Reactive Sigma-Aryliron Complexes or Iron-Promoted Coupling of

Jan 21, 2010 - Synopsis. Arylmagnesium bromides react with the high-spin cyclopentadienyliron(II) bromide 1a with carbon−carbon coupling to the dinu...
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Organometallics 2010, 29, 806–813 DOI: 10.1021/om900870j

Reactive Sigma-Aryliron Complexes or Iron-Promoted Coupling of Two Phenyl Anions to One Bis(cyclohexadienylidene) Ligand: Synthesis, Structure, Mass Spectrometry, and DFT Calculations Mark W. Wallasch, Daniel Weismann, Christoph Riehn, Stefan Ambrus, Gotthelf Wolmersh€ auser, Anita Lagutschenkov, Gereon Niedner-Schatteburg,# and Helmut Sitzmann* FB Chemie der TU Kaiserslautern, Erwin-Schr€ odinger-Strasse 54, D-67663 Kaiserslautern, Germany. # FB Chemie der TU Kaiserslautern and Landesforschungszentrum OPTIMAS. Received October 7, 2009

A reaction of the bulky alkylcyclopentadienyliron(II) high-spin complex [Cp000 Fe(μ-Br)]2 (1a) (Cp000= C5H2(CMe3)3-1,2,4) with phenylmagnesium bromide produced the deep blue dinuclear complex [{Cp000 Fe}2(μ,η5:η5-H5C6dC6H5)] (2) with a bridging bis(cyclohexadienylidene) ligand. Its structural analysis shows a centrosymmetric dimer. Each tri(tert-butyl)cyclopentadienyliron fragment is η5coordinated to a cyclohexadienylidene moiety in which one carbon atom is bent out of the plane by 0.39 A˚, exhibiting a bond length of 1.370 A˚ to its symmetry equivalent. Electrospray ionization mass spectra (ESI-MS) from acetonitrile solution confirm nicely the elemental composition of 2 by way of their isotope patterns. Reaction of 1a or its tetraisopropylcyclopentadienyl analogue [4CpFe(μ-Br)]2 (1b) (4CpdC5H(CHMe2)4) with 2,6-diisopropylphenylmagnesium bromide affords the extremely air-sensitive, paramagnetic σ-aryl complexes [Cp000 Fe(C6H3iPr2)] (3a) or [4CpFe(C6H3iPr2)] (3b), whose 4Cp-Fe distance of 1.92 A˚ is typical for cyclopentadienyliron high-spin complexes. In reactions with copper(I) halides 3a is rearranged to a diamagnetic π complex and coordinated via the ipso carbon atom of the six-membered ring to copper(I) halide fragments to form heterodinuclear complexes [Cp000 Fe(μ,η5:η1-C6H3iPr2)CuCl] (4-Cl) and [Cp000 Fe( μ,η5:η1-C6H3iPr2)CuBr] (4-Br). ESI mass spectra of complexes 4 do not show the molecular cations, but fragmentation to cyclopentadienyliron arene cations and formation of the hexa(tert-butyl)ferrocenium cation on one hand and fusion of complex fragments to oligonuclear complexes with or without inclusion of oxygen or fragments of solvent molecules on the other hand. Three of these oligonuclear complexes formed under the conditions of the ESI-MS experiment, whose elemental composition could be derived from isotope patterns, have been interpreted as [Cp000 Fe(μ,η5:η1-C6H3iPr2)Cu(μ,η1:η5-OC6H3iPr2)FeCp000 ]þ and [{Cp000 Fe(μ,η5:η1-C6H3iPr2)Cu}2X]þ (X=Cl, Br). DFT calculations support the structural analysis of 2 and predict the structure of the dication 22þ. The crystal structures obtained by X-ray diffraction for 2 and 3b are reported.

Introduction In a recent publication a reaction of the half-sandwich iron(II) bromide [Cp000 Fe(μ-Br)]2 with phenylmagnesium bromide was briefly mentioned, and the product from a reaction of the same starting compound with mesitylmagnesium bromide was described as a σ aryl complex [Cp000 Fe(C6H2Me3-2,4,6)] with four unpaired electrons. This finding was based on proton NMR spectra, a magnetic moment of 5.50 μB, and a weak set of X-ray data, which could not be refined.1 With copper(I) chloride the novel σ mesityl complex rearranged to the dinuclear π complex [Cp000 Fe(μ-C6H2Me32,4,6)CuCl]. For the hypothetical unsubstituted derivative [CpFe(μ-C6H5)CuCl] the bridging aryl ligand has been

modeled by DFT calculations to possess 88% aryl and 12% cyclohexadienyl-ylidene character.2 We are interested in the reactivity of high-spin cyclopentadienyliron complexes in general and in the behavior of the novel type of σ aryl complexes, whose hypothetical σ/π rearrangement products carry about 30% carbene character based on DFT calculations.2 In this publication full details on the preparation and characterization of the coupling product [(Cp000 Fe)2(C12H10)] (2, Cp000 =1,2,4-tri(tert-butyl)cyclopentadienyl) obtained from [Cp000 Fe(μ-Br)]2 and phenylmagnesium bromide will be given, including a crystal structure determination, ESI mass spectra, and DFT calculations. Furthermore, with [4CpFeC6H3(CHMe2)2-2,6] (3b, 4Cp=tetraisopropylcyclopentadienyl), with

*Corresponding author. E-mail: [email protected]. (1) Wallasch, M. W.; Rudolphi, F.; Wolmersh€auser, G.; Sitzmann, H. Z. Naturforsch. 2009, 64b, 11–17.

(2) Wallasch, M. W.; Vollmer, G. Y.; Kafiyatullina, A.; Wolmersh€auser, G.; Jones, P. G.; Mang, M.; Meyer, W.; Sitzmann, H. Z. Naturforsch. 2009, 64b, 18–24.

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a σ-diisopropylphenyl ligand, the second example of a σ aryl cyclopentadienyliron complex will be presented along with a well-resolved crystal structure analysis. The copper(I) bromide complex [Cp000 Fe(μ,η5:η1-C6H3(CHMe2)2-2,6)CuBr] (4) obtained from [Cp000 FeC6H3(CHMe2)2-2,6] (3a) and copper(I) bromide reacts under the conditions of ESI mass spectrometry with fragmentation, but also with agglomeration to form complexes of higher nuclearity, as will be shown in the following section.

Results and Discussion Synthesis, X-ray Analysis, and ESI Mass Spectrometry of Complex 2. When one equivalent of phenylmagnesium bromide was added to a solution of tri(tert-butyl)cyclopentadienyliron(II) bromide3 [{Cp000 Fe(μ-Br)}2] (1a) in tetrahydrofuran, the mixture quickly turned light green and developed a deep blue color within two hours at room temperature. Dark blue blocks of the dinuclear complex [{Cp000 Fe}2(μ,η5:η5-H5C6dC6H5)] (2) could be obtained from pentane in good yield. The solid compound can be handled briefly in air, but turns brown and starts to decompose after several minutes in air. In an inert atmosphere it can be stored at room temperature. Its pentamethylcyclopentadienyl (Cp*) analogue [{Cp*Fe}2(μ,η5:η5-H5C6dC6H5)] was obtained by two-electron reduction of the dicationic biphenyl complex [{Cp*Fe}2(μ,η6:η6-H5C6-C6H5)]2þ.4,5 Characteristic 1H NMR signals of 2 are due to the hydrogen atoms of the coupled six-membered rings in ortho (2.87 ppm), meta (4.46 ppm), and para (6.29 ppm) position, which compare well with the signals (2.6, 4.1, and 5.6 ppm) reported for the Cp*Fe derivative [{Cp*Fe}2(μ,η5:η5-H5C6dC6H5)].5 In the 13 C NMR spectra the signals of the ipso carbon atoms of the six-membered rings are found at 105.4 ppm, and those of the ortho carbon atoms at 47.2 ppm. More data can be found in the Experimental Section. The dinuclear molecule resides on a crystallographic inversion center and exhibits a typical CdC double bond length of 1.37 A˚ between the ipso carbon atoms of the two six-membered rings, which are bent out of their respective ring plane by 0.55 A˚. Due to the different ring sizes, the distance of the iron atom to the dienyl plane (1.58 A˚) is shorter than the iron-Cp000 distance (1.70 A˚), as expected. The conformation of the molecule with pairs of adjacent tert-butyl substituents symmetrically facing the CdC double bond between the sixmembered rings can be seen in Figure 1, and the fold angle along the line C6 3 3 3 C11 is 27.1°, in good agreement with the 25° angle of the Cp* derivative.5 Coupling of two phenyl anions to a bis(cyclohexadienylidene) ligand is a process rarely encountered in transition metal chemistry, but has been documented for yttrium6 and holmium complexes.7 ESI mass spectra of 2 exhibit three signals originating from iron-containing species, as could be concluded from the (3) (a) Wallasch, M.; Wolmersh€auser, G.; Sitzmann, H. Angew. Chem. 2005, 117, 2653–2655. (b) Angew. Chem., Int. Ed. 2005, 44, 2597-2599. (4) Raba^ a, H.; Lacoste, M.; Delville-Desbois, M.-H.; Ruiz, J.; Gloaguen, B.; Ardoin, N.; Astruc, D.; Le Beuze, A.; Saillard, J.-Y.; Linares, J.; Varret, F.; Dance, J.-M.; Marquestaudt, E. Organometallics 1995, 14, 5078–5092. (5) Lacoste, M.; Raba^a, H.; Astruc, D.; Ardoin, N.; Varret, F.; A.; Saillard, J.-Y.; Le Beuze, A. J. Am. Chem. Soc. 1990, 112, 9548–9557. (6) Fryzuk, M. D.; Love, J. B.; Rettig, S. J. J. Am. Chem. Soc. 1997, 119, 9071–9072. (7) Fryzuk, M. D.; Jafarpour, L.; Kerton, F.; Love, J. B.; Patrick, B.; Rettig, S. J. Organometallics 2001, 20, 1387–1396.

Figure 1. Crystal structure of the bis(cyclohexadienyl-ylidene) complex 2. Selected distances/A˚: Fe1-C1 2.101(3), Fe1-C2 2.099(3), Fe1-C3 2.058(3), Fe1-C4 2.097(3), Fe1-C5 2.064(3), Fe1-C6 2.148(3), Fe1-C7 2.044(4), Fe1-C8 2.029(3), Fe1-C9 2.041(3), Fe1-C10 2.138(3), Fe1 3 3 3 C11 2.537(4), Fe-Cp ring plane (C1-C5) 1.70, Fe-dienyl plane (C6-C10) 1.58, C6-C7 1.397(5), C7-C8 1.400(5), C8-C9 1.372(5), C9-C10 1.403(5), C10-C11 1.442(5), C11-C6 1.450(5), C11-C110 1.370(6), C11-dienyl plane (C6-C10) 0.548, fold angle along C6 3 3 3 C11 27.1°. For details on the crystal structure determinations see Table 1. Table 1. Details on the Crystal Structure Determination of Complexes 2 and 3b

formula fw/g mol-1 crystal size/mm space group unit cell a/A˚ b/A˚ c/A˚ R/deg β/deg γ/deg V/A˚3 Z T (K) Dcal./g cm-3 μ/cm-1 transmission theta limits/deg reflns unique reflns solution program refinement data/restraints/params R1 wR2 (all data) GooF (all data) max./min. diff peak/e A˚-3

2

3b

C46H68Fe2 732.40 0.40  0.37  0.09 P1 triclinic 9.0560(11) 9.8081(12) 12.6486(18) 103.924(15) 91.979(16) 110.895(15) 1009.7(2) 1 293(2) 1.205 7.48 0.75217-0.93498 2.76-25.68 14 182 3582 direct methods SHELXS-97 SHELXL-97 43582/0/226 0.0420 0.0995 0.861 0.351, -0.276

C29H46Fe 450.51 0.24  0.33  0.08 P1 triclinic 8.5442(13) 9.047(2) 19.018(4) 95.835(18) 100.718(15) 107.787(17) 1355.6(5) 2 150(2) 1.104 11.04 0.0258-0.9559 2.81-33.38 20 312 9508 direct methods SIR97 SHELXL-97 9508/0/283 0.0371 0.0868 0.996 0.429, -0.269

almost perfect coincidence of the experimentally determined isotope pattern with theoretical expectation (Figure 2). The parent signal was detected at an m/z ratio of 366.2 and corresponds to the dication 22þ derived from 2 by ionization of neutral complex 2 either in acetonitrile solution by traces of air or during the electrospray process. The corresponding monocation 2þ was not detected. The signal

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Figure 2. ESI mass spectrum of 2. Comparison of experiment and simulation of isotope patterns. (a) Overview spectrum. Experiment (black) and simulation for three peaks (colored). (b) Enlarged region of mass peaks assigned to 22þ. Experiment (black) and simulation (red). Spectra obtained with an ESI-FT-ICR mass spectrometer (see Experimental Section).

at m/z 443.2 is most probably due to loss of an undetected Cp000 Feþ cation from 22þ, and the m/z 522.4 peak has been assigned to the hexa(tert-butyl)ferrocenium cation [Cp000 2Fe]þ, which is probably formed under the conditions of the ESI-MS experiment. It is interesting to note that the two fragment peaks at m/z 443.2 and 522.4 appeared also as the strongest and second strongest peak in the electron impact mass spectrum. DFT Calculations on Complex 2 and the Cations 2þ and 2þ 2 . In order to gain insight into the structural changes upon ionization, we have investigated the structure of compound 2 as well as those of the related monocation 2þ and dication 22þ by ab initio calculations on the DFT level. Using the Gaussian 03 program package chemical structure optimizations (8) Wachters, J. H. J. Chem. Phys. 1970, 52, 1033.

have been performed at the B3LYP/6-311G (Fe) and B3LYP/ cc-pVDZ levels of theory as suggested in refs 8-10. The NBO analysis was implemented similarly to the analysis provided by Meyer and Mang.2 Moreover the calculations have also been performed with the B3LYP/SDD basis sets similar to ref 11. The detailed results of the calculations are given in the Supporting Information. Here we would like to point out only the salient features of the structures and the changes imposed by the increased positive charge. The structural results of the DFT calculations are compared in Table 2 with data from X-ray diffraction. Although (9) P. J. Hay, P. J. J. Chem. Phys. 1977, 66, 4377. (10) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007. (11) Kirgan, R. A.; Rillema, D. P. J. Phys. Chem. A 2007, 111, 13157– 13162.

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Table 2. Comparison of Structural Data for 2, 2þ, and 22þ from X-ray Analysis and DFT Calculations cation 2þ

dication 22þ

DFT B3LYP/6-311G, cc-pVDZ (B3LYP/SDD)

DFT B3LYP/6-311G, cc-pVDZ (B3LYP/SDD)

DFT B3LYP/6-311G, cc-pVDZ (B3LYP/SDD)

2.103 (2.136) 2.146 (2.177) 2.070 (2.099) 2.065 (2.095) 2.071 (2.105) 2.146 (2.183) 2.590 (2.604) 1.69 (1.75) 1.60 (1.65) 1.413 (1.422) 1.424 (1.436) 1.422 (1.435) 1.414 (1.422) 1.470 (1.478) 1.470 (1.477) 1.367 (1.373)

2.116 (2.150) 2.146 (2.186) 2.096 (2.138) 2.092 (2.133) 2.107 (2.153) 2.160 (2.206) 2.370 (2.401) 1.71 (1.83) 1.60 (1.65) 1.412 (1.420) 1.416 (1.428) 1.417 (1.429) 1.410 (1.418) 1.442 (1.450) 1.442 (1.451) 1.433 (1.435) 1.49a

2.103 (2.125) 2.124 (2.157) 2.115 (2.150) 2.130 (2.168) 2.127 (2.162) 2.133 (2.166) 2.183 (2.213) 1.75 (1.72) 1.60 (1.63) 1.414 (1.424) 1.412 (1.424) 1.411 (1.423) 1.414 (1.422) 1.422 (1.432) 1.425 (1.432) 1.498 (1.495)

33.6 (30.2)

13.5 (12.7) 5.5a

3.2(2.2)

neutral complex 2 distances/A˚ and angles/deg

X-ray

Fe1-CX (X = 1-5) average Fe1-C6 Fe1-C7 Fe1-C8 Fe1-C9 Fe1-C10 Fe1-C11 Fe-Cp ring plane C1-C5 Fe-dienyl plane C6-C10 C6-C7 C7-C8 C8-C9 C9-C10 C10-C11 C6-C11 C11-C110 C11-dienyl plane (C6-C10) fold angle between C6-C10-C11 and dienyl plane

2.084 2.148(3) 2.044(4) 2.029(3) 2.041(3) 2.138(3) 2.537(4) 1.70 1.58 1.397(5) 1.400(5) 1.372(5) 1.403(5) 1.442(5) 1.450(5) 1.370(6) 1.37a 0.548 27.1 25a

a

Data from the X-ray analysis of [Fe2(μ2,η10-biphenyl)Cp2]nþ (n = 0, 1) taken from lit.12 and references therein.

Table 3. Development of NBO Charge Distribution As a Function of Overall Charge Statea

tri(tert-butyl)cyclopentadienyl metal center half biphenyl complete biphenyl half biphenyl0 metal center0 tri(tert-butyl)cyclopentadienyl0

C17H29 Fe C6H5 C12H10 C6H50 Fe0 C17H290

2

2þ

22þ

-0.37 þ0.84 -0.47 -0.94 -0.47 þ0.84 -0.37

-0.09 þ0.62 -0.03 -0.06 -0.03 þ0.62 -0.09

þ0.13 þ0.87 0.00 0.00 -0.01 þ0.87 þ0.12

a Point charges are given in units of electron charge e. All structures were fully optimized at the B3LYP level of theory, using the 6-311G basis on Fe and cc-pVDZ on all other atoms.

the calculated absolute bond lengths for the neutral complex 2 differ from the X-ray data by þ2% on average, the overall structure is described very well. In particular, the doublebond character of C11-C110 , the relatively long Fe-C11 length (indicating η5-coordination of the six-membered ring), and the fold angle of ca. 25-30° are nicely reproduced. These features develop upon increase of the overall charge (þ1, þ2) in the following way: the distances of Fe to the C atoms of the six-membered rings equalize (η6-coordination), the carbon-carbon bond between the two six-membered rings is elongated towards a single bond, and the fold angle is strongly reduced to approximately 5°. The increasing biphenyl character of the bridging ligand is also reflected in the change of the charge distribution as obtained by NBO analysis of the optimized structures (Table 3). A note of caution is mandatory for the interpretation of the results of such an arbitrary scheme for the partition of the total electron density. The calculated partial charges reflect neither an experimentally observable quantity nor formal charges or oxidation states of chemical reasoning. As a result of the NBO analysis, the neutral 2 contains one extra elemental charge in the bridging ligand equally distributed over the two rings. This charge is reduced nearly to zero for 22þ, whereas the positive charge on the iron atoms remains the same. The intermediate state 2þ, which we have not observed here experimentally, already displays an almost neutral bridging ligand of biphenyl character, its positive

charge being delocalized over both iron centers. Our computational results (structure, partial charges) for 2 compare very well with the results obtained in ref 2 on a compound closely related to 4-Cl. Moreover, our results agree also quite well with the X-ray structural results obtained by Astruc and co-workers5,12 for similar compounds. The model of stepwise electron transfer accompanied by a structural rearrangement between 2 and 2þ, as put forward by Astruc et al.5 based on cyclic voltammetry, X-ray, and M€ ossbauer data, is clearly confirmed by our calculations. Synthesis, X-ray Analysis, and ESI Mass Spectrometry of Complexes 3 and 4. In order to suppress carbon-carbon coupling during the reaction of the aryl Grignard reagent with the dinuclear iron bromide complex 1a and in order to sterically protect the envisaged reaction product, 2,6-diisopropylphenylmagnesium bromide was employed. The paramagnetic σ aryl complex [Cp000 Fe(C6H3iPr2-2,6)] (3a) resulting from this reaction is extremely sensitive, like the corresponding mesityl derivative [Cp000 Fe(C6H2Me3-2,4,6)] described recently.1 During the isolation procedure decomposition is already visible as a darkening process of the brownish-orange precipitate, which looks crystalline in the beginning and quickly turns sticky. For this reason 3a could not be isolated nor analyzed. NMR spectra of the paramagnetic material showed contamination with diamagnetic impurities probably arising from the decomposition process. The same problems were encountered with the tetraisopropylcyclopentadienyl derivative [4CpFe(C6H3iPr2-2,6)] (3b) obtained from bromide [{4CpFe(μ-Br)}2] (1b).13 3b was somewhat more stable than 3a, showing less, but still significant contamination with diamagnetic material in the 1H NMR spectrum. 3b could, however, be crystallized from pentane solution and characterized by an X-ray crystal structure analysis at low temperature. (12) Astruc, D. Acc. Chem. Res. 1997, 30, 383–391. (13) (a) Sitzmann, H.; Dezember, T.; Kaim, W.; Baumann, F.; Stalke, D.; K€archer, J.; Dormann, E.; Winter, H.; Wachter, C.; Kelemen, M. Angew. Chem. 1996, 108, 3013–3016. (b) Angew. Chem., Int. Ed. Engl. 1996, 35, 2872-2874.

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Figure 3. Crystal structure of the σ-aryl complex 3b. Selected distances/A˚ and angles/deg: Fe1-C1 2.2496(13), Fe1-C2 2.2781(12), Fe1-C3 2.2866(13), Fe1-C4 2.2711(13), Fe1-C5 2.2688(13), Fe1-C60 2.0317(13), Fe-Cp ring plane (C1-C5) 1.915, angle between cyclopentadienyl ring plane and line Fe1-C60 84.3, angle between two ring planes 89.8. For details on the structure determination see Table 1.

3b is a rare example of an iron half-sandwich complex with pogo stick structure.14 The distance between the iron atom and the cyclopentadienyl ring plane of 1.915 A˚ is almost identical with the 1.92 A˚ found for 1a and indicates a highspin electron configuration at the iron center, as has been shown by magnetic susceptibility measurements on the mesityl complex [Cp000 Fe(C6H2Me3-2,4,6)]1 and on 1a.3 The 2.032 A˚ bond length Fe1-C60 is significantly shorter than the iron-carbon bonds in the high-spin complex [Fe(1-naphthyl)4]2- (2.147(7) and 2.104(7) A˚).15 This relatively short bond approaches the bond length of 1.979 A˚ found for the diamagnetic cycloheptatrienylidene complex [CpFe(CO)2(C7H6)]þ,16 where a substituted tropylium ion or an iron-carbene system may be discussed. The conformation of the molecule seen in Figure 3 places one isopropyl substituent of the aryl group underneath the substitution gap of the tetraisopropylcyclopentadienyl ligand. The second alkyl is accommodated between two adjacent isopropyl substituents of the cyclopentadienyl ligand. The bond Fe-C60 extends at an angle of 84.3° against the cyclopentadienyl plane. By bending out of a perpendicular direction steric strain is reduced so much that the two nearby isopropyl substituents in 2- and 3-position of the cyclopentadienyl ligand are not even rotated away from each other, as has been found in other cases like the dinuclear carbonyl complexes [{4CpM(μ-CO)}2] (M=Co17 or Ni18). The σ aryl complex 3a was reacted with copper(I) halides to furnish good yields of the corresponding copper complexes [Cp000 Fe(η5:η1-C6H3iPr2-2,6)CuX] (X = Cl, 4-Cl; X = Br, 4-Br). The corresponding mesityl derivatives of (14) Siemeling, U.; Vorfeld, U.; Neumann, B.; Stammler, H.-G. Organometallics 1998, 17, 483. (15) Bazhenova, T. A.; Lobkovskaya, R. M.; Shibaeva, R. P.; Shilova, A. K.; Gruselle, M.; Leny, G.; Deschamps, E. J. Organomet. Chem. 1983, 244, 375–382. (16) Petz, W. Iron-Carbene Complexes; Springer: Berlin, 1993; p 103. (17) Baumann, F.; Dormann, E.; Ehleiter, Y.; Kaim, W.; K€archer, J.; Kelemen, M.; Krammer, R.; Saurenz, D.; Stalke, D.; Wachter, C.; Wolmersh€ auser, G.; Sitzmann, H. J. Organomet. Chem. 1998, 587, 267–283. (18) Sitzmann, H.; Wolmersh€auser, G. Z. Naturforsch. 1995, 50b, 750–756.

Wallasch et al.

complexes 4 have been discussed in detail.2 According to theoretical calculations and crystal structure determinations, these complexes contain a bridging six-membered-ring ligand belonging to an aryl(chloro)cuprate π-coordinated to a cyclopentadienyliron moiety. There is, however, some admixture of cyclohexadienyl-ylidene character (ca. 11%), corresponding to a small participation of a carbene complex of copper(I) halide as a second resonance structure.2 ESI mass spectra of 4-Br reveal a number of complexes of different nuclearity emerging from reactions occurring during or after the electrospray process. Only singly positively charged species are observed. In mass spectra of 4-Br two groups of signals have been observed (m/z 450-550 and m/z 950-1150, see Figure 4), whereas the molecular cation of 4-Br is not observed. The first group of peaks can be assigned from their isotope patterns to fragments containing only one metal, i.e., iron atom. The signal with the lowest m/z ratio (451.66) corresponds to the cationic sandwich complex [Cp000 Fe(C6H4iPr2)]þ, where the copper bromide fragment of the starting material has been replaced by a proton. Presumably the copper halide is lost during the spray process, and the strongly basic ipso carbon atom picks up a proton, e.g., from impurities present in the 10-6 molar acetonitrile solution or from the solvent itself. The parent peak at m/z 467.70 corresponds to the cationic phenol complex [Cp000 Fe(C6H3iPr2OH)]þ, which may have been formed in the presence of traces of oxygen or water. Along this line the peaks detected at m/z 483.75 and 499.84 could be tentatively assigned to higher oxidized species of composition [Cp000 Fe(C6H3iPr2O2H)]þ and [Cp000 Fe(C6H3iPr2O3H)]þ, respectively. However, the isotope patterns do not allow for an unequivocal interpretation. The last peaks in this group at m/z 532.07 clearly reveal a bromine isotope pattern and can thus nicely be fitted with a composition [Cp000 Fe(C6H3iPr2)HBr]þ (see Supporting Information). The second group of peaks in the mass spectrum of 4-Br can be assigned from their isotope patterns to bridged species containing additional oxygen or halogen atoms and at least two iron and one or two copper atoms. The peaks at m/z 979.57 can be nicely fitted to a composition [(Cp000 )2Fe2Cu(C6H3iPr2)2O]þ. Although the chemical structure of this species is not obvious, the composition points toward a condensation of two units of 4-Br. This formation of bridged species is more clearly seen from the analysis of the peaks at m/z 1063.31 and 1107.22. The former is adequately assigned by a superposition of two isotope patterns, i.e., [{Cp000 Fe(C6H3iPr2)Cu}2Cl - 4H]þ and [{Cp000 Fe(C6H3iPr2)Cu}2Cl]þ, whereas the latter can be perfectly explained by [{Cp000 Fe(C6H3iPr2)Cu}2Br]þ (Supporting Information). We assume that these species possess halogen-bridged structures of type [Cp000 Fe(C6H3iPr2)Cu-X-Cu(C6H3iPr2)FeCp000 ]þ (X= Cl, Br), as shown in Figure 5.

Conclusions The coupling of two phenyl anions to a bis(cyclohexadienyl-ylidene) ligand observed upon reaction of bis{tri(tert-butyl)cyclopentadienyliron bromide} (1a) with phenylmagnesium bromide is unusual for transition metal compounds and resembles reactions of lanthanide complexes. One reason for such reactivity may be seen in the unusual high-spin electron configuration of the cyclopentadienyliron starting material. If the σ-aryl complex formed

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Figure 4. ESI mass spectrum of 4-Br. Groups of peaks in the region m/z 450-550 (black: experiment; colored: simulation of isotope pattern). Groups of peaks in the region m/z 950-1150 (black: experiment; colored: simulation of isotope pattern). Spectra obtained with an ion-trap ESI mass spectrometer (see Experimental Section).

Figure 5. Suggested structure of [{Cp000 Fe(C6H3iPr2)Cu}2Br]þ (m/z 1107.22). The structural similarity of the species related to the mass peaks at m/z 1063.31 and 1107.22 is further supported by the results of MS-MS experiments, leading to the same fragment ions (m/z 393.30; 449.41; 531.82) upon isolation and subsequent collision-induced dissociation with He.

with 2,6-diisopropylphenylmagnesium bromide can be taken as a model compound for the first step in the coupling reaction, the next steps should be σ/π rearrangement followed by dimerization or vice versa. DFT calculations reproduce the structural parameters of the resulting complex 2 very nicely and show that upon oxidation of the dinuclear bis(cyclohexadienyl-ylidene) complex 2 the first electron is taken mainly from the biphenyl ligand and leads to a pronounced structural reorganization, whereas upon further oxidation the second electron stems from the cyclopentadienyliron fragments. In our investigation electrospray ionization mass spectrometry (ESI-MS) served as a valuable tool for the characterization of neutral multinuclear metal organic compounds. Their charging occurs simply during the spraying process, and their composition could be unequivocally identified by analysis of the related isotope patterns.

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Moreover, the observation of new aggregates in the electrospray process inspires chemists to search for new synthetic pathways. For instance, the occurrence of tetranuclear complexes [LCu-X-CuL]þ (L=Cp000 FeC6H3R2) in the ESI mass spectrometer calls for experiments aiming at halide abstraction from copper halide complexes of the [Cp000 Fe(C6H3R2)CuX] type by means of Lewis acids in order to synthesize such cations on a preparative scale.

Experimental Section Synthetic work has been carried out under inert gas in rigorously dried and deoxygenated solvents using Schlenk-line techniques for the preparation of 2,6-diisopropylmagnesium bromide and an argon-filled glovebox from MBraun (Garching) for preparation and handling of the iron complexes. The cyclopentadienyliron bromide starting compounds 1a1 and 1b13 have been prepared according to procedures documented in the literature. (μ,η5:η5-Dicyclohexadienylidene)bis{tri(tert-butyl)cyclopentadienyliron} (2). To a solution of 1 (300 mg, 0.41 mmol) in tetrahydrofuran (5 mL) was added a solution of phenylmagnesium bromide diethyl ether complex (173 mg, 0.82 mmol) in tetrahydrofuran (5 mL) with stirring at room temperature. The solution turned from green to deep blue within 3 h. Solvent removal in vacuo, extraction with pentane (10 mL), centrifugation, evaporation to a residual volume of 3 mL, and cooling to -70 °C afforded 210 mg (0.51 mmol, 63%) of blue-black crystals. Single crystals suitable for X-ray diffraction were grown from a toluene solution layered with pentane. Anal. Calcd for C46H68Fe2: C, 75.27; H, 9.34. Found: C, 74.18; H, 9.33. 1H NMR (400 MHz, 298 K, C6D6): δ 6.29 (1H, para), 4.46 (2H, meta), 3.90 (2H, Cp ring H), 2.87 (2H, ortho), 1.28 (s, 18H, C(CH3)3). 13C NMR (100 MHz, 298 K, C6D6): δ 105.4 (CdC), 99.9 (2C, ring-C-CMe3), 98.6 (1C, ring-C-CMe3), 78.8 (2C, meta), 75.7 (1C, para), 70.3 (2C, ring-C-H), 47.2 (2C, ortho), 34.4 (2C, CMe3), 33.8 (6C, C(CH3)3), 32.7 (1C, CMe3), 31.1 (3C, C(CH3)3). 1-Bromo-2,6-diisopropylbenzene. 2,6-Diisopropylaniline (30 g, 169 mmol) was added to concentrated hydrobromic acid (48%, 150 mL) with vigorous stirring. The pale yellow suspension was cooled to -50 °C, and solid sodium nitrite (20 g, 289 mmol) was added with a spatula within an interval of about 10 minutes, accompanied by a color change to brownish. Stirring was continued for one hour; then diethyl ether precooled to the same temperature was added, and the solution was allowed to warm slowly to -15 °C. At -22 °C the evolution of a brown gas started, which continued for a while at -15 °C. When the gas evolution slowed, the mixture was cooled to -50 °C again. Water (20 mL) and sodium carbonate decahydrate (100 g) were added, and the mixture was allowed to thaw to room temperature. Gas evolution was observed, and stirring was continued overnight. The phases were separated, the aequeous phase was extracted twice with diethyl ether (2  150 mL), and the combined organic phases were subjected to rotary evaporation of the diethyl ether solvent. Distillation of the remaining reddish-yellow oil at 0.01 mbar yielded the product fraction (29.6 g, 122.7 mmol, 73%) as a colorless liquid at a boiling temperature of 65 °C. 1H NMR (600 MHz, 298 K, C6D6): δ 7.06 (t, 1H, para, 3 JH,H=7.62), 6.96 (d, 2H, meta, 3JH,H=7.62), 3.58 (2H, CHMe2, 3 JH,H=6.88), 1.14 (12H, CH3, 3JH,H=6.88). 13C{1H}NMR (150 MHz, 298 K, C6D6): δ 147.8 (1C, ipso-CBr), 127.6 (2C, metaCH), 126.7 (1C, para-CH), 124.4 (2C, meta-CH), 33.7 (2C, CHMe2), 22.9 (4C, CH3). 2,6-Diisopropylphenylmagnesium Bromide Tetrahydrofuran Adduct. A mixture of 2-bromo-1,3-diisopropylbenzene (25 g, 104 mmol), magnesium (2.6 g, 107 mmol), tetrahydrofuran (150 mL), and a trace of iodine was heated to 65 °C oil bath temperature for 3 h. After almost complete consumption of

Wallasch et al. magnesium the gray-black suspension was filtered through a medium porosity glass frit, and the filtrate was evaporated to dryness. The colorless residue was washed two times with petroleum ether (100 mL each) and dried in an oil-pump vacuum to yield a colorless powder (32.4 g, (96 mmol, 93%). Anal. Calcd for the tetrahydrofuran adduct C16H25BrMgO: C, 56.93; H, 7.46. Found: C, 57.16; H, 7.79. Tri(tert-butyl)cyclopentadienyl(2,6-diisopropylphenyl)iron (3a). To a green solution of 1a (500 mg, 0.68 mmol) in tetrahydrofuran (10 mL) was added a solution of 2,6-diisopropylphenylmagnesium bromide bis(diethyl ether) complex (460 mg, 1.36 mmol) in tetrahydrofuran (5 mL) with stirring at room temperature. The solution turned from moss green to yellow. Solvent removal in vacuo was started five minutes after the addition of the Grignard compound. Two-fold extraction with pentane (2  10 mL), centrifugation, and evaporation to dryness afforded 256 mg (0.56 mmol, 42%) of product, which precipitated as an initially brownish-orange and crystalline powder and turned darker and sticky within minutes. Due to the instability of this complex and its tetraisopropylcyclopentadienyl analogue 3b, we have been unable to obtain satisfactory elemental analyses for both of these. The compound was used immediately for the preparation of the copper complex 4-Cl (see below). Tetraisopropylcyclopentadienyl(2,6-diisopropylphenyl)iron (3b). To a green solution of bis{(μ-bromo)(tetraisopropylcyclopentadienyl)iron(II)} (1b) (500 mg, 0.68 mmol) in tetrahydrofuran (10 mL) was added a solution of 2,6-diisopropylphenylmagnesium bromide tetrahydrofuran complex (460 mg, 1.36 mmol) in tetrahydrofuran (5 mL) with stirring at room temperature. The solution turned from moss green to yellow. Solvent removal in vacuo was started after five minutes of stirring at room temperature. Two-fold extraction with pentane (10 mL each), centrifugation, and solvent evaporation afforded a yellow microcrystalline powder (496 mg, 1.10 mmol, 81%), which started to become brownish and sticky during the workup procedure. Pale yellow crystals suitable for X-ray diffraction were grown from pentane solution. 1H NMR (400 MHz, 298 K, C6D6): δ (Δν1/2) 188.0 (704), 72.2 (2216), 13.7 (577), -19.2 (1524), -28.5 (143), -118.1 (2030). Satisfactory elemental analyses could not be obtained due to the thermal instability of the complex. Tri(tert-butyl)cyclopentadienyliron(II)(μ,η5:η1-2,6-diisopropylphenyl)copper(I) Chloride (4-Cl). The starting compound 3a was prepared immediately before the following procedure was carried out. After the addition of copper(I) chloride (22 mg, 0.22 mmol) to a magnetically stirred tetrahydrofuran solution (5 mL) of 3a (100 mg, 0.22 mmol) at room temperature the green solution turned intense red. Stirring was continued for about 30 min; then the solvent was removed in vacuo, and the residual solid was extracted twice with toluene (2  5 mL). Toluene removal and washing with pentane left a red, crystalline solid (85 mg, 0.15 mmol, 70%). Anal. Calcd for C29H48ClCuFe: C, 63.38; H, 8.44. Found: C, 62.48; H, 8.21. 1H NMR (600 MHz, 298 K, C6D6): δ 5.96 (1H, para), 5.61 (2H, meta), 4.31 (2H, Cp ring H), 3.38 (2H, CHMe2), 1.48 (6H, CH3), 1.45 (s, 9H, C(CH3)3), 1.44 (s, 18H, C(CH3)3), 1.28 (6H, CH3). 13C NMR (150 MHz, 298 K, C6D6): δ 123.5 (s, 1C, ipso), 120.5 (s, 2C, ortho), 106.6 (s, 1C, Cp000 -C-CMe3), 99.7 (s, 2C, Cp000 -C-CMe3), 81.9 (d, 1C, para, 1 JC,H=170 Hz), 80.3 (d, 2C, meta, 1JC,H=165 Hz), 68.8 (dd, 2C, Cp000 -CH, 1JC,H=171 Hz, 3JC,H=6.5 Hz), 39.0 (d, 2C, CHMe2, 1 JC,H=129 Hz), 33.4 (q, 6C, C(CH3)3, 1JC,H=125 Hz), 32.8 (s, 2C, CMe3), 31.6 (q, 3C, C(CH3)3, 1JC,H =125 Hz), 30.9 (s, 1C, CMe3), 28.2 (q, 2C, CH3, 1JC,H = 126 Hz), 21.6 (q, 2C, CH3, 1 JC,H =127 Hz). Tri(tert-butyl)cyclopentadienyliron(II)(μ,η5:η1-2,6-diisopropylphenyl)copper(I) Bromide (4-Br). Soon after the addition of copper(I) bromide (32 mg, 0.22 mmol) to a magnetically stirred solution of 3a (100 mg, 0.22 mmol) at room temperature the pale yellow solution turned intense red. After 45 min the solvent was removed in vacuo, and the residual solid was extracted twice with toluene (2  5 mL). Toluene removal and washing with pentane

Article left a red, crystalline solid (86 mg, 0.16 mmol, 66%). Anal. Calcd for C29H48BrCuFe: C, 58.64; H, 7.81. Found: C, 57.56; H, 7.74. 1 H NMR (600 MHz, 298 K, C6D6): δ 5.95 (t, 1H, para, 3JH,H= 5.70 Hz), 5.62 (2H, meta, 3JH,H=5.70 Hz), 4.33 (2H, Cp ring H), 2.87 (“sep”, 2H, CHMe2), 1.49 (d, 6H, CH3, 3JH,H =6.60 Hz), 1.47 (s, 9H, C(CH3)3), 1.44 (s, 18H, C(CH3)3), 1.30 (d, 6H, CH3, 3 JH,H =6.54 Hz). 13C NMR (150 MHz, 298 K, C6D6): δ 123.6 (1C, ipso), 120.6 (2C, ortho), 106.7 (1C, para) 99.7 (2C, meta), 81.7 (1C, ring-C-CMe3), 80.2 (2C, ring-C-CMe3), 68.8 (2C, ringCH), 39.0 (2C, CHMe2), 33.4 (6C, C(CH3)3), 32.8 (2C, CMe3), 31.6 (3C, C(CH3)3), 30.9 (1C, CMe3), 28.1 (2C, CH3), 21.6 (2C, CH3). Electrospray ionization mass spectrometry (ESI-MS) was performed with two different instruments. (A) A modified Bruker APEX III ESI-FT-ICR (7 T) mass spectrometer equipped with a Bruker Apollo I electrospray source used in the positive ionization mode. Nitrogen was used as drying gas at a pressure of 10 psi and a temperature of 150 °C. The solutions were sprayed at a nebulizer pressure of 26 psi, and the electrospray needle was typically held at 2.0 kV. The instrument was controlled by the Bruker XMASS software. (B) A Bruker Esquire 3000plus ion trap instrument. The ion source was used in positive electrospray ionization mode. Scan speed was 13 000 m/z/s in maximum resolution scan mode (0.3 fwhm/m/z), scan range was 50 to 1500 m/z. All spectra were accumulated for at least five minutes. Sample solutions in acetonitrile at concentrations of 10-5 M were filtered through a PVDF filter (0.45 μm-13 mm) and continuously infused into the ESI chamber at a flow rate of 4 mL/min using a syringe pump. Nitrogen was used as drying gas with flow rate of 5.0 L/ min at 300 °C. The solutions were sprayed at a nebulizer pressure of 10 psi, and the electrospray needle was typically held at 2.0 kV. The instrument was controlled by Bruker Esquire Control 5.3 software, and data analysis was performed using Bruker Data Analysis 3.4 software. All computations were carried out with the GAUSSIAN03 program19 by applying density functional theory (DFT) to describe correlation effects, using the B3LYP functional. Two

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different combinations of basis sets were applied: Either firstrow elements were described by Dunning’s correlation consistent double-ζ plus polarization basis for DFT (cc-pVDZ),10 together with the Wachters-Hay 6-311G basis8,9 for the firstrow transition element Fe, or all atoms, including Fe, were described by the SDD basis set as previously applied in ref 11. Charge distributions were taken from natural bond orbital (NBO) analyses carried out using the NBO 3.1 program20 as implemented in GAUSSIAN03.

Acknowledgment. This work was funded by the Deutsche Forschungsgemeinschaft (DFG grant Si 366/ 12-1). H.S. is grateful to Prof. O. J. Scherer for many years of friendly support. Supporting Information Available: Crystallographic information files (CIF) as well as results of the ab initio calculations and ESI mass spectra. This material is available free of charge via the Internet at http://pubs.acs.org. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Akrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; M. A. Al-Laham, Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. GAUSSIAN 03 (Revision C.01); Gaussian, Inc.: Wallingford, CT, 2004. (20) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO Version 3.1; Theoretical Chemistry Institute, University of Wisconsin: Madison, 1995.