Article pubs.acs.org/Organometallics
Benzylidyne Bridges from Diphenylacetylene and a Methylidyne Bridge from Methylmagnesium Chloride Guy Y. Vollmer, Mark W. Wallasch, Dirk Saurenz, Tobias R. Eger, Heiko Bauer, Gotthelf Wolmershaü ser, Marc H. Prosenc, and Helmut Sitzmann* FB Chemie der TU, Erwin-Schrödinger-Straße 54, 67663 Kaiserslautern, Germany S Supporting Information *
ABSTRACT: Preparation of the dimeric starting compound [(C5H2tBu31,2,4)Fe(μ-Br)]2 (1) gave a small amount of its green sodium bromide adduct [(C5H2tBu3-1,2,4)Fe(μ-Br)2Na(DME)2] (2). Reduction of 1 under argon in the presence of diphenylacetylene yielded dark red [(C5H2tBu31,2,4)Fe(μ-CPh)]2 (3) containing two bridging benzylidyne ligands resulting from diphenylacetylene scission. The dicobalt methylidyne complex [{(C5HiPr4)Co}2(μ-CH)(μ-H)] (5) was obtained in high yield from the reaction of [(C5HiPr4)Co(μ-Cl)]2 (4) with two equivalents of methylmagnesium chloride. According to DFT calculations the dinuclear alkylidyne complexes 3 and 5 contain d6 metal centers, namely, iron(II) (3) and cobalt(III) (5). Whereas the benzylidyne ligands of the iron complex 3 display an empty orbital of pz character at the bridging carbon atom, the pz orbital of the methylidyne carbon in the dicobalt complex 5 is filled with a lone pair of electrons.
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the second one in a μ-alkyne-like bridging position has been characterized by a very accurate crystal structure determination of the trinuclear iron complex [Fe3(CO)9(μ3,η2-⊥-EtC CEt)].48 An example of reversible coupling of two methoxycarbyne ligands to a dimethoxyacetylene ligand has been observed for the cationic dimolybdenum complex [(CpMo)2(μ-COMe)2(μ-PCy2)]+ upon addition of two carbonyl ligands, whose elimination by UV irradiation restores the bis(μ-methoxycarbyne) precursor.49 This article reports on a reaction of bulky alkylcyclopentadienyliron fragments with diphenylacetylene that is closely related to the scission of a coordinated diphosphorus ligand during photolytic elimination of carbon monoxide from the dinuclear diphosphorus complex [{(C5H2tBu3-1,2,4)Fe}2(μ,η2:η2-P2)(μ-CO)] with formation of the bis(phosphido) complex [{(C5H2tBu3-1,2,4)Fe}2(μ-P)2]50 (Scheme 1). The diphosphorus molecule closely resembles an alkyne and coordinates as a four-electron donor like the 2,2,5,5tetramethyl-3-hexyne ligand in the related diiron complex [(OC)3Fe(μ,η2:η2-C2R2)Fe(CO3] (FeFe; R = CMe3).51 Loss
ALKYLIDYNE COMPLEXES After the discoveries of the bridging alkylidyne ligand in the [(Me3SiCH2)2Nb(μ-CSiMe3)]2 dimer,1 the mononuclear carbyne complexes trans-[X(CO)4MCMe)] (M = Cr, Mo, W; X = Cl, Br, I),2 and the tantalum(V) neopentylidyne complex [(Me3CCH2)3TaCCMe3]·Li(DMP)3 (DMP = N,N′-dimethylpiperazine), a large body of carbyne and alkylidyne complexes has been investigated. Fischer4−11 and Schrock12−24 24 concentrated on complexes with terminal carbyne ligands and reported almost 800 molecules of the [LnMCR] type. An early monograph was published in 1988,25 additional review articles covering complexes with terminal carbyne ligands are those of Kim and Angelici,26 Mayr27,28 with Hoffmeister,27 and Caldwell.29 Besides about 100 complexes with terminal carbyne ligands, Stone30−37 described many complexes with doubly (ca. 450 examples) or triply bridging carbyne ligands (ca. 300 representatives). Acetylene scission by highly reactive metal complexes has been discovered for the ditungsten hexaalkoxide [W2(OtBu)6] with a WW triple bond38 and produces alkylidyne complexes [(tBuO)3WCR] in clean reactions with a variety of substituted alkynes under mild conditions.39 Recent review articles on the utility of alkyne metathesis with alkylidyne complexes in synthetic organic chemistry have been provided by Fürstner and other authors.40−44 A combined coupling and scission reaction of diphenylacetylene ligands in a mononuclear tungsten complex aided by a fullerene ligand has been observed recently.45 Diphenylacetylene scission with formation of the μ4-benzylidyne ligands of the hexanuclear cluster [(CpMo)2Co4(μ4-CPh)2(μ-CO)2(CO)8] has been reported in 2002,46 and more alkyne scission reactions in cluster chemistry are the subject of a review article.47 A diethylacetylene ligand trapped in a bonding situation with one alkyne carbon atom in an alkylidyne-like μ3 coordination and © 2015 American Chemical Society
Scheme 1. Formation of the Bis(μ-phosphido) Diiron Complex [(C5H2tBu3-1,2,4)Fe(μ-P)]250
Received: December 3, 2014 Published: January 26, 2015 644
DOI: 10.1021/om501236h Organometallics 2015, 34, 644−652
Article
Organometallics
EI mass spectra of 1 show a signal for the molecular ion with very low intensity, the monomeric [(C5H2tBu3-1,2,4)FeBr]+ moiety with 12.6% intensity, and a methyl cleavage product [C16H26Fe]+ with 54.1% intensity, respectively, which is the second largest signal after the parent peak representing tBu+. Other signals correspond to the hexa(tert-butyl)ferrocenium cation (22.1%) or to the cation [(C5H2tBu3-1,2,4)Fe]+ (2.9%). Low-spin iron(II)−Cp ring plane distances are rarely exceeding 1.7 Å (e.g., 1.72 Å in octaisopropylferrocene,62 1,1′,2,2′,4,4′-hexa(tert-butyl)ferrocene,63,64 or 1.74 Å in decaphenylferrocene66), while the known high-spin Fe−Cp distances are either close to 1.9 Å or even longer.52−58 A magnetic moment of 5.09(20) μB55 has been determined by NMR spectroscopy using a modification67 of the method of Evans68 on solutions of 1 in benzene (C6H6) at 298 K. Compound 1, like its tetraisopropylcyclopentadienyl isomer, shows reversible color changes with coordinating solvents such as tetrahydrofuran (green) or acetonitrile (violet). The solvent coordination is reversible, and the red color of 1 can be restored by solvent removal in vacuo. In one experiment formation of a small amount of green crystals was observed during evaporation of the pentane extract of 1 (Scheme 2). The amount of material was not enough for full characterization, but an X-ray crystal structure analysis has been carried out (Figure 1) and revealed the heteronuclear
of the carbonyl ligand is compensated by scission of the fourelectron donor P2 in two phosphide ligands.
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BULKY ALKYLCYCLOPENTADIENYLIRON COMPLEXES During the preparation of transition metal half-sandwich complexes of the [CpFeR] type with a very bulky alkylcyclopentadienyl ligand problems with oily or tarry consistency were encountered, which could be solved by switching the glovebox atmosphere from nitrogen to argon. The suspicion of complex formation with nitrogen could be confirmed in the meantime52,53 and will be the subject of a subsequent publication.54 Initially we resorted, however, to diphenylacetylene as another triply bonded reaction partner in order to learn more about the reactivity of bulky alkylcyclopentadienyliron complexes such as [(C 5 H 2 t Bu 3 -1,2,4)Fe(μ-Br)] 2 (1), 55,56 [(C5HiPr4)Fe(μ-Br)]2,57 or [(C5iPr5)Fe(μ-Br)]2.57,58 Compounds of the [CpFeX]2 type have been isolated only with alkylcyclopentadienyl derivatives bulky enough to hinder ferrocene formation. While the pentaisopropylcyclopentadienyl ligand (C5iPr5) seems to be too bulky to form decaisopropylferrocene,59,60 the tetraisopropylcyclopentadienyl ligand (C5HiPr4) is able to form the sterically hindered ferrocene [(C5HiPr4)2Fe]61,62 as well as the dimeric bromide [(C5HiPr4)Fe(μ-Br)]2.57 The 1,2,4-tri(tert-butyl)cyclopentadienyl ligand can also form the sterically congested ferrocene [(C5H2tBu31,2,4)2Fe]63,64 as well as the bromide 1 or the corresponding iodide.64 Bis{tri(tert-butyl)cyclopentadienyliron(II) bromide} (1) and Its Reaction with Diphenylacetylene under Reducing Conditions. In a short communication the crystal structure of the bromo-bridged dimer 1 has been reported.55 In this article the preparation of 1 is described in detail. A side product of 1 will be mentioned, which explains earlier observations of alkali halide precipitates from pentane solutions of 1 (Scheme 2). A similar observation has been made in the iron(II) amide [{(Me3Si)2N}2Fe(μ-Cl)Li(THF)3].65 From the dark green reaction solution of iron(II) bromide dimethoxyethane adduct and sodium tri(tert-butyl)cyclopentadienide the red, paramagnetic dimer [(C5H2tBu31,2,4)Fe(μ-Br)]2 (1) was isolated (see Scheme 1 and Experimental Procedures).
Figure 1. Crystal and molecular structure of the sodium bromide complex 2 with ellipsoids plotted at the 50% probability level. Selected bond distances (Å) and angles (deg): Fe1−C1 2.343(3), Fe1−C2 2.378(3), Fe1−C3 2.258(3), Fe1−C4 2.257(3), Fe1−C5 2.314(3), Fe1−ring plane 1.967, Fe1−Br1 2.4633(7), Fe1−Br2 2.4316(7), Na1−Br1 2.9542(15), Na1−Br2 2.9072(15), Na1−O1 2.362(3), Na1−O2 2.419(3), Na1−O3 2.404(3), Na1−O4 2.359(3), Br1− Na1−Br2 81.18(4), Br1−Fe1−Br2 102.36(2), Fe1−Br1−Na1 86.32(3), Fe1−Br2−Na1 87.96(4), fold angle of the FeBr2Na ring along Br···Br 163.8.
Scheme 2. Preparation of the Starting Compound 1a
complex [(C5H2tBu3-1,2,4)Fe(μ-Br)2Na(DME)2] (2), a sodium bromide adduct of 1. The (C5H2tBu3-1,2,4)cent−Fe distance (1.97 Å) is in the typical range for iron(II) high-spin complexes, and the Fe−Br distances are slightly shorter than comparable distances in 1.55 The sodium bromide distances are negligibly shorter than the sum of ionic radii of sodium and bromide (2.98 Å69), respectively. The precipitation of alkali halide from filtered pentane extracts of complex 1 or its tetra- and pentaisopropylcyclopentadienyl analogues57 had been observed before and can now be explained as a consequence of alkali halide complexation.
a
The sodium bromide adduct 2 is formed in small amounts as a side product. 645
DOI: 10.1021/om501236h Organometallics 2015, 34, 644−652
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Table 1. 13C Chemical Shift Values for Bridging Alkylidyne Ligands
A pentane solution of 1 with equimolar amounts of diphenylacetylene and trisodium heptaantimonide powder gave a 63% yield of the red-black, crystalline complex 3 (Scheme 3). A similar experiment with potassium graphite as a
13
C chemical shift/ppm
formula
Scheme 3. Formation of the Bis(benzylidyne) Complex 3
μ-carbyne carbon
aryl ipso carbon
literature ref
391.7 351.3 333.9 334.6 342.3 341.7 350.6 381.7 387.6
171.7 not assigned
this work 70 71 71 71 71 71 71 71
364.9
166.4/165.4
72
341.4
166.8/165.8
72
[Cp‴Fe(μ-CPh)]2 [(tBuO)4W2(μ-CPh)2] [(tBuO)4Re2(μ-CtBu)2] [(tBuO)4Re2(μ-CCMe2Ph)2] [(F3CMe2CO)4Re2(μ-CtBu)2] [(F3CMe2CO)4Re2(μ-CCMe2Ph)2] [{(F3C)2MeCO}4Re2(μ-CtBu)2] [(tBuO)4Re2(CO)2(μ-CtBu)2] [(F3CMe2CO)4Re2(CO)2(μCtBu)2] [{(OC)3Re}(CpW)2(μ-Br)2(μCR)(μ3-CR)(μ-CO)]a [{(OC)3Re}(CpW)2(μ-Br)2(μ-CR) (μ3-CR)(μ-O)]a [{CpMo(CO)}2(μ-PCy2)(μCCOOMe)] [{CpMo(CO)}2(μ-PCy2)(μCCOOEt)] [(CpMo)2(μ-PCy2) (μ-CO)(μCCOOMe)] [(CpMo){CpMo(CO)} {(OC)3Ru}(μ-PCy2)(μ-COMe)2] [(μ-H)Fe4(μ,η2-CH)(CO)11PPh3] [Fe2(CO)6(μ-CPh)(μ-tBuPOEt)] [{CpMo(N2CPh2)} {CpMo(CO)}(μ-PCy2)(μ-CPh)] [{CpMo(CO)}2(μ-PCy2)(μ-CPh)] [{HB(pz3)W(CO)2}{(Ind) Rh(CO)}(μ-CR)]a,b [{HB(pz3)W(CO)2} {(Me3P)2Pt}(μ-CR)]a [{HB(pz3)W(CO)2}{(Me3P) Pt(CO)}(μ-CR)]a [(CpW){Fe(CO)3}2{μ3N(OH)}(μ-CO)(μ-CR)]a [(CpW){Fe(CO)3}2(μ3-NR)(μCO)(μ-CR)]a
reducing agent instead of trisodium heptaantimonide afforded a slightly lower yield of 3 (60%), and with sodium amalgam a 40% yield was obtained. 13C NMR spectra of the crude product mixtures show the signals of 3 as the main product and of 1,3,5tri(tert-butyl)cyclopentadiene as a significant byproduct. While sodium heptaantimonide or potassium graphite produced just these two compounds in approximately equimolar amounts, the reduction with sodium amalgam produced other side products as well, which could not be assigned. These side products can, however, be removed by filtration over silica gel. Proton NMR spectra show the three signals in an 18:9:2 ratio typical for the tri(tert-butyl)cyclopentadienyl ligand and small signals between 7.5 and 7.7 ppm for the phenyl protons. A characteristic 13C NMR resonance for the benzylidyne carbon atoms was found at 391.7 ppm and corresponds to a 302 ppm shift to lower field compared to the starting compound diphenylacetylene (89.4 ppm). Table 1 presents 13C NMR data for a small selection of transition metal complexes with bridging alkylidyne ligands. All of these exhibit their bridging carbyne 13C NMR signal at very low field, and most of the μ-carbyne resonances have been found between 300 and 400 ppm. Some examples even exceed this range, especially the dinuclear iron complex [Fe2(CO)6(μCPh)(μ-tBuPOEt)] with a bridging benzylidyne ligand, whose carbyne carbon signal has been found at 449 ppm.75 The ipso carbon atoms of the aryl substituents of benzylidyne and pmethylbenzylidyne bridges show chemical shift values in a range between 150 and 170 ppm and compare well with the 172 ppm value found for 3. X-ray structural analysis of 3 reveals a centrosymmetric dimer with an Fe−Fe distance of 2.3558(4) Å bridged by two benzylidyne ligands. Figure 2 shows that the phenyl ring of each benzylidyne unit is not residing in a symmetric position, but displaced toward one of the iron atoms. The short Fe−Fe distance seems to be determined by the bridges, because the related diphosphide complex [{(C5H2tBu3-1,2,4)Fe}2(μ-P)2] exhibits a 2.50 Å Fe−Fe distance.50 The results of DFT calculations show that there is no direct Fe−Fe bond, which has been discussed in the following section on DFT calculations. The Fe−centroid distance of 1.736 Å is comparable to the 1.73 Å found for the bis(phosphide) analogue.50 The 18 valence electron rule calls for an FeFe double bond. The (carbyne)C−C(ipso) bond length of 1.46 Å is approximately in the middle between a typical single bond and a bond in the sixmembered aromatic ring. The PhC carbyne ligands are slightly tilted toward the Cp ligand on account of attractive C−H···C π interactions between a tert-butyl substituent and the phenyl ring. This also seems to be a favored orientation in solution. The high field shift of one
a
402.8
49
403.3
49
361.1
49
385.2
49
333.8 448.9 373.5
153.6 165.4
74 75 73
415.3 296.6
161.8 156.8
73 76
332.0
161.5
76
322.4
158.8
76
302.1
not given
77
303.1
not given
77
R = p-C6H4Me; bInd = η5-indenyl.
tert-butyl group, whose resonance was found at 0.73 ppm, can be attributed to the anisotropy effect of the phenyl ring. Chemical shift values for the protons of tert-butyl groups in other (C5H2tBu3-1,2,4)Fe(II) complexes for comparison are between 1.1 and more than 1.5 ppm; examples are [(C5H2tBu31,2,4)Fe(μ,η 5 :η 1 -C 6 H 2 Me 3 )CuX] (X = Cl, Br) 78 and [(C5H2tBu3-1,2,4)2Fe].64 One mechanistic hypothesis assumes coordination of diphenylacetylene before the reduction takes place. There is, however, no visible change upon addition of diphenylacetylene to a solution of 1 in pentane. Presumably the steric bulk of the dinuclear complex 1 prevents its facile reaction with diphenylacetylene at room temperature. According to another hypothesis, reduction of 1 takes place first. In this case an unknown reduction product could serve as a highly reactive intermediate capable of diphenylacetylene scission. In contrast, reactions of the anionic iron naphthalene complex [(C5Me5)Fe(η4-C10H8)]− with diphenylacetylene led to the observation of iron-mediated cyclodimerization of diphenylacetylene to tetraphenylcyclobutadiene ligands.79 μ-Hydrido-μ-methylidynebis(tetraisopropylcyclopentadienylcobalt) 5. Formation of the dinuclear cobalt 646
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the solid starting compounds were mixed before a small quantity of solvent was added. The gas was, however, not identified by analytical means. The dicobalt complex 5, containing the rare combination of a methylidyne and a hydride bridge, is available in an overall yield of well above 50% from cobalt(II) chloride, sodium tetraisopropylcyclopentadienide, and methylmagnesium chloride in two steps. As 5 is too soluble in pentane for crystallization and not stable enough at temperatures above 100 °C for sublimation, we have not been able to grow single crystals, and the characterization rests mainly on NMR spectra. The tetraisopropylcyclopentadienyl ligands show the expected 6:6:6:6:2:2:1 pattern. Diastereotopic methyl positions give rise to four CH3 signals, the isopropyl CH groups account for two signals, and the ring protons account for one signal.61 Characteristic for the bridging ligands are the methylidyne hydrogen signal at low field (20.14 ppm) and the hydride signal at high field (−0.95 ppm) as well as the 13C NMR signal for the methylidyne carbon atom, which shows up at 392 ppm as a doublet with a 1JC,H coupling constant of 156 Hz. With pentamethylcyclopentadienylcobalt acetylacetonate and methyllithium a similar reaction has been observed before, which led to the trinuclear complex [{(Me5C5)Co}3(μ3-CH)(μ3-H)].81 The closely related nickel complex [{(C5HiPr4)Ni}2(μ-CH2)], obtained in a very similar reaction from the corresponding tetraisopropylcyclopentadienylnickel bromide dimer [(C5HiPr4)Ni(μ-Br)]2 and methylmagnesium chloride, has been discussed in terms of the 18 valence electron rule.82 The conversion of two methyl anions to methane, methylidyne, and hydride as seen in the reaction of 4 to 5 (Scheme 4) obviously involves oxidative addition of one C−H bond to the dicobalt core and could be rationalized as a consequence of the lower electron count of cobalt compared to nickel. Again the results of DFT calculations are not supporting the assumption of a direct cobalt−cobalt bond, as has been outlined in the following section on DFT calculations. Although the 18 valence electron (VE) rule calls for a cobalt−cobalt double bond for complex 5, there are also other cases known where the 18 VE rule did not lead to a correct bonding description.83
Figure 2. Crystal structure of the bis(benzylidyne) complex 3 with ellipsoids plotted at the 50% probability level. Selected bond distances (Å) and angles (deg): Fe1···Fe1A 2.3558(4), Fe1−C1 2.1633(14), Fe1−C2 2.1489(15), Fe1−C3 2.0908(14), Fe1−C4 2.0874(14), Fe1− C5 2.1020(14), Fe1−C6 1.8076(14), Fe1A−C6 1.7983(14), C6− C(ipso) 1.462(2), Fe1−C6−Fe1A 81.59(6), C6−Fe−C6A 98.42(6), Fe−Cp ring plane 1.736, interplanar angle Cp ring plane−Fe2C2 plane 94.5, interplanar angle Fe2C2−phenyl ring 62.3.
complex 5 with a bridging methylidyne ligand and a hydride bridge could be observed upon reaction of the chloro(tetraisopropylcyclopentadienyl)cobalt(II) [(C5HiPr4)Co(μCl)]2 (4) starting material80 with methylmagnesium chloride (Scheme 4). Gas evolution could be observed when in one run Scheme 4. Formation of the Dinuclear Methylidyne Complex 5
Figure 3. Eight occupied and one empty molecular orbital attributed to the Fe−Fe and Fe−C orbitals of σ- and π-symmetry for the bis(benzylidyne) complex 3. Top: Three bonding cyclopentadienyl−iron interactions. 647
DOI: 10.1021/om501236h Organometallics 2015, 34, 644−652
Article
Organometallics DFT Calculations. In order to gain deeper insights into the electronic and geometric structure of complexes 3 and 5, DFT calculations were performed, which had been successfully employed for calculations on related structures before.84−86 Geometry optimization of complex 3 revealed a nearly C2symmetric structure with geometric parameters in good proximity to those obtained from X-ray structure analysis. An Fe−Fe distance of 2.374 Å, distances Fe−C6 of 1.803 and 1.809 Å, angles of Fe−C6−Fe and C6−Fe−C6′ of 83.3° and 96.7°, and a torsion angle Fe−C6−Fe−C6′ of 0° are indicative of a square planar four-membered ring structure in the gas phase. A distance between the centroid of the C5 rings and Fe atoms of 1.77 Å and an Fe−Fe−centroid angle of 177° indicate a nearly linear arrangement of the Cp−Fe···Fe−Cp moieties. For an accurate description of chemical bonding in complex 3 an NBO analysis was performed.92 For complex 3 the metalcentered nonbonding and the metal−ligand bonding orbitals as well as the empty pz orbital at the benzylidyne carbon atom are depicted in Figure 3. For complex 3, covalent bonds were found for all four Fe− carbenoid (C6) interactions (Fe dxy (50%), C sp2 orbitals with NBO occupancies of ca. 1.58e and three orbitals for bonding interactions of the iron atom to the cyclopentadienyl ligands, where filled Cp orbitals donate electron density into empty d orbitals of the iron atom, Figure 3). In addition three filled nonbonding d orbitals at each iron atom were found (Figure 3). However, within the limits of the NBO program no direct Fe− Fe bond was found, which is in contrast to expectations from the Lewis structure. The NPA charges were calculated for the Fe center to be −0.48e and for the benzylidyne carbon atom to be +0.36e, which indicates a charge transfer from the carbyne carbon atoms to the iron atoms. With three lone pairs at the iron atoms the benzylidynebridged complex 3 can be viewed as a low-spin iron(II) complex. With the cyclopentadienyl ligands as monoanions, the bridging benzylidyne ligands must also be regarded as monoanionic. With their empty pπ orbital (Figure 3) they should be regarded as deprotonated phenylcarbene species with a sextet of electrons (Figure 4). Two pairs of electrons are used
Figure 5. Calculated structure of complex 5. The Co atoms (blue), carbon atoms (dark gray), and hydrogen atoms (white) are aligned to the Co−carbynoid−Co−hydrido plane. Alkyl hydrogen atoms have been omitted for clarity. Selected bond distances (Å): Co···Co 2.402, Co−C(carbyne) 1.755, Co−Cpcent 1.69, Co−H 1.62.
atoms, because the ionic radius for low-spin Co(II) is expected to be even a little bit larger than for low-spin Fe(II).87 The following analysis of the results of DFT calculations on this compound will confirm that the cobalt atoms of 5 can be regarded as Co(III). The distances to the bridging hydrido ligand Co−H are calculated to be 1.619 and 1.622 Å and are in accord with related hydride bridges reported in the literature.88−91 From the data calculated for complex 5 a similar geometry to complex 3 was found to be a minimum on the potential energy surface and in accord with NMR spectroscopic data. An NBO analysis92 revealed metal−ligand bonding orbitals for complex 5 depicted in Figure 6.
Figure 6. Metal-based NBOs for one cobalt center and the lone pair at the methylidyne ligand.
Figure 4. With three lone pairs at the low-spin iron(II) central atoms complex 3 may in a formal sense be assembled from two Cp‴Fe+ cations and two monoanionic PhC− ligands possessing only six valence electrons and resembling a deprotonated benzylidene moiety.
For complex 5 according to the 18 valence electron rule a double bond between the Co atoms is suggested. As for the Fe complex 3, from the NBO analysis three Co−Cp bonding orbitals were found as well as one Co−carbynoid σ bond. In addition, three filled d orbitals were located at each Co center with lone pair character, suggesting a d6 electron configuration and, thus, a Co(III) oxidation state. The NPA charges calculated for Co (−0.26e), C (−0.08e), and H (0.11e) indicate a smaller charge transfer from the carbynoid ligand to the metal center. Also, the small positive charge on the bridging hydrido ligand points toward a covalent three-center−two-electron bond.81 For complex 5 the calculation again provided no
for benzylidyne C−Fe bonds to both iron atoms. This interpretation leaves the iron atoms with only 16 valence electrons and the benzylidyne carbon atoms with electron sextets and is supported by the calculations. The geometry optimization of the cobalt complex 5 revealed the structure depicted in Figure 5. A distance Co−Co of 2.402 Å is calculated to be slightly longer than the Fe−Fe distance for complex 3. The distances Co−C6 of 1.755 Å and Co−Cp (centroid) of 1.69 Å are calculated to be shorter than the corresponding distances in complex 3 (1.736 Å), which is an indication for Co(III) central 648
DOI: 10.1021/om501236h Organometallics 2015, 34, 644−652
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Two of these engage in Co−C bonds, while the lone pair in the π orbital participates in a three-center interaction with both cobalt atoms similar to the bridging hydride. The formation of the methylidyne complex 5 from [4CpCo(μ-Cl)]2 and methylmagnesium chloride accounts for the formation of a dianionic methylene ligand CH22− like the one found in the nickel complex [{4CpNi}2(μ-CH2)].82 It appears to be generated according to the equation 2CH3− → CH22− + CH4. The transformation of CH22− to one hydride anion and a methylidyne trianion requires two electrons, which are taken from the cobalt atoms in an oxidative addition reaction: CH22− + 2e− → CH3− + H−.
evidence for a direct metal−metal bond between the cobalt atoms within the limits of the NBO program. Second-order perturbation analysis92 revealed a strong donor−acceptor three-center interaction between the two cobalt atoms via the methylidyne bridge. Thus, a delocalized Co−Co interaction is suggested.81 The reason for the lack of a direct metal−metal bond can be illustrated by analyzing the molecular orbitals (MOs) of complex 5, which are depicted in Figure 7. The σ- and the πbonding orbital contribution is compensated by filled Co−Co antibonding orbitals in close energetic proximity. Thus, direct Co−Co bonding is negligible.
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CONCLUSION The structure of the sodium bromide adduct 2 of the cyclopentadienyliron bromide 1 illustrates the difference between closed-shell organometallics with 18 valence electrons, which are not expected to add halide, and open-shell complexes with a central ion in its high spin state. The latter are characterized by weaker and longer bonds more ionic in character than bonds in typical organometallic 18 VE complexes. In the presence of the reducing agents sodium amalgam and potassium graphite, but even better with trisodium heptaantimonide, the dinuclear iron bromide derivative [(C5H2tBu31,2,4)Fe(μ-Br)]2 (1) cleaves diphenylacetylene in two benzylidyne bridges and forms the diamagnetic Fe(II) lowspin complex [(C5H2tBu3-1,2,4)Fe(μ-CPh)]2 (3). Although the 18 VE rule and the short Fe−Fe distance of 2.3558(4) Å support the assumption of an FeFe double bond, DFT calculations provide no evidence for such a bond. The benzylidyne ligands are covalently bound to the iron atoms and may be regarded as monoanionic PhC− moieties possessing only six valence electrons and an empty π orbital on the benzylidyne carbon atom. The related dicobalt starting compound [(C5HiPr4)Co(μ-Cl)]2 (4) converts two equivalents of methyl anions supplied as methylmagnesium chloride into methane, methylidyne, and hydride. The two latter species are found as bridging ligands in the dinuclear complex [{(C5HiPr4)Co}2(μ-CH)(μ-H)] (5). The methylidyne ligand, although covalently bound to both cobalt(III) central atoms, can be described in a formal sense as a methylidyne trianion CH3−, which is not only donating one electron pair to each cobalt(II) atom but also engaged in three-center bonding with both cobalt atoms. An 18 valence electron count results for both cobalt atoms from the cyclopentadienyl anion as a 6π donor, Co(III) with a d6 configuration and participation of each cobalt atom in three additional metal−ligand bonds: one twocenter−two-electron bond and two three-center−two-electron bonds. Complexes 3 and 5 with their planar core and electronic unsaturation may be viewed as kinetically stabilized models for highly reactive intermediates of cluster formation reactions. The bulky alkylcyclopentadienyl ligands prevent agglomeration to complexes of higher nuclearity. It will be interesting to proceed toward even more steric bulk sufficient to hinder dinuclear complex formation and to generate monomeric half-sandwich complexes in order to find out more about the mechanism of the methyl transformation observed.
Figure 7. Occupied molecular orbitals attributed to the Co−Co orbitals of σ- and π-symmetry.
According to the calculations on cobalt complex 5, the methylidyne carbon atom possesses a lone pair in an orbital of pz character that is empty in the iron complex 3. Thus, the carbyne ligand in complex 5 formally carries three negative charges and in complex 3 only one. This is supported by the NPA analyses revealing a more negative charge at the alkylidyne carbon atom (Δ = −0.4e) for complex 5, indicating a shift of electron density from the metal atom to the ligand. However, the total number of electrons calculated for the alkylidyne carbon atom does not point toward a highly ionic interaction. Due to the strongly covalent and delocalized character of the metal−carbon interaction, the electron density is well balanced between the metal and carbon atoms. The cobalt atoms possess three lone pairs just like the iron atoms of complex 3. With the cyclopentadienyl ligands and the bridging hydride as monoanions, the bridging methylidyne ligand can be regarded as a trianion. In contrast to the monoanionic benzylidyne ligand of complex 3 this methylidyne ligand carries three pairs of electrons besides the C−H bond.
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Figure 8. With its Co(III) central atoms the methylidyne ligand of complex 5 has to be regarded as a trianionic CH unit, which donates one electron pair to each cobalt atom. The lone pair in a p orbital of the methylidyne carbon atom perpendicular to the Co2C plane participates in a three-center−two-electron bond just like the electron pair of the hydride bridge.
EXPERIMENTAL PROCEDURES
Synthetic work has been carried out under inert gas in rigorously dried and deoxygenated solvents using Schlenk line techniques for the preparation of alkali cyclopentadienide salts and an argon-filled 649
DOI: 10.1021/om501236h Organometallics 2015, 34, 644−652
Article
Organometallics
which was filtered over silica gel (3 cm in diameter, 2 cm thick). Evaporation of the deep red solution yielded a dark red solid (117 mg) containing complex 3 (0.123 mmol, 63%) and 1,3,5-tri(tert-butyl)cyclopentadiene HCp‴ (0.1 mmol, 25%), according to the integral ratio in the proton NMR spectra. The product was recrystallized from pentane at −30 °C. Anal. Calcd for C48H68Fe2: C, 76.18; H, 9.06. Found: C, 76.09; H, 9.16. 1H NMR (400 MHz, 298 K, C6D6): δ = 8.44 (d, 4H, o-CH, 3JH,H = 7.8 Hz), 7.59 (t, 4H, m-CH, 3JH,H = 7.5 Hz), 7.40 (t, 2H, p-CH, 3JH,H = 7.5 Hz), 5.72 (s, 4H, (C5H2tBu3-1,2,4) ring H), 1.00 (s, 36H, CH3, tBu groups in 1- and 2-position), 0.65 (s, 18H, CH3, 4-tBu). 13C {1H}NMR (100 MHz, 298 K, C6D6): δ = 391.7 (2C, μ-C), 171.7 (2C, ipso-C), 126.7 (4C, m- or o-C), 125.5 (4C, o- or mC), 123.6 (2C, p-C), 107.3 (4C, Cp‴, C1, C2), 105.2 (2C, Cp‴, C4), 90.9 (4C, Cp2‴, C3, C5), 33.7 (16C, overlap of CH3 and CMe3 signals for tBu groups in 1,1′,2, and 2′ position), 32.6 (6C, CH3 for 4- and 4′-tBu), 32.5 (2C, CMe3 for 4- and 4′-tBu). μ - H yd r id e -μ - m e t h y l i d y n e b i s { t e t ra is opro py lcy c lopentadienylcobalt}, 5. To a black solution of bis{chloro(tetraisopropylcyclopentadienyl)cobalt(II)} (4, 350 mg, 0.53 mmol) in tetrahydrofuran (20 mL) was added at once solid methylmagnesium chloride diethyl ether adduct (162 mg, 1.09 mmol). The turbid mixture was stirred at ambient temperature for 14 h; then the solvent was removed in vacuo. Extraction of the black-brown residue with petroleum ether, centrifugation, and evaporation of the brown-black organic solution gave a sticky, black residue, which was extremely soluble in pentane. The tarry solid was washed several times with small portions of acetonitrile (combined volume 10 mL) and isolated as a dry powder after removal of residual solvent in vacuo (256 mg, 0.43 mmol, 81%). Anal. Calcd for C35H60Co2: C, 70.21; H, 10.10. Found: C, 69.33; H, 10.26. 1H NMR (400 MHz, 298 K, C6D6): δ = 20.14 (s, 1H, CH bridge), 4.95 (s, 2H, C5iPr4H, ring H), 2.88 (br, 4H, CHMe2), 2.29 (br, 4H, CHMe2) 1.36 (d, 12H, 3JH,H 7.0 Hz, CH3), 1.32 (d, 12H, 3 JH,H 7.0 Hz, CH3), 1.13 (d, 12H, 3JH,H 6.6 Hz, CH3), 1.03 (d, 12H, 3 JH,H 6.9 Hz, CH3). −0.95 (s, 1H, Co-H-Co). 13C NMR (100 MHz, 298 K, C6D6): δ = 392.0 (d, 1C, 1JC,H 156 Hz, μ-CH), 104.7 (4C, ring CiPr), 103.5 (4C, ring CiPr), 72.6 (d, 2C, ring-CH, 1JC,H 168 Hz), 27.1 (q, 4C, 1JC,H 125 Hz, CH3), 25.9 (q, 4C, 1JC,H 125 Hz, CH3), 24.6 (q, 4C, 1JC,H 126 Hz, CH3), 23.4 (q, 4C, 1JC,H 126 Hz, CH3). EI-MS: m/z (%) found 598 (100) M+, 234 (3) [C17H30]+, 233 (3) [C17H29]+, 191 (4) [C14H23]+, 149 (5.5) [C11H17]+, 107 (11) [C8H11]+, 43 (27.5) [C3H7]+. Computational Details. Geometry optimizations of all complexes were performed at the unrestricted DFT level of theory employing the B3LYP hybrid functional.94 For all atoms the def2-TZVP basis set was used.95 For all calculations the program system Gaussian0996 was used with the NBO 3.1 module92 embedded within the program. All stationary points were checked by frequency calculations, revealing no imaginary frequency for minima.
glovebox from MBraun company, Garching, for preparation and handling of the iron complexes. Solvents have been dried under inert gas with molten alkali metals such as potassium (tetrahydrofuran) or sodium/potassium alloy (diethyl ether, petroleum ether, pentane) and distilled prior to use. NMR spectra were obtained on a Bruker DPX 400 NMR spectrometer and referenced to the residual solvent signals: Chemical shifts δ are given in ppm, coupling constants J in Hz. Mass spectra have been recorded by Prof. G. Hornung with a Finnigan MAT 90 mass spectrometer. Elemental analyses have been carried out in the analytical laboratory of the chemistry department of the TU Kaiserslautern. 1,3,5-Tri(tert-butyl)cyclopentadiene was prepared according to ref 93 and metalated to the sodium salt with sodium amide in boiling tetrahydrofuran. For the determination of the magnetic moment of 1 a solution with known concentration of 1 in benzene and a sealed capillary containing benzene were combined in an NMR tube. From the chemical shift difference between the two benzene signals obtained at 298 K the magnetic moment of complex 1 was derived according to Sur67 and Evans.68 Trisodium Heptaantimonide. Caution: A well-ventilated hood is essential for this procedure in order to avoid exposure to ammonia. A three-necked round-bottomed 500 mL flask equipped with a stir bar, dry ice condenser, and two gas inlets for inert gas and ammonia was evacuated while being warmed with a heat gun. The flask was allowed to cool to room temperature and filled with nitrogen. A small piece of sodium (0.45 g, 19.6 mmol) was placed in the flask, and the condenser was filled with dry ice and acetone. Ammonia (130 mL) was condensed into the flask and stirred with the sodium to yield a deep blue solution of sodium in ammonia. The ammonia was then condensed into a second 500 mL flask with the same equipment charged with antimony powder (40.0 g, 328 mmol). Sodium pieces (3.20 g, 139 mmol) were introduced under inert gas within a few minutes with magnetic stirring of the mixture, which was continued for 4 h. The ammonia was allowed to evaporate, and the residue was suspended in toluene (250 mL). The mixture was stirred and heated under reflux for 2 h, then allowed to cool to room temperature. The solvent was decanted, and the residue was washed with pentane (2 × 100 mL), dried in vacuo, isolated as a dark gray powder (41.0 g, 43.4 mmol, 94%), and used as a reducing agent. Bis{(μ-bromo)tri-tert-butylcyclopentadienyliron}, 1. To a stirred suspension of very light orange, almost colorless iron(II)bromide dimethoxyethane complex (917 mg, 3.0 mmol) in dimethoxyethane (40 mL) cooled to −30 °C was added via cannula a solution of sodium tri(tert-butyl)cyclopentadienide (769 mg, 3.0 mmol) in dimethoxyethane (40 mL). Stirring at −30 °C was continued for another 60 min; then the dark green reaction mixture was allowed to thaw to ambient temperature. The solvent was removed in vacuo, and the residue was extracted with pentane (10 mL). After centrifugation the red solution was evaporated to dryness to yield 775 mg (1.05 mmol, 70%) of an orange solid. Anal. Calcd for C34H58Br2Fe2: C, 55.31; H, 7.92. Found: C, 54.70; H, 7.95. 1H NMR (400 MHz, 298 K, C6D6): δ 47.2 (4H, Δν1/2 974 Hz, ring H), −7.5 (broad, CH3), −12.6 (broad, CH3). The two high-field signals are partially superimposed with a combined integral corresponding to the 54 protons of the six tert-butyl groups. The signal at −7.5 ppm represents the four tert-butyl groups in 1,2,1′,2′-position, and the high field signal represents the two tert-butyl groups in 4- and 4′-position. EI-MS: m/z (%) found 738.1 (
