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Synthesis, characterisation and properties of bis(permethylindenyl) iron and tin complexes
Accepted Manuscript Synthesis, characterisation and properties of bis(permethylindenyl) iron and tin complexes Paul Ransom, Thomas A.Q. Arnold, Amber L. Thompson, Jean-Charles Buffet, Dermot O’Hare PII:
S0022-328X(14)00464-1
DOI:
10.1016/j.jorganchem.2014.10.001
Reference:
JOM 18742
To appear in:
Journal of Organometallic Chemistry
Received Date: 31 July 2014 Revised Date:
25 September 2014
Accepted Date: 1 October 2014
Please cite this article as: P. Ransom, T.A.Q. Arnold, A.L. Thompson, J.-C. Buffet, D. O’Hare, Synthesis, characterisation and properties of bis(permethylindenyl) iron and tin complexes, Journal of Organometallic Chemistry (2014), doi: 10.1016/j.jorganchem.2014.10.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Synthesis,
Characterisation
and
Properties
of
bis(permethylindenyl) Iron and Tin complexes Paul Ransom, Thomas A. Q. Arnold, Amber L. Thompson, Jean-Charles Buffet, and
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Dermot O’Hare* Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford, OX1 3TA, UK.
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*Corresponding author: Tel: +44(0) 1865 272 686. Fax +44 1865 272690 E-mail address: [email protected] (Dermot O’Hare)
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ABSTRACT
Neutral iron complex, (EBI*)Fe (EBI* = ethylenebis(1-hexamethylindenyl), and its cationic analogue, [(EBI*)Fe](BF4), have been synthesised. Both rac- and meso-(EBI*)Fe can be isolated but the isomeric distribution of the reaction products was found to be highly sensitive to the reaction conditions employed. The redox behaviour of rac-(EBI*)Fe in solution has been studied by cyclic voltammetry, it
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shows a one-electron reversible oxidation at –0.7 V vs. (Cp)2Fe/(Cp)2Fe+. Chemical oxidation of (EBI*)Fe with AgBF4 gives [(EBI*)Fe](BF4). The molecular structures of rac-(EBI*)Fe and meso-[(EBI*)Fe](BF4) have been determined by single crystal
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X-ray structure analysis. (EBI*)(SnMe3)2 was synthesised by the reaction of [(EBI*)]Na2 with ClSnMe3 in THF. In solution, only two of the many possible isomers are observed by 1H NMR spectroscopy. Single crystals of (EBI*)(SnMe3)2
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were grown as pale, plates by the slow cooling to –78°C of a saturated hexane solution of a mixture of the isomers. 1H NMR analysis of the crystals showed them to be the isomer with C-SnMe3 peaks at a chemical shift of – 0.27ppm.
Keywords Metallocene, Iron, Tin, Permethylindenyl, Cyclic voltammetry
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Introduction Although cyclopentadienyl ligands have played a pivotal role in the development of organometallic chemistry [1], the related indenyl ligand demonstrates additional features largely based on its flexible hapticity [2-5]. For example, Basolo, Marder and co-workers have exploited this flexible ligation ability with rhodium containing
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complexes [5] while Bradley and co-workers, amongst others, have reported a series of bis(indenyl) iron complexes [6].
Ansa-bridged indenyl metal complexes have also been the subject of intense synthetic activity [8]. It is possible to introduce an unsubstituted ethylene bridge between
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indenyl rings by a salt elimination reaction between 1,2-dibromoethane and two equivalents of an alkali metal indenyl (or cyclopentadienyl) salt as shown by
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Buchwald and co-workers [9a]. For example the synthesis of the ethylenebis(1tetrahydroindenyl) (EBTHI)H2, the unmethylated ligand precursor ethylenebis(1indenyl) (EBI)H2, and the ethylenebis-(alkylsiloxyindenyl) (EBIOSiMe2tBu)H2, which are then deprotonated twice with BuLi to yield the desired dilithium ligand salts [9]. Alternatively, indene can be mixed with 1,2-dibromoethane and an alkylmagnesium compound in heptane, presumably forming an indenyl Grignard compound, which
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then couples with another unit to give (EBI)H2 [10]. These synthetic strategies all involve nucleophilic substitutions by the indenyl moiety. Another route for generating ansa-metallocene complexes with substituted ethylene bridges is the reductive
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coupling of fulvenes. It is mediated by a number of activated metal species from groups 2, 4, 8 and the lanthanides, useful due to the facile synthesis of a wide range of differently substituted fulvenes. Alkali metal reduction of fulvenes has been reported
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to give a variety of products, including coupled fulvenes [11]. We have previously reported the synthesis of an unbridged [12] and silane bridged permethylindenyl (Ind* = C9Me7) iron complexes [7], as well as the synthesis of group 4 [13a], cobalt [13b], and cerium[14] EBI* complexes. Not long after the discovery of ferrocene, cyclopentadienyl tin complexes were reported in this journal by Fritz et al.[15a] While interesting complexes in their own right, a major use was their ability to act as mild (non reducing) ligand transfer reagents. For example, the use of Pn*(SnMe3)2 (where Pn* = C8Me6) is required for the synthesis of Pn*TiCl2 where the Pn*Li2 was found to be too reducing.[15b,c]
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ACCEPTED MANUSCRIPT Herein, we report the synthesis of new iron and tin complexes based on bridged permethylindenyl based ligands.
Synthesis and characterisation of (EBI*)(SnMe3)2
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Results and Discussion
(EBI*)(SnMe3)2, 1, was synthesised by the reaction of (EBI*)Na2 with ClSnMe3 in THF. An immediate colour change from the red (EBI*)Na2 in THF to yellow was observed. The
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reaction of (EBI*)Li2 with ClSnMe3 was also performed both in Et2O and hexane, as shown in Scheme 1, the colour change being less obvious due to the initial insolubility of the lithium
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salt. After filtration and removal of the solvent under vacuum, 1H NMR spectroscopy analysis of the lithium salt reaction residue indicated two main resonances in a region indicative of C-SnMe3 coordination, at –0.27 and –0.29 ppm in benzene-d6. Recrystallisation from hexane afforded two solids, the least hexane soluble enriched in the –0.29 ppm species and the more soluble in the –0.27 ppm compound, in yields of approximately 63 and 14%
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respectively.
Scheme 1. Synthesis of (EBI*)(SnMe3)2, 1; a number of isomers are possible
Several possible regioisomers of (EBI*)(SnMe3)2 may be envisaged. On reaction with [EBI*]Li2, the SnMe3 unit can bond in an η1 fashion at two points on each five-membered ring, at the 1- or 3-position; the 2-position would lead to a loss of aromaticity of the aryl unit and so substitution at this position is expected to be unfavourable, as shown in Scheme 2.
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ACCEPTED MANUSCRIPT These 2-substituted, iso-indenes are not unknown, however, and can be trapped as Diels-
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Alder adducts.[15d]
Scheme 2. Possible coordination sites of the SnMe3 unit on EBI* framework while
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maintaining the aromaticity of the aryl unit. A third site results in unfavourable loss of aromaticity.
In addition, the SnMe3 units may coordinate to the same position or the alternate position on the second indenyl moiety, resulting in the three combinations 1,1′, 1,3′ (or 3,1′), and 3,3′. Lastly, assuming the SnMe3 unit can approach from above or below
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each ring, two possible stereochemistries are possible for each site. This results in a total of ten possible isomers shown schematically in Scheme 3, six of which are diastereomers, and hence NMR distinct (each row of Scheme 3), since identical NMR
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spectra are expected for each image/mirror-image enantiomeric pair.
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Scheme 3 Schematic of the possible isomers of (EBI*)(SnMe3)2, stereochemistry of SnMe3 units shown by Sn wedges and dashes
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It is therefore difficult to predict how many unique environments of SnMe3 and hence peaks would be observed in the NMR spectrum of (EBI*)SnMe3. This is further complicated by the literature precedence for fluxionality in η1-cyclopentadienyl and indenyl compounds of maingroup elements; one possible mechanism being [1,5]-silatropic shifts [16a-c]. These shifts
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take place suprafacially, maintaining the stereochemistry of the C-Si centre. The dynamic properties of a silyl bridged Al ansa-indenyl compound have been studied, and it was found
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that the rac-1,1′ species converts into the rac-1,3′ rapidly at 23 °C (and slowly at –60 °C) via two [1,5]-Al shifts and an anti-aromatic intermediate [16d]. The different 1,3′ species also interconvert (slow at 23 °C, rapid at 70 °C) via [1,5]-H shifts, and the rac-1,1′ to meso-1,1′ via [1,5]-Si (of the silyl bridge) and [1,5]-H shifts. However, in the case of (EBI*)(SnMe3)2, there are no H atoms on the ring periphery, limiting some of the possible interconversion pathways. In ethylidene bridged species, it is often possible to distinguish between 1- and 3- substituted positions using NMR spectroscopy. For example, with the SnMe3 unit in the 3-position the 1H will be a multiplet due to coupling to the protons of the bridge. This NMR approach is not
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ACCEPTED MANUSCRIPT as accessible in (EBI*)(SnMe3)2, due to the permethylation of the ring periphery although a correlated 1H-119Sn approach may be possible. A variable temperature 1H NMR study was performed on a sample of (EBI*)(SnMe3)2, in toluene-d8 between 80 to –80 °C in 20°C intervals (Figure S1). At 20 °C, the resonances ascribed to C-SnMe3 appear at –0.29 and – 0.31 ppm. The spectra are very complicated; at low temperatures these signals separate
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further and move upfield to values of –0.33 and –0.37 ppm at –80 °C. At higher temperatures these peaks coalesce to a low broad signal further upfield, at –0.21 ppm at 80 °C. The resonances ascribed to the EBI* methyl protons lie between approximately 1.61 and 2.50 ppm.
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Single crystals of (EBI*)(SnMe3)2 were grown as very pale yellow, plates by the slow cooling to –78 °C of a saturated hexane solution of a mixture of isomers. The crystal analysed
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was 1R,1'R-(EBI*)(SnMe3)2, 1. 1H NMR analysis of the crystals showed them to be the isomer with C-SnMe3 peaks at a chemical shift of –0.27 ppm. The molecular structure of 1
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obtained is shown in Fig. 1 and selected bond lengths and angles are given in Table 1.
Fig. 1. Molecular structure of 1R,1'R-(EBI*)(SnMe3)2, 1, with H atoms omitted for clarity and thermal ellipsoids are drawn at 50%.
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ACCEPTED MANUSCRIPT Table 1. Selected bond lengths and angles for (EBI*)(SnMe3)2, 1R,1’R-1. Lengths (Å)
Fold Angle Sn1-C3-C4 αa Rotation Angle a
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1.508(6) 1.411(6) 1.463(6) 1.353(6) 1.504(6) 1.535(5) 1.506 1.518 2.226(4)
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1.494(6) C6 - C11 1.405(6) C11 - C12 1.478(6) C12 - C21 1.334(6) C21 - C22 1.510(6) C22 - C6 1.559(5) C6 - C5 1.528(5) 1.506 Avg. C5 - Me 1.518 Avg. C6 - Me 2.225(4) Sn7 - C6 Angles (°) 2.6 112.0(3) 6.1
Fold Angle Sn7-C6-C5 -
2.5 112.7(3)
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C3 - C25 C25 - C32 C32 - C33 C33 - C34 C34 - C3 C3 - C4 C4 - C5 Avg. C5 - Me Avg. C6 - Me Sn1 - C3
ca. 12.6
It can be clearly seen from Fig. 1 that in the isomer of (EBI*)(SnMe3)2 which was analysed by single-crystal X-ray diffraction, each SnMe3 unit is bound in an η1 manner to the carbon in
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the 1-position. This bonding mode can also be inferred from analysis of the intra-ring C-C bond distances; within the C6 ring C-C bond distances are all very similar, and the C33-C34 distance is the shortest within the skeleton at 1.334(6)Å, due to it being a localised double bond. In contrast to the 1,1′ bonding mode observed in Fig. 1, examples of bis-SnMe3
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substituted ansa-indenyl compounds in the literature have the SnR3 units in the 3position.[17] The reasons behind this are unclear, however, a possible explanation is that,
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with the SnMe3 coordinated to the 3-position at positions C21 and C33, C6 and C3 would be sp2 hybridised with planar bonding to the ethylidene bridge. This may result in the two permethylated rings of EBI* becoming too sterically congested. As shown in Fig. 1, the two indenyl moieties of the EBI* unit are relatively planar and almost parallel to one another, with only a small fold angle (FA) observed at the C6-C5 ring junctions. The SnMe3 units are approximately tetrahedral, with angles close to the ideal tetrahedral angle of 109.5°. No structurally characterised ansa-indenyl or cyclopentadienyl Sn species exist in the literature. The closest two species for comparison are the permethylated indenyl and fluorenyl species (Ind*)(SnMe3) [18] and (Flu*)(SnMe3) [19].
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ACCEPTED MANUSCRIPT These have Sn-Cring distances of 2.232(6) and 2.221(3) Å respectively. The value for 1 falls within this range, at 2.225(4) and 2.226(4) Å for the two indenyl moieties. Syntheses and characterisation of ethylenebis(1-hexamethylindenyl) iron complexes
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A number of different syntheses of (EBI*)Fe, 2, were attempted, as shown in Scheme 4. The rac-/meso- isomeric distribution of the reaction products was found to be highly sensitive to the reaction conditions employed, varying with the choice of alkali metal ligand salt, source of iron starting materials, reaction temperature, and crystallisation procedures.
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The stoichiometric reaction of (EBI*)K2 with FeCl2·THF1.5 in THF at –78 °C resulted after work-up in the isolation of isomerically pure meso-(EBI*)Fe, meso-2. The use of
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(EBI*)Li2·THF0.38 with FeCl2·THF1.5 in THF at –78 °C afforded, after work-up, a mixture of rac- and meso-isomers; however, the pure rac-form was obtained at – 196 °C. Similarly, the use of EBI*Na2 with FeCl2·THF1.5 in THF at –78 °C resulted in a mixture of rac- and meso-(EBI*)Fe. However, reaction of Fe(acac)2 with EBI*Na2 at
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70 °C in THF was also found to yield pure rac-(EBI*)Fe.
Scheme 4. Various syntheses of rac- and meso-(EBI*)Fe, 2.
The literature details only five ansa-bridged indenyl Fe compounds, all synthesised by reaction of an appropriate ligand precursor with FeCl2. One of these has the ansa-bridge in a position which imparts no rac/meso stereochemistry upon the products.[20] In all the other four cases, a mixture of isomers was obtained upon reaction, which were separated and 8
ACCEPTED MANUSCRIPT isolated either through crystallization [7,21] or via an alumina chromatography column [22,23]. The unbridged analogue (Ind*)2Fe is synthesised by the reaction of the lithium salt with FeCl2·2THF in THF at room temperature [12]. Each of the 1H NMR spectra of rac- and meso-(EBI*)Fe show six distinct resonances attributed to the methyl groups on the EBI* ring periphery, together with a AA'BB' multiplet
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ascribed to the ethylene bridge. The methyl resonances are within the range 1.35 - 2.66 ppm. The 13C{1H} NMR spectrum for each isomer shows six resonances in the range 8 – 18 ppm which can be ascribed to the methyl carbon atoms, and those of the ethylene bridge at approximately 35 ppm. For each species, five signals are seen between 70 – 95 ppm from the
membered ring observed between 125 – 135 ppm.
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carbon atoms of the five-membered ring, with the four carbon atoms at the back of the six-
Fig. 2 depicted the various geometric parameters used in the crystallographic discussion
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throughout.
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Fig. 2. Schematic depictions defining the various geometric parameters. Single crystals suitable for X-ray diffraction were grown in two different ways; as dark red needles from the slow evaporation of an isomerically pure benzene-d6
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solution, or as large brown hexagonal plates from cooling a mixture of rac- and mesoisomers in benzene-d6 to 5 °C. The molecular structure of rac-(EBI*)Fe, rac-2, is depicted Fig. 3. The compound crystallises in the orthorhombic space group Fdd2.
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Alternate views are shown in Fig. S2, and selected bond lengths and angles are given in Table 2. As shown in Fig. 3, rac-2 is only pseudo-C2 symmetric; the molecule does not possess a C2 axis and only one EBI* moiety exists in the asymmetric unit, with each indenyl ring crystallographically distinct. The rac-bonding mode of the EBI* ligand to the Fe centre results in a rotation angle (RA) of 148.9(3)°. This value is only slightly less than that observed in the unbridged bisheptamethylindenyl species [(Ind*)2Fe] at 151.3°;[21] hence, it appears that, in this case, the ethylene bridge does not restrain the conformation of the two indenyl rings of the EBI* moiety about the Fe atom. The indenyl rings exhibit only a slight fold angle of 4.5°. The average Fe-Cpcent distance is 0.065 Å shorter than in the 19-valence electron species rac-(EBI*)Co 9
ACCEPTED MANUSCRIPT [13b], despite the decreasing atomic size across the period, attributed to the extra electron in an antibonding orbital for the cobalt congener. The average Fe-C distance, 2.04806 Å, agrees well, however, with that reported for SBI*Fe, SBI* = Me2Si(C9Me6)2, 2.0458 Å.[7] The tilt angle, α, is 16.75(11)°, lower than that of two other
ethylene-bridged
Fe
species
in
the
literature;
23.0°
for
{(4,7-
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Me2C9H4)2(CHPh)2}Fe [21] and 21.6° for {(C5H4)2(CH2)2}Fe [25], assumed to be because the indenyl groups are sterically bulkier. This value is larger than α reported for the SiMe2-bridged analogue (13.4°),[7] where we can infer that the ethylene bridge
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imparts more strain than its silicon counterpart.
Fig. 3. Molecular structure of rac-(EBI*)Fe, rac-2. For clarity, all hydrogens atoms have been omitted (displacement ellipsoids are drawn at 50% probability). Selected distances
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Fe(1)-Cpcent 1.6386(9) Å and Fe(1)-Cpcent’ 1.6381(9) Å.
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ACCEPTED MANUSCRIPT Table 2. Selected bond lengths and angles for rac-(EBI*)Fe, rac-2. Lengths (Å) 1.9880(18) 2.0477(18) 2.0619(19) 2.0724(19) 2.0735(18) 1.436(3) 1.443(3) 1.452(3) 1.454(3) 1.453(3) 1.528(3)
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2.0736(19) Fe1 - C18 2.0789(19) Fe1 - C19 2.0634(18) Fe1 - C21 2.0428(18) Fe1 - C23 1.9864(18) Fe1 - C32 1.459(3) C18 - C19 1.451(3) C19 - C21 1.433(3) C21 - C23 1.444(3) C23 - C32 1.450(3) C32 - C18 1.527(3) C18 - C17 1.554(2) 1.502 Avg. C5 - Me 1.514 Avg. C6 - Me 1.6386(9) Fe1 - Cpcent Angles (°) 4.64(10) 16.75(11) 6.30(19)
Fold Angle Hinge Angle -
148.9(3)
1.497 1.512 1.6381(9)
4.4(10) 7.08(18)
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Fold Angle α Hinge Angle Rotation Angle
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Fe1 - C1 Fe1 - C10 Fe1 - C11 Fe1 - C13 Fe1 - C15 C1 - C10 C10 - C11 C11 - C13 C13 - C15 C15 - C1 C15 - C16 C16 - C17 Avg. C5 - Me Avg. C6 - Me Fe1 - Cpcent
Numerous attempts directed towards the chemical oxidation of rac-/meso- isomeric mixture of (EBI*)Fe were made. In the unbridged case, the dropwise addition of [(Cp)2Fe](PF6) in MeCN to a solution of (Ind*)2Fe in THF yields [(Ind*)2Fe](PF6)
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[12]. This was not successful in the case of (EBI*)Fe across a range of solvents, nor was the alternate use of [(Cp)2Fe](BF4) as oxidising agent. However, upon addition of
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AgBF4 in THF to a red-brown THF solution of (EBI*)Fe, an immediate grey-green precipitate was observed. Filtration and washing with hexane afforded a black solid of [(EBI*)Fe](BF4), 3. The 1H NMR spectrum of 3 was paramagnetically broadened, and extremely air sensitive which made the EPR and Mossbauer studies very difficult to perform. Sufficient single crystals of meso-[(EBI*)Fe](BF4), meso-3, suitable for Xray diffraction were grown as black plates by the slow diffusion of Et2O into a MeCN solution of [(EBI*)Fe](BF4) synthesised from a rac-/meso- isomeric mix of (EBI*)Fe, the molecular structure is depicted Fig. 4. We were unable to obtain a bulk sample or separate the rac- and meso isomers.
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Fig. 4. Molecular structure of meso-[(EBI*)Fe](BF4), meso-3. For clarity, hydrogens atoms
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and the [BF4]− counterion were omitted (displacement ellipsoids are drawn at 50% probability. Selected distances (Å) Fe1-Cpcent 1.6812(19) and Fe1-Cpcent’ 1.6381(9). Table 3. Selected bond lengths and angles for meso-[(EBI*)Fe](BF4), meso-3. Lengths (Å)
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2.106(4) Fe1 - C17 2.078(4) Fe1 - C18 2.065(4) Fe1 - C19 2.010(4) Fe1 - C20 2.117(4) Fe1 - C21 1.458(6) C17 - C18 1.431(6) C18 - C19 1.430(6) C19 - C20 1.461(5) C20 - C21 1.455(5) C21 - C17 1.551(5) C17 - C16 1.552(6) 1.497 Avg. C5 - Me 1.506 Avg. C6 - Me 1.6812(19) Fe1 - Cpcent Angles (°)
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Fe1 - C2 Fe1 - C3 Fe1 - C4 Fe1 - C5 Fe1 - C6 C2 - C3 C3 - C4 C4 - C5 C5 - C6 C6 - C2 C5 - C15 C15 - C16 Avg. C5 - Me Avg. C6 - Me Fe1 - Cpcent
C6 - C5 planes α Hinge Angle Rotation Angle
7.3(2) 19.2(2) 9.4(4)
C6 - C5 planes Hinge Angle -
21.7(5)
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2.022(4) 2.148(4) 2.095(4) 2.076(4) 2.074(4) 1.452(6) 1.453(6) 1.468(6) 1.414(6) 1.449(6) 1.548(6) 1.496 1.510 1.6719(19)
6.0(2) 7.2(4)
ACCEPTED MANUSCRIPT The compound crystallises in the triclinic space group P-1, and the asymmetric unit contains one EBI* moiety and one disordered BF4 molecule. Alternate views are shown in Fig. S3, and relevant bond distances and angles are given in Table 3. Figure 4 shows the EBI* ligand bonding to the Fe centre in a meso- mode, corresponding to a RA of 21.7(5)°. This is only 4.2° higher than the value in meso-[(EBI*)Co](BF4)
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[13b], and may be due to differences in the packing of molecules and counterions in the crystal.
Unlike in the Co analogue, oxidation affords a 17-valence electron species and occurs via the removal of an electron from a bonding molecular orbital. Thus, as expected,
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the Fe-Cpcent distance increases compared to rac-(EBI*)Fe upon oxidation, by an average of 0.038 Å due to the weakening of the Fe-EBI* bonding interaction. Oxidation also results in other structural changes within the ligand framework such as
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the increase in hinge angle (HA) by an average of 1.6°. This is concomitant with the slight loss of aromaticity and reduction from an ideal η5 bonding situation. The unbridged analogue, (Ind*2)Fe has been oxidised to form [(Ind*)2Fe](PF6); however, its crystal structure was not determined. The structure of the charge-transfer salt [(Ind*)2Fe][TCNQ] has, however, been reported [28]. In this species, the RA is consistent with those reported for other [(Ind*)2M]+ cations [29], as the value of meso-
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3 is with meso-[(EBI*)Co]+.
The electrochemical behaviour of rac-(EBI*)Fe was studied as a solution in CH2Cl2 with 0.1 molL–1 [NnBu4][PF6] as the supporting electrolyte. Data were recorded at room at
scan
rates
of
50/100/200 mVs–1,
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temperature
referenced
internally
to
the
[(Cp)2Fe]/[(Cp)2Fe+] couple at +0.46 V vs SCE under identical conditions [24]. Selected
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examples of the cyclic voltammograms recorded are shown in Fig. 5.
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Fig. 5. Cyclic voltammograms in CH2Cl2 of rac-(EBI*)Fe (top) and rac-(EBI*)Fe with
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internal (Cp)2Fe reference (bottom). Thermodynamic half-potentials were determined by
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square-wave voltammetry
The thermodynamic half-potentials were obtained by square-wave voltammetry, and rac-(EBI*)Fe showed a one-electron reversible oxidation at –0.24 V vs SCE. Interestingly, closer analysis of the cyclic voltammetric trace of rac-(EBI*)Fe shows a slight shoulder; although the sample used was mainly rac-(EBI*)Fe by NMR analysis, this shoulder may be attributed to the sample containing a small amount of the meso-isomer. A general trend can clearly be seen that upon permethylation of the cyclopentadiene (Cp) or indene (Ind) ligands, the oxidation potentials for the metallocenes formed move to more negative values, demonstrating the greater electron releasing power of the methylated species. This effect is more pronounced in the Cp case [24]. Furthermore, (Ind)2Fe has a 14
ACCEPTED MANUSCRIPT lower oxidation potential than (Cp)2Fe, and similarly (Ind*)2Fe is more readily electrochemically oxidised than (Cp*)2Fe [25]. The oxidation potential of (Ind*)2Fe can be used as an unbridged, and hence unstrained, reference compound for rac-(EBI*)Fe, which has an intermediary value between those of (Ind*)2Fe and (Cp*)2Fe. All the species given in Table 3 contain 18-valence electrons, and the HOMO for (Ind*)2Fe is regarded as being the
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antibonding A1 orbital [12]. This would suggest that the effect of the ansa-bridge is to lower the energy of the HOMO of rac-(EBI*)Fe relative to that of (Ind*)2Fe, leading to a less negative oxidation potential. This agrees with the work of Green who suggested that ansabridges make metallocenes more electrophilic.[26] (Ind*)2Fe showed a second reversible oxidation was observed in the case of rac-(EBI*)Fe.
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oxidation at +1.28 V vs SCE,[12] rare for 18-electron metallocenes; however, no such second
Table 4. Selected bond lengths and angles for rac-(EBI*)Fe.
[(Cp)2Fe] [(Cp*)2Fe] [(Ind)2Fe] [(Ind*)2Fe] rac-[(EBI*)Fe]
Ref
24 24 12 12 This work
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Conclusions
E1/2 (V vs SCE) 0.46 -0.13 0.13 -0.32 -0.24
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Compound
We report the synthesis and characterisation of the new ansa-bis(peralkylindenyl) iron and tin complexes, (EBI*)(SnMe3)2, 1, (EBI*)Fe, 2, and [(EBI*)Fe](BF4), 3. Only two of the many possible isomers of 1 were observed by 1H NMR spectroscopy Both
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rac- and meso-(EBI*)Fe can be isolated but the isomeric distribution of the reaction products was found to be highly sensitive to the reaction conditions employed.
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Electrochemical studies show that rac-(EBI*)Fe, rac-2, has a redox couple which is intermediate between that of (Ind*)2Fe and [(Cp*)2Fe]. The extremely air- and moisture-sensitive 17-electron cation [(EBI*)Fe](BF4) can be prepared by chemical oxidation of a solution of (EBI*)Fe to obtain meso-3. One isomer of each new material was characterised by X-ray crystallography: 1R,1′R-(EBI*)(SnMe3)2, rac(EBI*)Fe and meso-[(EBI*)Fe](BF4).
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Materials and methods General procedure: Air and moisture sensitive reactions were performed on a dual-manifold vacuum/N2 line using standard Schlenk techniques, or in a N2 filled MBraun Unilab glovebox. Hexane and toluene were dried using a Braun SPS-800 solvent purification system.
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Et2O and THF were dried at reflux over Na/benzophenone and distilled under N2. Hexane, toluene and Et2O were stored over K mirrors. THF was stored over activated 3 Å molecular sieves. Benzene-d6 (99%) was obtained from Goss Scientific, dried before being vacuum transferred prior to use.
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over K and preactivated 3 Å molecular sieves, freeze-pump-thaw degassed three times
Solution NMR samples were prepared in the glovebox under N2 atmosphere in 13
C{1H} NMR spectra were recorded on 300 MHz
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Young’s tap NMR tubes. 1H and
Varian VX-Works spectrometers. All chemical shifts were expressed as δ, in parts per million (ppm) relative to TMS (δ = 0). Crystals were mounted on MiTeGen MicroMounts using perfluoropolyether oil, and cooled rapidly to 150 K in a stream of cold nitrogen using an Oxford Cryosystems CRYOSTREAM unit [30]. Data collections were performed using an Enraf-Nonius FR590 Kappa CCD diffractometer,
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utilising graphite-monochromated Mo Kα X-ray radiation (λ = 0.71073 Å). Raw frame data were collected at 150(2) K using a Nonius Kappa CCD diffractometer, reduced using DENZO-SMN [31] and corrected for absorption using SORTAV [32]. The structure was solved using SuperFlip [33] and refined using full matrix least-squares
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using CRYSTALS [34,35]. Interplanar distances and angles were calculated using PLATON [36,37].
AC C
Synthesis of (EBI*)(SnMe3)2
(EBI*)Li2 (0.39 g, 0.00089 mol) was slurried in hexane and cooled to –78°C. To this buff coloured slurry was added a solution of ClSnMe3 (0.35 g, 0.00177 mol) in hexane. The reaction mixture was allowed to warm to room temperature and stirred for 15 hours affording a cream-yellow suspension. On settling and filtration, an off white powder with yellow supernatant was obtained. Removal of the solvent under vacuum afforded 0.32 g yellow solid, shown by NMR analysis to be an isomer with C-SnMe3 shifts at –0.27 ppm. The offwhite solid was extracted with 60 °C toluene and the solution filtered through celite. Removal of the solvent from this very light yellow solution afforded 0.07 g off-white solid, shown by
16
ACCEPTED MANUSCRIPT 1
H NMR spectroscopy analysis to be consistent of an isomer with C-SnMe3 peaks at –
0.29 ppm. Total yield: 0.39 g, 77%. 1
H NMR (benzene-d6, 25 °C, 300.1 MHz) δ (ppm): Isomer I: –0.27 (s, 18H, SnMe3), 2.03,
2.09, 2.20, 2.30, 2.40, 2.50 (all s, 6H, Me), 2.55 (bs, 4H, C2H4). Isomer II –0.29 (s, 18h, SnMe3), 1.91, 2.17, 2.29, 2.41, 2.44, 2.50 (all s, 6H, Me), 2.58 (bs, 4H, C2H4). MS (EI): Calc:
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752.26 Found: 752.23. Anal. Calc for C38H58Sn2: C, 60.67; H, 7.77. Found: C, 60.58; H, 7.83. Synthesis of (EBI*)Fe
(EBI*)Na2 + Fe(acac)2: One equivalent of (EBI*)Na2 (0.015 g, 0.032 mmol) and one equivalent of Fe(acac)2 (0.008 g, 0.032 mol) were added to an NMR tube. After
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addition of 1 mL of THF, the tube was sonicated for 30 minutes then heated to 70 °C for 15 hours. Removal of the solvent left a brown solid, which was dissolved in benzene-d6 and filtered through celite. NMR analysis showed pure rac-(EBI*)Fe.
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(EBI*)Li2·THF0.38 + FeCl2·THF1.5: Route A: One equivalent of (EBI*)Li2 (0.25 g, 0.057 mmol) and one equivalent of FeCl2·THF1.5 (0.13 g, 0.057 mmol) were added to a Schlenk, which was cooled to –196 °C and THF added. The frozen mixture was allowed to warm to room temperature with stirring, and the red-brown mixture stirred for 15 hours. Removal of the solvent under vacuum left a red-brown solid which was
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extracted with hexane. The resulting brown solution was reduced to a minimum volume and cooled to –78 °C. The supernatant was removed via cannula and the black solid washed with –78 °C hexane. NMR analysis showed pure rac-(EBI*)Fe. Single crystals of rac-(EBI*)Fe suitable for X-ray diffraction were obtained as dark red
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needles by the slow evaporation of this pure benzene-d6 solution. Yield of rac-(EBI*)Fe: 0.11 g, 40%. Route B: One equivalent of FeCl2·THF1.5 (0.25 g,
AC C
1.07 mmol) was slurried in THF, cooled to –78 °C and added to a slurry of one equivalent of (EBI*)Li2·THF0.38 (0.50 g, 1.07 mmol) in THF at –78 °C. The reaction mixture was stirred and allowed to warm to room temperature, going dark red-brown above approximately –5 °C. After stirring at room temperature for 15 hours the solvent was removed under vacuum affording a black-brown solid, which was extracted with hexane at 60 °C, reduced to a minimum volume and cooled to –78 °C. The resulting solid was washed with hexane at –78 °C to leave a black crystalline solid. Analysis by NMR showed a 1.35:1 rac/meso mixture, Yield of a rac-/meso- isomeric mix of EBI*Fe: 0.23 g, 45%. Single crystals of rac-(EBI*)Fe suitable for X-ray diffraction grew as large brown hexagons from cooling this benzene-d6 rac/meso mixture to 5 °C.
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ACCEPTED MANUSCRIPT (EBI*)K2 + FeCl2·THF1.5 One equivalent of (EBI*)K2 (0.15 g, 0.30 mmol) was slurried in THF and cooled to –78 °C, and one equivalent of slurry of FeCl2·THF1.5 (0.070g, 0.30 mmol) in THF was added. The initial beige slurry darkened immediately, and was allowed to warm to room temperature with stirring. The redbrown reaction mixture was stirred for 15 hours, and the solvent removed under
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vacuum. The resulting brown solid was extracted with hexane at 60 °C to give a dark brown solution, which was reduced to a minimum volume and cooled to –78 °C. A black-brown solid was deposited and filtered, shown by NMR analysis to be isomerically pure meso-(EBI*)Fe. The supernatant was further reduced in volume and
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cooled to –78 °C, and the resulting solid collected and washed with hexane at –78 °C. NMR analysis showed a 1.44:1 mix of rac/meso-isomers. Yield of a rac-/mesoisomeric mix of (EBI*)Fe: 0.025 g, 0.041 g, total 46%.
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(EBI*)Na2 + FeCl2·THF1.5 One equivalent of FeCl2·THF1.5 (0.075 g, 0.32 mmol) was slurried in THF, cooled to –78 °C and added to a yellow slurry of one equivalent of (EBI*)Na2 (0.15g, 0.32 mmol) in THF at –78 °C. The stirred reaction mixture darkened, becoming dark brown upon warming to room temperature, and was stirred for a further 15 hours. Removal of the solvent under vacuum and extraction with
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hexane at 60 °C afforded a dark brown solution, which was reduced to a minimum volume and cooled to –78 °C. The black solid deposited was filtered and washed with hexane at –78 °C and was shown by NMR analysis to be a 1:1.75 rac/meso mixture. Yield of rac-/meso- isomeric mixture of (EBI*)Fe: 0.064 g, 42%.
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rac-(EBI*)Fe:1H NMR (benzene-d6, 25 °C, 300.1 MHz) δ (ppm): 1.35, 1.65, 2.21, 2.23, 2.46, 2.66 (all s, 6H, Me), 3.42-3.57, 3.77-3.92 (m, 4H, C2H4).
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C{1H} NMR
AC C
(benzene-d6, 25 °C, 75 MHz) δ (ppm): 8.4, 12.0, 16.6, 17.2, 17.4, 17.5 (Me), 35.1 (C2H4), 70.1, 81.0, 85.0, 89.7, 90.6 (Cs of five-membered ring), 127.9, 129.0, 132.0, 133.3 (Cs at back of six-membered ring). meso-(EBI*)Fe: 1H NMR (benzene-d6, 25 °C, 300.1 MHz) δ (ppm): 1.90, 1.99, 2.03, 2.10, 2.20, 2.38 (all s, 6H, Me), 3.30-3.46, 3.56-3.72 (m, 4H, C2H4).
13
C{1H} NMR
(benzene-d6, 25 °C, 75 MHz) δ (ppm): 11.1, 13.5, 15.4, 16.4, 16.6, 17.1 (Me), 33.9 (C2H4), 72.5, 79.0, 83.4, 88.9, 93.5 (Cs of five-membered ring), 125.6, 125.9, 129.9, 129.3 (Cs at back of six-membered ring). HRMS (EI): Calculated: 480.2479 Found: 480.2482. Elemental Analysis Calculated for C32H40Fe: C, 79.99; H, 8.39. Found: C, 79.87; H, 8.31.
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ACCEPTED MANUSCRIPT Synthesis of [(EBI*)Fe](BF4) AgBF4 and subsequent reactant mixtures were kept in the absence of the light. One equivalent of a rac-/meso- isomeric mix of EBI*Fe (0.050 g, 0.10 mmol) was dissolved in THF, affording a red-brown solution. To this was added with stirring a colourless solution of one equivalent of AgBF4 (0.021 g, 0.10 mmol) in THF, and an immediate grey-green precipitate was formed. The reaction mixture was
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stirred for 1 hour and then uncovered and allowed to stir in the presence of light for a further 15 minutes. The solvent was removed under vacuum and the residue washed with hexane to leave a green-yellow solid, which was further extracted with CH2Cl2 as a red-brown solution. Single crystals of the meso- isomer were obtained as dark black
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plates from the slow diffusion of Et2O into a MeCN solution of [(EBI*)Fe](BF4). X-ray crystallography details
Single crystals of (EBI*)(SnMe3)2, 1R,1’R-1 were grown from a hexane solution,
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C38H58Sn2, Mr = 725.26, Triclinic, P-1, a = 11.8662(3) Å, b = 13.1892(4) Å, c = 13.6148(5) Å, α =75.8139(15), β =85.9655(16), γ =70.9717(13)°, V = 1952.83(11) Å3, Z = 2, T =150 K, plate, colourless, 8140 independent reflections, R1 = 0.0656 wR2 = 0.1069 [I >2σ(I)]. CCDC 1016875.
Single crystals of (EBI*)Fe, rac-2, were grown from a benzene-d6 solution, C32H40Fe, Mr = 480.49, Orthorhombic, Fdd2, a =
28.8919(2) Å, b = 32.1302(3) Å,
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c=10.5664(1) Å, α = β = γ = 90°, V = 9808.81(15) Å3, Z = 16, T =150 K, block, red, 5539 independent reflections, R(int) = 0.050, R1 = 0.029 wR2 = 0.071 [I >2σ(I)]. CCDC 941730.
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Single crystals of meso-[(EBI*)Fe](BF4), meso-3, were grown from the slow diffusion of Et2O into MeCN, C32H40FeBF4, Mr = 567.32, triclinic, P-1, a = 9.6208(2) Å, b =
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10.8366(3) Å, c = 13.5504(3) Å, α = 94.0268(12)°, β = 107.5100(12)° γ = 91.3824(13)°, V = 1342.41(6) Å3, Z = 2, T = 150 K, plate, black,, 5402 independent reflections, R(int) = 0.081, R1 = 0.068 wR2 = 0.182 [I >2σ(I)]. CCDC 941731.
Electrochemistry
Electrochemical experiments were performed in anhydrous CH2Cl2 or THF containing 0.1 molL–1 [NnBu4][PF6] as supporting electrolyte, using a Princeton Applied Research VersaSTAT 3 potentiostat controlled using a PC running V3-Studio software. Cyclic voltammetric and square-wave experiments were performed using a three-electrode configuration with a N2 inlet/outlet bubbler connected to an external oil bubbler and a
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ACCEPTED MANUSCRIPT side-neck fitted with a rubber septum for addition of the samples (G. Glass, Australia). The working electrode used was a Pt disc BASi MF-2013 of 1.6mm diameter, the counter electrode a Pt wire, with a Ag wire as the pseudo-reference electrode. The electrodes were polished prior to each use. Before each sample was run, the empty cell was thoroughly purged with N2, the electrolyte solution thoroughly degassed with N2,
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and a background scan of the solvent window recorded. The sample was dissolved in a small amount of electrolyte, transferred via syringe into the cell, and further degassed. After recording the CV traces, (Cp)2Fe was added as an internal reference and acquisition of the voltammograms repeated. The Ag wire pseudo-reference electrode
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was calibrated to the [(Cp)2Fe]/[(Cp)2Fe]+ couple at +0.46V vs the saturated calomel electrode (SCE) in CH2Cl2 and +0.56V in THF vs SCE, and all potentials are reported vs SCE. A number of different scan rates were used and measurements were
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performed under an inert N2 atmosphere. Thermodynamic half-potentials were obtained by square-wave voltammetry, and reversibility of the redox process was tested by a plot of the maximum anodic (or minimum cathodic) peak current against the square root of the scan rate, giving a straight line in the reversible cases. Comparison of the anodic to cathodic potential difference with that of the internal
Table 5
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[(Cp)2Fe] reference
Crystallographic details for (EBI*)(SnMe3)2, (EBI*)Fe and [(EBI*)Fe](BF4). [(EBI*)Fe](BF4) meso-3 C32H40Fe·BF4
752.26 Triclinic P¯ 1 150 11.8662(3) 13.1892(4) 13.6148(5) 75.8139(15) 85.9655(16) 70.9717(13) 1952.83(11) 2 Mo Kα
480.49 Orthorhombic, Fdd2 150 28.8919 (2) 32.1302 (3) 10.5664 (1)
567.32 Triclinic, P¯ 1 150 9.6208 (2) 10.8366 (3) 13.5504 (3) 94.0268 (12) 107.5100 (12) 91.3824 (13) 1342.41 (6) 2 Mo Kα
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(EBI*)Fe rac-2 C32H40Fe
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Chemical formula Mr Crystal system, space group Temperature (K) a, b, c (Å)
(EBI*)(SnMe3)2 1R,1’R-1 C38H58Sn2
α, β, γ (°) V (Å3) Z Radiation type
9808.81 (15) 16 Mo Kα 20
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5–26
5.1–27.5
5–26
0.62 0.06 × 0.11 × 0.18
0.63 0.32 × 0.10 × 0.08
0.61 0.12 × 0.03 × 0.03
KappaCCD diffractometer Multi-scan Multi-scan from symmetry-related measurements using SORTAV
KappaCCD diffractometer Multi-scan Multi-scan from symmetry-related measurements using SORTAV (Blessing 1995). 0.823, 0.951 55703, 5539, 5165
Area diffractometer Multi-scan DENZO/SCALEPAC K (Otwinowski & Minor, 1997)
0.050 0.649
0.081 0.625
0.029, 0.071, 1.05
0.068, 0.182, 0.98
5539
5402
310
371
1
80
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0.85, 0.92 31472 8808 8140
81040
0
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4904
0.92, 0.98 15333, 5402, 3999
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Tmin, Tmax No. of measured, independent and observed [I > 2σ(I)] reflections Rint (sin θ/λ)max (Å– 1 ) R[F2 > 2s(F2)], wR(F2), S No. of reflections No. of parameters No. of restraints
67533
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Absorption correction
8140
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No. of reflections for cell measurement θ range (°) for cell measurement µ (mm–1) Crystal size (mm) Diffractometer
Acknowledgements
J.-C.B. and T.A.Q.A. would like to thank SCG Chemicals for funding. We thank Chemical Crystallography (University of Oxford) for the use of the diffractometer.
Notes and references
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ACCEPTED MANUSCRIPT Electronic Supplementary Information (ESI) available: [crystallography details]. See DOI: 10.1039/b000000x/
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C. Gibson, S. K. Spitzmesser, Chem. Rev. 103 (2003) 283. (f) G. J. P. Britovsek, V. C. Gibson, D. F. Wass, Angew. Chem., Int. Ed. 38 (1999) 429. (g) L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 100 (2000) 1253.
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Organometallics 30 (2011) 800. (b) P. Ransom, A. E. Ashley, A. L. Thompson, D. O’Hare, J. Organomet. Chem. 694 (2009) 1059.
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New permethylindenyl tin and iron complexes were synthesised and characterised by NMR spectroscopy, X-ray crystallography and the electrochemical behaviour of rac-(EBI*)Fe was studied.
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ELECTRONIC SUPPLEMENTARY DATA
Synthesis,
Characterisation
and
Properties
of
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bis(permethylindenyl) Iron and Tin complexes Paul Ransom, Thomas A. Q. Arnold, Amber L. Thompson, Jean-Charles Buffet, and
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Dermot O’Hare*
1. VT NMR Spectroscopy Figure S1
of 1
2. X-ray crystallography Figures S2-S3
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List of Contents
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3. Depiction of the crystallographic parameters
S1
S2
S3 S3 S4
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1. VT NMR Spectroscopy of 1
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Figure S1: variable temperature NMR spectra of EBI*(SnMe3)2, 1.
S2
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2. X-ray crystallography
Figure S2 Alternate views of rac-(EBI*)Fe, rac-2, with H atoms omitted for clarity (thermal
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ellipsoids drawn at 50%); grey: carbon and brown: iron.
Figure S3 Alternate views of meso-[(EBI*)Fe](BF4), meso-3, with H atoms omitted for clarity (thermal ellipsoids drawn at 50%); grey: carbon and brown: iron.
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ACCEPTED MANUSCRIPT 3. Depiction of the parameters Rotation Angle (RA), Hinge Angle (HA) and α. Alpha (α): An unweighted least-squares plane was calculated for the five Cp carbon atoms of each indenyl moiety (C1, C10, C11, C13, C15 and C18, C19, C21, C23, C32 for rac-(EBI*)Fe shown below) and the angle measured between the two planes calculated calculated using Platon. This is the
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angle complementary to that calculated between the normals to the planes.
Figure S4 Mercury depiction of the planes used to calculate α.
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HA: The angle between the unweighted least-squares plane for the Cp omitting the ipso-C and the unweighted least-squares plane for the ipso-C and its two ortho-C neighbours within the Cp as calculated with Platon. i.e. the angle between the plane defined by C19, C21, C23, C32 below and
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C18, C19, C32. This is the angle complementary to that calculated between the normals to the planes.
Figure S5 Mercury depiction of the planes used to calculate HA.
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ACCEPTED MANUSCRIPT RA: The angle between the two unweighted least-squares planes fitting the central metal and the bridgehead and terminal Cp atoms as calculated by Platon. This plane runs the length of the Ind fragment, approximately perpendicular to it. In the example below the planes are defined by Fe1, C1, C10, C13 and Fe1, C19, C23, C32. This angle is complementary to that calculated between the
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normals to the planes.
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Figure S6 Mercury depiction of the planes used to calculate RA.
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S0022-328X(14)00464-1
DOI:
10.1016/j.jorganchem.2014.10.001
Reference:
JOM 18742
To appear in:
Journal of Organometallic Chemistry
Received Date: 31 July 2014 Revised Date:
25 September 2014
Accepted Date: 1 October 2014
Please cite this article as: P. Ransom, T.A.Q. Arnold, A.L. Thompson, J.-C. Buffet, D. O’Hare, Synthesis, characterisation and properties of bis(permethylindenyl) iron and tin complexes, Journal of Organometallic Chemistry (2014), doi: 10.1016/j.jorganchem.2014.10.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Synthesis,
Characterisation
and
Properties
of
bis(permethylindenyl) Iron and Tin complexes Paul Ransom, Thomas A. Q. Arnold, Amber L. Thompson, Jean-Charles Buffet, and
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Dermot O’Hare* Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford, OX1 3TA, UK.
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*Corresponding author: Tel: +44(0) 1865 272 686. Fax +44 1865 272690 E-mail address: [email protected] (Dermot O’Hare)
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ABSTRACT
Neutral iron complex, (EBI*)Fe (EBI* = ethylenebis(1-hexamethylindenyl), and its cationic analogue, [(EBI*)Fe](BF4), have been synthesised. Both rac- and meso-(EBI*)Fe can be isolated but the isomeric distribution of the reaction products was found to be highly sensitive to the reaction conditions employed. The redox behaviour of rac-(EBI*)Fe in solution has been studied by cyclic voltammetry, it
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shows a one-electron reversible oxidation at –0.7 V vs. (Cp)2Fe/(Cp)2Fe+. Chemical oxidation of (EBI*)Fe with AgBF4 gives [(EBI*)Fe](BF4). The molecular structures of rac-(EBI*)Fe and meso-[(EBI*)Fe](BF4) have been determined by single crystal
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X-ray structure analysis. (EBI*)(SnMe3)2 was synthesised by the reaction of [(EBI*)]Na2 with ClSnMe3 in THF. In solution, only two of the many possible isomers are observed by 1H NMR spectroscopy. Single crystals of (EBI*)(SnMe3)2
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were grown as pale, plates by the slow cooling to –78°C of a saturated hexane solution of a mixture of the isomers. 1H NMR analysis of the crystals showed them to be the isomer with C-SnMe3 peaks at a chemical shift of – 0.27ppm.
Keywords Metallocene, Iron, Tin, Permethylindenyl, Cyclic voltammetry
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Introduction Although cyclopentadienyl ligands have played a pivotal role in the development of organometallic chemistry [1], the related indenyl ligand demonstrates additional features largely based on its flexible hapticity [2-5]. For example, Basolo, Marder and co-workers have exploited this flexible ligation ability with rhodium containing
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complexes [5] while Bradley and co-workers, amongst others, have reported a series of bis(indenyl) iron complexes [6].
Ansa-bridged indenyl metal complexes have also been the subject of intense synthetic activity [8]. It is possible to introduce an unsubstituted ethylene bridge between
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indenyl rings by a salt elimination reaction between 1,2-dibromoethane and two equivalents of an alkali metal indenyl (or cyclopentadienyl) salt as shown by
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Buchwald and co-workers [9a]. For example the synthesis of the ethylenebis(1tetrahydroindenyl) (EBTHI)H2, the unmethylated ligand precursor ethylenebis(1indenyl) (EBI)H2, and the ethylenebis-(alkylsiloxyindenyl) (EBIOSiMe2tBu)H2, which are then deprotonated twice with BuLi to yield the desired dilithium ligand salts [9]. Alternatively, indene can be mixed with 1,2-dibromoethane and an alkylmagnesium compound in heptane, presumably forming an indenyl Grignard compound, which
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then couples with another unit to give (EBI)H2 [10]. These synthetic strategies all involve nucleophilic substitutions by the indenyl moiety. Another route for generating ansa-metallocene complexes with substituted ethylene bridges is the reductive
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coupling of fulvenes. It is mediated by a number of activated metal species from groups 2, 4, 8 and the lanthanides, useful due to the facile synthesis of a wide range of differently substituted fulvenes. Alkali metal reduction of fulvenes has been reported
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to give a variety of products, including coupled fulvenes [11]. We have previously reported the synthesis of an unbridged [12] and silane bridged permethylindenyl (Ind* = C9Me7) iron complexes [7], as well as the synthesis of group 4 [13a], cobalt [13b], and cerium[14] EBI* complexes. Not long after the discovery of ferrocene, cyclopentadienyl tin complexes were reported in this journal by Fritz et al.[15a] While interesting complexes in their own right, a major use was their ability to act as mild (non reducing) ligand transfer reagents. For example, the use of Pn*(SnMe3)2 (where Pn* = C8Me6) is required for the synthesis of Pn*TiCl2 where the Pn*Li2 was found to be too reducing.[15b,c]
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ACCEPTED MANUSCRIPT Herein, we report the synthesis of new iron and tin complexes based on bridged permethylindenyl based ligands.
Synthesis and characterisation of (EBI*)(SnMe3)2
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Results and Discussion
(EBI*)(SnMe3)2, 1, was synthesised by the reaction of (EBI*)Na2 with ClSnMe3 in THF. An immediate colour change from the red (EBI*)Na2 in THF to yellow was observed. The
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reaction of (EBI*)Li2 with ClSnMe3 was also performed both in Et2O and hexane, as shown in Scheme 1, the colour change being less obvious due to the initial insolubility of the lithium
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salt. After filtration and removal of the solvent under vacuum, 1H NMR spectroscopy analysis of the lithium salt reaction residue indicated two main resonances in a region indicative of C-SnMe3 coordination, at –0.27 and –0.29 ppm in benzene-d6. Recrystallisation from hexane afforded two solids, the least hexane soluble enriched in the –0.29 ppm species and the more soluble in the –0.27 ppm compound, in yields of approximately 63 and 14%
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respectively.
Scheme 1. Synthesis of (EBI*)(SnMe3)2, 1; a number of isomers are possible
Several possible regioisomers of (EBI*)(SnMe3)2 may be envisaged. On reaction with [EBI*]Li2, the SnMe3 unit can bond in an η1 fashion at two points on each five-membered ring, at the 1- or 3-position; the 2-position would lead to a loss of aromaticity of the aryl unit and so substitution at this position is expected to be unfavourable, as shown in Scheme 2.
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Alder adducts.[15d]
Scheme 2. Possible coordination sites of the SnMe3 unit on EBI* framework while
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maintaining the aromaticity of the aryl unit. A third site results in unfavourable loss of aromaticity.
In addition, the SnMe3 units may coordinate to the same position or the alternate position on the second indenyl moiety, resulting in the three combinations 1,1′, 1,3′ (or 3,1′), and 3,3′. Lastly, assuming the SnMe3 unit can approach from above or below
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each ring, two possible stereochemistries are possible for each site. This results in a total of ten possible isomers shown schematically in Scheme 3, six of which are diastereomers, and hence NMR distinct (each row of Scheme 3), since identical NMR
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spectra are expected for each image/mirror-image enantiomeric pair.
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Scheme 3 Schematic of the possible isomers of (EBI*)(SnMe3)2, stereochemistry of SnMe3 units shown by Sn wedges and dashes
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It is therefore difficult to predict how many unique environments of SnMe3 and hence peaks would be observed in the NMR spectrum of (EBI*)SnMe3. This is further complicated by the literature precedence for fluxionality in η1-cyclopentadienyl and indenyl compounds of maingroup elements; one possible mechanism being [1,5]-silatropic shifts [16a-c]. These shifts
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take place suprafacially, maintaining the stereochemistry of the C-Si centre. The dynamic properties of a silyl bridged Al ansa-indenyl compound have been studied, and it was found
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that the rac-1,1′ species converts into the rac-1,3′ rapidly at 23 °C (and slowly at –60 °C) via two [1,5]-Al shifts and an anti-aromatic intermediate [16d]. The different 1,3′ species also interconvert (slow at 23 °C, rapid at 70 °C) via [1,5]-H shifts, and the rac-1,1′ to meso-1,1′ via [1,5]-Si (of the silyl bridge) and [1,5]-H shifts. However, in the case of (EBI*)(SnMe3)2, there are no H atoms on the ring periphery, limiting some of the possible interconversion pathways. In ethylidene bridged species, it is often possible to distinguish between 1- and 3- substituted positions using NMR spectroscopy. For example, with the SnMe3 unit in the 3-position the 1H will be a multiplet due to coupling to the protons of the bridge. This NMR approach is not
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ACCEPTED MANUSCRIPT as accessible in (EBI*)(SnMe3)2, due to the permethylation of the ring periphery although a correlated 1H-119Sn approach may be possible. A variable temperature 1H NMR study was performed on a sample of (EBI*)(SnMe3)2, in toluene-d8 between 80 to –80 °C in 20°C intervals (Figure S1). At 20 °C, the resonances ascribed to C-SnMe3 appear at –0.29 and – 0.31 ppm. The spectra are very complicated; at low temperatures these signals separate
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further and move upfield to values of –0.33 and –0.37 ppm at –80 °C. At higher temperatures these peaks coalesce to a low broad signal further upfield, at –0.21 ppm at 80 °C. The resonances ascribed to the EBI* methyl protons lie between approximately 1.61 and 2.50 ppm.
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Single crystals of (EBI*)(SnMe3)2 were grown as very pale yellow, plates by the slow cooling to –78 °C of a saturated hexane solution of a mixture of isomers. The crystal analysed
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was 1R,1'R-(EBI*)(SnMe3)2, 1. 1H NMR analysis of the crystals showed them to be the isomer with C-SnMe3 peaks at a chemical shift of –0.27 ppm. The molecular structure of 1
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obtained is shown in Fig. 1 and selected bond lengths and angles are given in Table 1.
Fig. 1. Molecular structure of 1R,1'R-(EBI*)(SnMe3)2, 1, with H atoms omitted for clarity and thermal ellipsoids are drawn at 50%.
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ACCEPTED MANUSCRIPT Table 1. Selected bond lengths and angles for (EBI*)(SnMe3)2, 1R,1’R-1. Lengths (Å)
Fold Angle Sn1-C3-C4 αa Rotation Angle a
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1.508(6) 1.411(6) 1.463(6) 1.353(6) 1.504(6) 1.535(5) 1.506 1.518 2.226(4)
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1.494(6) C6 - C11 1.405(6) C11 - C12 1.478(6) C12 - C21 1.334(6) C21 - C22 1.510(6) C22 - C6 1.559(5) C6 - C5 1.528(5) 1.506 Avg. C5 - Me 1.518 Avg. C6 - Me 2.225(4) Sn7 - C6 Angles (°) 2.6 112.0(3) 6.1
Fold Angle Sn7-C6-C5 -
2.5 112.7(3)
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C3 - C25 C25 - C32 C32 - C33 C33 - C34 C34 - C3 C3 - C4 C4 - C5 Avg. C5 - Me Avg. C6 - Me Sn1 - C3
ca. 12.6
It can be clearly seen from Fig. 1 that in the isomer of (EBI*)(SnMe3)2 which was analysed by single-crystal X-ray diffraction, each SnMe3 unit is bound in an η1 manner to the carbon in
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the 1-position. This bonding mode can also be inferred from analysis of the intra-ring C-C bond distances; within the C6 ring C-C bond distances are all very similar, and the C33-C34 distance is the shortest within the skeleton at 1.334(6)Å, due to it being a localised double bond. In contrast to the 1,1′ bonding mode observed in Fig. 1, examples of bis-SnMe3
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substituted ansa-indenyl compounds in the literature have the SnR3 units in the 3position.[17] The reasons behind this are unclear, however, a possible explanation is that,
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with the SnMe3 coordinated to the 3-position at positions C21 and C33, C6 and C3 would be sp2 hybridised with planar bonding to the ethylidene bridge. This may result in the two permethylated rings of EBI* becoming too sterically congested. As shown in Fig. 1, the two indenyl moieties of the EBI* unit are relatively planar and almost parallel to one another, with only a small fold angle (FA) observed at the C6-C5 ring junctions. The SnMe3 units are approximately tetrahedral, with angles close to the ideal tetrahedral angle of 109.5°. No structurally characterised ansa-indenyl or cyclopentadienyl Sn species exist in the literature. The closest two species for comparison are the permethylated indenyl and fluorenyl species (Ind*)(SnMe3) [18] and (Flu*)(SnMe3) [19].
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ACCEPTED MANUSCRIPT These have Sn-Cring distances of 2.232(6) and 2.221(3) Å respectively. The value for 1 falls within this range, at 2.225(4) and 2.226(4) Å for the two indenyl moieties. Syntheses and characterisation of ethylenebis(1-hexamethylindenyl) iron complexes
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A number of different syntheses of (EBI*)Fe, 2, were attempted, as shown in Scheme 4. The rac-/meso- isomeric distribution of the reaction products was found to be highly sensitive to the reaction conditions employed, varying with the choice of alkali metal ligand salt, source of iron starting materials, reaction temperature, and crystallisation procedures.
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The stoichiometric reaction of (EBI*)K2 with FeCl2·THF1.5 in THF at –78 °C resulted after work-up in the isolation of isomerically pure meso-(EBI*)Fe, meso-2. The use of
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(EBI*)Li2·THF0.38 with FeCl2·THF1.5 in THF at –78 °C afforded, after work-up, a mixture of rac- and meso-isomers; however, the pure rac-form was obtained at – 196 °C. Similarly, the use of EBI*Na2 with FeCl2·THF1.5 in THF at –78 °C resulted in a mixture of rac- and meso-(EBI*)Fe. However, reaction of Fe(acac)2 with EBI*Na2 at
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70 °C in THF was also found to yield pure rac-(EBI*)Fe.
Scheme 4. Various syntheses of rac- and meso-(EBI*)Fe, 2.
The literature details only five ansa-bridged indenyl Fe compounds, all synthesised by reaction of an appropriate ligand precursor with FeCl2. One of these has the ansa-bridge in a position which imparts no rac/meso stereochemistry upon the products.[20] In all the other four cases, a mixture of isomers was obtained upon reaction, which were separated and 8
ACCEPTED MANUSCRIPT isolated either through crystallization [7,21] or via an alumina chromatography column [22,23]. The unbridged analogue (Ind*)2Fe is synthesised by the reaction of the lithium salt with FeCl2·2THF in THF at room temperature [12]. Each of the 1H NMR spectra of rac- and meso-(EBI*)Fe show six distinct resonances attributed to the methyl groups on the EBI* ring periphery, together with a AA'BB' multiplet
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ascribed to the ethylene bridge. The methyl resonances are within the range 1.35 - 2.66 ppm. The 13C{1H} NMR spectrum for each isomer shows six resonances in the range 8 – 18 ppm which can be ascribed to the methyl carbon atoms, and those of the ethylene bridge at approximately 35 ppm. For each species, five signals are seen between 70 – 95 ppm from the
membered ring observed between 125 – 135 ppm.
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carbon atoms of the five-membered ring, with the four carbon atoms at the back of the six-
Fig. 2 depicted the various geometric parameters used in the crystallographic discussion
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throughout.
.
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Fig. 2. Schematic depictions defining the various geometric parameters. Single crystals suitable for X-ray diffraction were grown in two different ways; as dark red needles from the slow evaporation of an isomerically pure benzene-d6
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solution, or as large brown hexagonal plates from cooling a mixture of rac- and mesoisomers in benzene-d6 to 5 °C. The molecular structure of rac-(EBI*)Fe, rac-2, is depicted Fig. 3. The compound crystallises in the orthorhombic space group Fdd2.
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Alternate views are shown in Fig. S2, and selected bond lengths and angles are given in Table 2. As shown in Fig. 3, rac-2 is only pseudo-C2 symmetric; the molecule does not possess a C2 axis and only one EBI* moiety exists in the asymmetric unit, with each indenyl ring crystallographically distinct. The rac-bonding mode of the EBI* ligand to the Fe centre results in a rotation angle (RA) of 148.9(3)°. This value is only slightly less than that observed in the unbridged bisheptamethylindenyl species [(Ind*)2Fe] at 151.3°;[21] hence, it appears that, in this case, the ethylene bridge does not restrain the conformation of the two indenyl rings of the EBI* moiety about the Fe atom. The indenyl rings exhibit only a slight fold angle of 4.5°. The average Fe-Cpcent distance is 0.065 Å shorter than in the 19-valence electron species rac-(EBI*)Co 9
ACCEPTED MANUSCRIPT [13b], despite the decreasing atomic size across the period, attributed to the extra electron in an antibonding orbital for the cobalt congener. The average Fe-C distance, 2.04806 Å, agrees well, however, with that reported for SBI*Fe, SBI* = Me2Si(C9Me6)2, 2.0458 Å.[7] The tilt angle, α, is 16.75(11)°, lower than that of two other
ethylene-bridged
Fe
species
in
the
literature;
23.0°
for
{(4,7-
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Me2C9H4)2(CHPh)2}Fe [21] and 21.6° for {(C5H4)2(CH2)2}Fe [25], assumed to be because the indenyl groups are sterically bulkier. This value is larger than α reported for the SiMe2-bridged analogue (13.4°),[7] where we can infer that the ethylene bridge
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imparts more strain than its silicon counterpart.
Fig. 3. Molecular structure of rac-(EBI*)Fe, rac-2. For clarity, all hydrogens atoms have been omitted (displacement ellipsoids are drawn at 50% probability). Selected distances
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Fe(1)-Cpcent 1.6386(9) Å and Fe(1)-Cpcent’ 1.6381(9) Å.
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ACCEPTED MANUSCRIPT Table 2. Selected bond lengths and angles for rac-(EBI*)Fe, rac-2. Lengths (Å) 1.9880(18) 2.0477(18) 2.0619(19) 2.0724(19) 2.0735(18) 1.436(3) 1.443(3) 1.452(3) 1.454(3) 1.453(3) 1.528(3)
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2.0736(19) Fe1 - C18 2.0789(19) Fe1 - C19 2.0634(18) Fe1 - C21 2.0428(18) Fe1 - C23 1.9864(18) Fe1 - C32 1.459(3) C18 - C19 1.451(3) C19 - C21 1.433(3) C21 - C23 1.444(3) C23 - C32 1.450(3) C32 - C18 1.527(3) C18 - C17 1.554(2) 1.502 Avg. C5 - Me 1.514 Avg. C6 - Me 1.6386(9) Fe1 - Cpcent Angles (°) 4.64(10) 16.75(11) 6.30(19)
Fold Angle Hinge Angle -
148.9(3)
1.497 1.512 1.6381(9)
4.4(10) 7.08(18)
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Fold Angle α Hinge Angle Rotation Angle
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Fe1 - C1 Fe1 - C10 Fe1 - C11 Fe1 - C13 Fe1 - C15 C1 - C10 C10 - C11 C11 - C13 C13 - C15 C15 - C1 C15 - C16 C16 - C17 Avg. C5 - Me Avg. C6 - Me Fe1 - Cpcent
Numerous attempts directed towards the chemical oxidation of rac-/meso- isomeric mixture of (EBI*)Fe were made. In the unbridged case, the dropwise addition of [(Cp)2Fe](PF6) in MeCN to a solution of (Ind*)2Fe in THF yields [(Ind*)2Fe](PF6)
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[12]. This was not successful in the case of (EBI*)Fe across a range of solvents, nor was the alternate use of [(Cp)2Fe](BF4) as oxidising agent. However, upon addition of
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AgBF4 in THF to a red-brown THF solution of (EBI*)Fe, an immediate grey-green precipitate was observed. Filtration and washing with hexane afforded a black solid of [(EBI*)Fe](BF4), 3. The 1H NMR spectrum of 3 was paramagnetically broadened, and extremely air sensitive which made the EPR and Mossbauer studies very difficult to perform. Sufficient single crystals of meso-[(EBI*)Fe](BF4), meso-3, suitable for Xray diffraction were grown as black plates by the slow diffusion of Et2O into a MeCN solution of [(EBI*)Fe](BF4) synthesised from a rac-/meso- isomeric mix of (EBI*)Fe, the molecular structure is depicted Fig. 4. We were unable to obtain a bulk sample or separate the rac- and meso isomers.
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Fig. 4. Molecular structure of meso-[(EBI*)Fe](BF4), meso-3. For clarity, hydrogens atoms
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and the [BF4]− counterion were omitted (displacement ellipsoids are drawn at 50% probability. Selected distances (Å) Fe1-Cpcent 1.6812(19) and Fe1-Cpcent’ 1.6381(9). Table 3. Selected bond lengths and angles for meso-[(EBI*)Fe](BF4), meso-3. Lengths (Å)
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2.106(4) Fe1 - C17 2.078(4) Fe1 - C18 2.065(4) Fe1 - C19 2.010(4) Fe1 - C20 2.117(4) Fe1 - C21 1.458(6) C17 - C18 1.431(6) C18 - C19 1.430(6) C19 - C20 1.461(5) C20 - C21 1.455(5) C21 - C17 1.551(5) C17 - C16 1.552(6) 1.497 Avg. C5 - Me 1.506 Avg. C6 - Me 1.6812(19) Fe1 - Cpcent Angles (°)
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Fe1 - C2 Fe1 - C3 Fe1 - C4 Fe1 - C5 Fe1 - C6 C2 - C3 C3 - C4 C4 - C5 C5 - C6 C6 - C2 C5 - C15 C15 - C16 Avg. C5 - Me Avg. C6 - Me Fe1 - Cpcent
C6 - C5 planes α Hinge Angle Rotation Angle
7.3(2) 19.2(2) 9.4(4)
C6 - C5 planes Hinge Angle -
21.7(5)
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2.022(4) 2.148(4) 2.095(4) 2.076(4) 2.074(4) 1.452(6) 1.453(6) 1.468(6) 1.414(6) 1.449(6) 1.548(6) 1.496 1.510 1.6719(19)
6.0(2) 7.2(4)
ACCEPTED MANUSCRIPT The compound crystallises in the triclinic space group P-1, and the asymmetric unit contains one EBI* moiety and one disordered BF4 molecule. Alternate views are shown in Fig. S3, and relevant bond distances and angles are given in Table 3. Figure 4 shows the EBI* ligand bonding to the Fe centre in a meso- mode, corresponding to a RA of 21.7(5)°. This is only 4.2° higher than the value in meso-[(EBI*)Co](BF4)
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[13b], and may be due to differences in the packing of molecules and counterions in the crystal.
Unlike in the Co analogue, oxidation affords a 17-valence electron species and occurs via the removal of an electron from a bonding molecular orbital. Thus, as expected,
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the Fe-Cpcent distance increases compared to rac-(EBI*)Fe upon oxidation, by an average of 0.038 Å due to the weakening of the Fe-EBI* bonding interaction. Oxidation also results in other structural changes within the ligand framework such as
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the increase in hinge angle (HA) by an average of 1.6°. This is concomitant with the slight loss of aromaticity and reduction from an ideal η5 bonding situation. The unbridged analogue, (Ind*2)Fe has been oxidised to form [(Ind*)2Fe](PF6); however, its crystal structure was not determined. The structure of the charge-transfer salt [(Ind*)2Fe][TCNQ] has, however, been reported [28]. In this species, the RA is consistent with those reported for other [(Ind*)2M]+ cations [29], as the value of meso-
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3 is with meso-[(EBI*)Co]+.
The electrochemical behaviour of rac-(EBI*)Fe was studied as a solution in CH2Cl2 with 0.1 molL–1 [NnBu4][PF6] as the supporting electrolyte. Data were recorded at room at
scan
rates
of
50/100/200 mVs–1,
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temperature
referenced
internally
to
the
[(Cp)2Fe]/[(Cp)2Fe+] couple at +0.46 V vs SCE under identical conditions [24]. Selected
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examples of the cyclic voltammograms recorded are shown in Fig. 5.
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Fig. 5. Cyclic voltammograms in CH2Cl2 of rac-(EBI*)Fe (top) and rac-(EBI*)Fe with
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internal (Cp)2Fe reference (bottom). Thermodynamic half-potentials were determined by
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square-wave voltammetry
The thermodynamic half-potentials were obtained by square-wave voltammetry, and rac-(EBI*)Fe showed a one-electron reversible oxidation at –0.24 V vs SCE. Interestingly, closer analysis of the cyclic voltammetric trace of rac-(EBI*)Fe shows a slight shoulder; although the sample used was mainly rac-(EBI*)Fe by NMR analysis, this shoulder may be attributed to the sample containing a small amount of the meso-isomer. A general trend can clearly be seen that upon permethylation of the cyclopentadiene (Cp) or indene (Ind) ligands, the oxidation potentials for the metallocenes formed move to more negative values, demonstrating the greater electron releasing power of the methylated species. This effect is more pronounced in the Cp case [24]. Furthermore, (Ind)2Fe has a 14
ACCEPTED MANUSCRIPT lower oxidation potential than (Cp)2Fe, and similarly (Ind*)2Fe is more readily electrochemically oxidised than (Cp*)2Fe [25]. The oxidation potential of (Ind*)2Fe can be used as an unbridged, and hence unstrained, reference compound for rac-(EBI*)Fe, which has an intermediary value between those of (Ind*)2Fe and (Cp*)2Fe. All the species given in Table 3 contain 18-valence electrons, and the HOMO for (Ind*)2Fe is regarded as being the
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antibonding A1 orbital [12]. This would suggest that the effect of the ansa-bridge is to lower the energy of the HOMO of rac-(EBI*)Fe relative to that of (Ind*)2Fe, leading to a less negative oxidation potential. This agrees with the work of Green who suggested that ansabridges make metallocenes more electrophilic.[26] (Ind*)2Fe showed a second reversible oxidation was observed in the case of rac-(EBI*)Fe.
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oxidation at +1.28 V vs SCE,[12] rare for 18-electron metallocenes; however, no such second
Table 4. Selected bond lengths and angles for rac-(EBI*)Fe.
[(Cp)2Fe] [(Cp*)2Fe] [(Ind)2Fe] [(Ind*)2Fe] rac-[(EBI*)Fe]
Ref
24 24 12 12 This work
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Conclusions
E1/2 (V vs SCE) 0.46 -0.13 0.13 -0.32 -0.24
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Compound
We report the synthesis and characterisation of the new ansa-bis(peralkylindenyl) iron and tin complexes, (EBI*)(SnMe3)2, 1, (EBI*)Fe, 2, and [(EBI*)Fe](BF4), 3. Only two of the many possible isomers of 1 were observed by 1H NMR spectroscopy Both
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rac- and meso-(EBI*)Fe can be isolated but the isomeric distribution of the reaction products was found to be highly sensitive to the reaction conditions employed.
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Electrochemical studies show that rac-(EBI*)Fe, rac-2, has a redox couple which is intermediate between that of (Ind*)2Fe and [(Cp*)2Fe]. The extremely air- and moisture-sensitive 17-electron cation [(EBI*)Fe](BF4) can be prepared by chemical oxidation of a solution of (EBI*)Fe to obtain meso-3. One isomer of each new material was characterised by X-ray crystallography: 1R,1′R-(EBI*)(SnMe3)2, rac(EBI*)Fe and meso-[(EBI*)Fe](BF4).
15
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Materials and methods General procedure: Air and moisture sensitive reactions were performed on a dual-manifold vacuum/N2 line using standard Schlenk techniques, or in a N2 filled MBraun Unilab glovebox. Hexane and toluene were dried using a Braun SPS-800 solvent purification system.
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Et2O and THF were dried at reflux over Na/benzophenone and distilled under N2. Hexane, toluene and Et2O were stored over K mirrors. THF was stored over activated 3 Å molecular sieves. Benzene-d6 (99%) was obtained from Goss Scientific, dried before being vacuum transferred prior to use.
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over K and preactivated 3 Å molecular sieves, freeze-pump-thaw degassed three times
Solution NMR samples were prepared in the glovebox under N2 atmosphere in 13
C{1H} NMR spectra were recorded on 300 MHz
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Young’s tap NMR tubes. 1H and
Varian VX-Works spectrometers. All chemical shifts were expressed as δ, in parts per million (ppm) relative to TMS (δ = 0). Crystals were mounted on MiTeGen MicroMounts using perfluoropolyether oil, and cooled rapidly to 150 K in a stream of cold nitrogen using an Oxford Cryosystems CRYOSTREAM unit [30]. Data collections were performed using an Enraf-Nonius FR590 Kappa CCD diffractometer,
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utilising graphite-monochromated Mo Kα X-ray radiation (λ = 0.71073 Å). Raw frame data were collected at 150(2) K using a Nonius Kappa CCD diffractometer, reduced using DENZO-SMN [31] and corrected for absorption using SORTAV [32]. The structure was solved using SuperFlip [33] and refined using full matrix least-squares
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using CRYSTALS [34,35]. Interplanar distances and angles were calculated using PLATON [36,37].
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Synthesis of (EBI*)(SnMe3)2
(EBI*)Li2 (0.39 g, 0.00089 mol) was slurried in hexane and cooled to –78°C. To this buff coloured slurry was added a solution of ClSnMe3 (0.35 g, 0.00177 mol) in hexane. The reaction mixture was allowed to warm to room temperature and stirred for 15 hours affording a cream-yellow suspension. On settling and filtration, an off white powder with yellow supernatant was obtained. Removal of the solvent under vacuum afforded 0.32 g yellow solid, shown by NMR analysis to be an isomer with C-SnMe3 shifts at –0.27 ppm. The offwhite solid was extracted with 60 °C toluene and the solution filtered through celite. Removal of the solvent from this very light yellow solution afforded 0.07 g off-white solid, shown by
16
ACCEPTED MANUSCRIPT 1
H NMR spectroscopy analysis to be consistent of an isomer with C-SnMe3 peaks at –
0.29 ppm. Total yield: 0.39 g, 77%. 1
H NMR (benzene-d6, 25 °C, 300.1 MHz) δ (ppm): Isomer I: –0.27 (s, 18H, SnMe3), 2.03,
2.09, 2.20, 2.30, 2.40, 2.50 (all s, 6H, Me), 2.55 (bs, 4H, C2H4). Isomer II –0.29 (s, 18h, SnMe3), 1.91, 2.17, 2.29, 2.41, 2.44, 2.50 (all s, 6H, Me), 2.58 (bs, 4H, C2H4). MS (EI): Calc:
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752.26 Found: 752.23. Anal. Calc for C38H58Sn2: C, 60.67; H, 7.77. Found: C, 60.58; H, 7.83. Synthesis of (EBI*)Fe
(EBI*)Na2 + Fe(acac)2: One equivalent of (EBI*)Na2 (0.015 g, 0.032 mmol) and one equivalent of Fe(acac)2 (0.008 g, 0.032 mol) were added to an NMR tube. After
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addition of 1 mL of THF, the tube was sonicated for 30 minutes then heated to 70 °C for 15 hours. Removal of the solvent left a brown solid, which was dissolved in benzene-d6 and filtered through celite. NMR analysis showed pure rac-(EBI*)Fe.
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(EBI*)Li2·THF0.38 + FeCl2·THF1.5: Route A: One equivalent of (EBI*)Li2 (0.25 g, 0.057 mmol) and one equivalent of FeCl2·THF1.5 (0.13 g, 0.057 mmol) were added to a Schlenk, which was cooled to –196 °C and THF added. The frozen mixture was allowed to warm to room temperature with stirring, and the red-brown mixture stirred for 15 hours. Removal of the solvent under vacuum left a red-brown solid which was
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extracted with hexane. The resulting brown solution was reduced to a minimum volume and cooled to –78 °C. The supernatant was removed via cannula and the black solid washed with –78 °C hexane. NMR analysis showed pure rac-(EBI*)Fe. Single crystals of rac-(EBI*)Fe suitable for X-ray diffraction were obtained as dark red
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needles by the slow evaporation of this pure benzene-d6 solution. Yield of rac-(EBI*)Fe: 0.11 g, 40%. Route B: One equivalent of FeCl2·THF1.5 (0.25 g,
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1.07 mmol) was slurried in THF, cooled to –78 °C and added to a slurry of one equivalent of (EBI*)Li2·THF0.38 (0.50 g, 1.07 mmol) in THF at –78 °C. The reaction mixture was stirred and allowed to warm to room temperature, going dark red-brown above approximately –5 °C. After stirring at room temperature for 15 hours the solvent was removed under vacuum affording a black-brown solid, which was extracted with hexane at 60 °C, reduced to a minimum volume and cooled to –78 °C. The resulting solid was washed with hexane at –78 °C to leave a black crystalline solid. Analysis by NMR showed a 1.35:1 rac/meso mixture, Yield of a rac-/meso- isomeric mix of EBI*Fe: 0.23 g, 45%. Single crystals of rac-(EBI*)Fe suitable for X-ray diffraction grew as large brown hexagons from cooling this benzene-d6 rac/meso mixture to 5 °C.
17
ACCEPTED MANUSCRIPT (EBI*)K2 + FeCl2·THF1.5 One equivalent of (EBI*)K2 (0.15 g, 0.30 mmol) was slurried in THF and cooled to –78 °C, and one equivalent of slurry of FeCl2·THF1.5 (0.070g, 0.30 mmol) in THF was added. The initial beige slurry darkened immediately, and was allowed to warm to room temperature with stirring. The redbrown reaction mixture was stirred for 15 hours, and the solvent removed under
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vacuum. The resulting brown solid was extracted with hexane at 60 °C to give a dark brown solution, which was reduced to a minimum volume and cooled to –78 °C. A black-brown solid was deposited and filtered, shown by NMR analysis to be isomerically pure meso-(EBI*)Fe. The supernatant was further reduced in volume and
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cooled to –78 °C, and the resulting solid collected and washed with hexane at –78 °C. NMR analysis showed a 1.44:1 mix of rac/meso-isomers. Yield of a rac-/mesoisomeric mix of (EBI*)Fe: 0.025 g, 0.041 g, total 46%.
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(EBI*)Na2 + FeCl2·THF1.5 One equivalent of FeCl2·THF1.5 (0.075 g, 0.32 mmol) was slurried in THF, cooled to –78 °C and added to a yellow slurry of one equivalent of (EBI*)Na2 (0.15g, 0.32 mmol) in THF at –78 °C. The stirred reaction mixture darkened, becoming dark brown upon warming to room temperature, and was stirred for a further 15 hours. Removal of the solvent under vacuum and extraction with
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hexane at 60 °C afforded a dark brown solution, which was reduced to a minimum volume and cooled to –78 °C. The black solid deposited was filtered and washed with hexane at –78 °C and was shown by NMR analysis to be a 1:1.75 rac/meso mixture. Yield of rac-/meso- isomeric mixture of (EBI*)Fe: 0.064 g, 42%.
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rac-(EBI*)Fe:1H NMR (benzene-d6, 25 °C, 300.1 MHz) δ (ppm): 1.35, 1.65, 2.21, 2.23, 2.46, 2.66 (all s, 6H, Me), 3.42-3.57, 3.77-3.92 (m, 4H, C2H4).
13
C{1H} NMR
AC C
(benzene-d6, 25 °C, 75 MHz) δ (ppm): 8.4, 12.0, 16.6, 17.2, 17.4, 17.5 (Me), 35.1 (C2H4), 70.1, 81.0, 85.0, 89.7, 90.6 (Cs of five-membered ring), 127.9, 129.0, 132.0, 133.3 (Cs at back of six-membered ring). meso-(EBI*)Fe: 1H NMR (benzene-d6, 25 °C, 300.1 MHz) δ (ppm): 1.90, 1.99, 2.03, 2.10, 2.20, 2.38 (all s, 6H, Me), 3.30-3.46, 3.56-3.72 (m, 4H, C2H4).
13
C{1H} NMR
(benzene-d6, 25 °C, 75 MHz) δ (ppm): 11.1, 13.5, 15.4, 16.4, 16.6, 17.1 (Me), 33.9 (C2H4), 72.5, 79.0, 83.4, 88.9, 93.5 (Cs of five-membered ring), 125.6, 125.9, 129.9, 129.3 (Cs at back of six-membered ring). HRMS (EI): Calculated: 480.2479 Found: 480.2482. Elemental Analysis Calculated for C32H40Fe: C, 79.99; H, 8.39. Found: C, 79.87; H, 8.31.
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ACCEPTED MANUSCRIPT Synthesis of [(EBI*)Fe](BF4) AgBF4 and subsequent reactant mixtures were kept in the absence of the light. One equivalent of a rac-/meso- isomeric mix of EBI*Fe (0.050 g, 0.10 mmol) was dissolved in THF, affording a red-brown solution. To this was added with stirring a colourless solution of one equivalent of AgBF4 (0.021 g, 0.10 mmol) in THF, and an immediate grey-green precipitate was formed. The reaction mixture was
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stirred for 1 hour and then uncovered and allowed to stir in the presence of light for a further 15 minutes. The solvent was removed under vacuum and the residue washed with hexane to leave a green-yellow solid, which was further extracted with CH2Cl2 as a red-brown solution. Single crystals of the meso- isomer were obtained as dark black
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plates from the slow diffusion of Et2O into a MeCN solution of [(EBI*)Fe](BF4). X-ray crystallography details
Single crystals of (EBI*)(SnMe3)2, 1R,1’R-1 were grown from a hexane solution,
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C38H58Sn2, Mr = 725.26, Triclinic, P-1, a = 11.8662(3) Å, b = 13.1892(4) Å, c = 13.6148(5) Å, α =75.8139(15), β =85.9655(16), γ =70.9717(13)°, V = 1952.83(11) Å3, Z = 2, T =150 K, plate, colourless, 8140 independent reflections, R1 = 0.0656 wR2 = 0.1069 [I >2σ(I)]. CCDC 1016875.
Single crystals of (EBI*)Fe, rac-2, were grown from a benzene-d6 solution, C32H40Fe, Mr = 480.49, Orthorhombic, Fdd2, a =
28.8919(2) Å, b = 32.1302(3) Å,
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c=10.5664(1) Å, α = β = γ = 90°, V = 9808.81(15) Å3, Z = 16, T =150 K, block, red, 5539 independent reflections, R(int) = 0.050, R1 = 0.029 wR2 = 0.071 [I >2σ(I)]. CCDC 941730.
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Single crystals of meso-[(EBI*)Fe](BF4), meso-3, were grown from the slow diffusion of Et2O into MeCN, C32H40FeBF4, Mr = 567.32, triclinic, P-1, a = 9.6208(2) Å, b =
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10.8366(3) Å, c = 13.5504(3) Å, α = 94.0268(12)°, β = 107.5100(12)° γ = 91.3824(13)°, V = 1342.41(6) Å3, Z = 2, T = 150 K, plate, black,, 5402 independent reflections, R(int) = 0.081, R1 = 0.068 wR2 = 0.182 [I >2σ(I)]. CCDC 941731.
Electrochemistry
Electrochemical experiments were performed in anhydrous CH2Cl2 or THF containing 0.1 molL–1 [NnBu4][PF6] as supporting electrolyte, using a Princeton Applied Research VersaSTAT 3 potentiostat controlled using a PC running V3-Studio software. Cyclic voltammetric and square-wave experiments were performed using a three-electrode configuration with a N2 inlet/outlet bubbler connected to an external oil bubbler and a
19
ACCEPTED MANUSCRIPT side-neck fitted with a rubber septum for addition of the samples (G. Glass, Australia). The working electrode used was a Pt disc BASi MF-2013 of 1.6mm diameter, the counter electrode a Pt wire, with a Ag wire as the pseudo-reference electrode. The electrodes were polished prior to each use. Before each sample was run, the empty cell was thoroughly purged with N2, the electrolyte solution thoroughly degassed with N2,
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and a background scan of the solvent window recorded. The sample was dissolved in a small amount of electrolyte, transferred via syringe into the cell, and further degassed. After recording the CV traces, (Cp)2Fe was added as an internal reference and acquisition of the voltammograms repeated. The Ag wire pseudo-reference electrode
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was calibrated to the [(Cp)2Fe]/[(Cp)2Fe]+ couple at +0.46V vs the saturated calomel electrode (SCE) in CH2Cl2 and +0.56V in THF vs SCE, and all potentials are reported vs SCE. A number of different scan rates were used and measurements were
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performed under an inert N2 atmosphere. Thermodynamic half-potentials were obtained by square-wave voltammetry, and reversibility of the redox process was tested by a plot of the maximum anodic (or minimum cathodic) peak current against the square root of the scan rate, giving a straight line in the reversible cases. Comparison of the anodic to cathodic potential difference with that of the internal
Table 5
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[(Cp)2Fe] reference
Crystallographic details for (EBI*)(SnMe3)2, (EBI*)Fe and [(EBI*)Fe](BF4). [(EBI*)Fe](BF4) meso-3 C32H40Fe·BF4
752.26 Triclinic P¯ 1 150 11.8662(3) 13.1892(4) 13.6148(5) 75.8139(15) 85.9655(16) 70.9717(13) 1952.83(11) 2 Mo Kα
480.49 Orthorhombic, Fdd2 150 28.8919 (2) 32.1302 (3) 10.5664 (1)
567.32 Triclinic, P¯ 1 150 9.6208 (2) 10.8366 (3) 13.5504 (3) 94.0268 (12) 107.5100 (12) 91.3824 (13) 1342.41 (6) 2 Mo Kα
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(EBI*)Fe rac-2 C32H40Fe
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Chemical formula Mr Crystal system, space group Temperature (K) a, b, c (Å)
(EBI*)(SnMe3)2 1R,1’R-1 C38H58Sn2
α, β, γ (°) V (Å3) Z Radiation type
9808.81 (15) 16 Mo Kα 20
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5–26
5.1–27.5
5–26
0.62 0.06 × 0.11 × 0.18
0.63 0.32 × 0.10 × 0.08
0.61 0.12 × 0.03 × 0.03
KappaCCD diffractometer Multi-scan Multi-scan from symmetry-related measurements using SORTAV
KappaCCD diffractometer Multi-scan Multi-scan from symmetry-related measurements using SORTAV (Blessing 1995). 0.823, 0.951 55703, 5539, 5165
Area diffractometer Multi-scan DENZO/SCALEPAC K (Otwinowski & Minor, 1997)
0.050 0.649
0.081 0.625
0.029, 0.071, 1.05
0.068, 0.182, 0.98
5539
5402
310
371
1
80
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0.85, 0.92 31472 8808 8140
81040
0
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4904
0.92, 0.98 15333, 5402, 3999
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Tmin, Tmax No. of measured, independent and observed [I > 2σ(I)] reflections Rint (sin θ/λ)max (Å– 1 ) R[F2 > 2s(F2)], wR(F2), S No. of reflections No. of parameters No. of restraints
67533
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Absorption correction
8140
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No. of reflections for cell measurement θ range (°) for cell measurement µ (mm–1) Crystal size (mm) Diffractometer
Acknowledgements
J.-C.B. and T.A.Q.A. would like to thank SCG Chemicals for funding. We thank Chemical Crystallography (University of Oxford) for the use of the diffractometer.
Notes and references
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ACCEPTED MANUSCRIPT Electronic Supplementary Information (ESI) available: [crystallography details]. See DOI: 10.1039/b000000x/
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[28] V. J. Murphy, D. O’Hare, Inorg. Chem., 1994, 33, 1833. [29] J. L. Robbins, N. M. Edelstein, S. R. Cooper, J. C. Smart, J. Am. Chem. Soc. 101 (1979)
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New permethylindenyl tin and iron complexes were synthesised and characterised by NMR spectroscopy, X-ray crystallography and the electrochemical behaviour of rac-(EBI*)Fe was studied.
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ELECTRONIC SUPPLEMENTARY DATA
Synthesis,
Characterisation
and
Properties
of
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bis(permethylindenyl) Iron and Tin complexes Paul Ransom, Thomas A. Q. Arnold, Amber L. Thompson, Jean-Charles Buffet, and
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Dermot O’Hare*
1. VT NMR Spectroscopy Figure S1
of 1
2. X-ray crystallography Figures S2-S3
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List of Contents
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3. Depiction of the crystallographic parameters
S1
S2
S3 S3 S4
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1. VT NMR Spectroscopy of 1
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Figure S1: variable temperature NMR spectra of EBI*(SnMe3)2, 1.
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2. X-ray crystallography
Figure S2 Alternate views of rac-(EBI*)Fe, rac-2, with H atoms omitted for clarity (thermal
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ellipsoids drawn at 50%); grey: carbon and brown: iron.
Figure S3 Alternate views of meso-[(EBI*)Fe](BF4), meso-3, with H atoms omitted for clarity (thermal ellipsoids drawn at 50%); grey: carbon and brown: iron.
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ACCEPTED MANUSCRIPT 3. Depiction of the parameters Rotation Angle (RA), Hinge Angle (HA) and α. Alpha (α): An unweighted least-squares plane was calculated for the five Cp carbon atoms of each indenyl moiety (C1, C10, C11, C13, C15 and C18, C19, C21, C23, C32 for rac-(EBI*)Fe shown below) and the angle measured between the two planes calculated calculated using Platon. This is the
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angle complementary to that calculated between the normals to the planes.
Figure S4 Mercury depiction of the planes used to calculate α.
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HA: The angle between the unweighted least-squares plane for the Cp omitting the ipso-C and the unweighted least-squares plane for the ipso-C and its two ortho-C neighbours within the Cp as calculated with Platon. i.e. the angle between the plane defined by C19, C21, C23, C32 below and
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C18, C19, C32. This is the angle complementary to that calculated between the normals to the planes.
Figure S5 Mercury depiction of the planes used to calculate HA.
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ACCEPTED MANUSCRIPT RA: The angle between the two unweighted least-squares planes fitting the central metal and the bridgehead and terminal Cp atoms as calculated by Platon. This plane runs the length of the Ind fragment, approximately perpendicular to it. In the example below the planes are defined by Fe1, C1, C10, C13 and Fe1, C19, C23, C32. This angle is complementary to that calculated between the
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normals to the planes.
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Figure S6 Mercury depiction of the planes used to calculate RA.
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