 126.5 (o-C6H3), 2984

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ASSOCIATED CONTENT

S Supporting Information *

Details of the DFT computations and information on the X-ray crystal structure analysis of compounds 2a−c and 4−6 (CIF files). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (20771103 and 20972169).

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REFERENCES

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dx.doi.org/10.1021/om200868j | Organometallics 2012, 31, 2978−2985