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Organometallics 2009, 28, 2112–2118
Bis[(trimethylsilyl)methyl]manganese: Structural Variations of Its Solvent-Free and TMEDA-, Pyridine-, and Dioxane-Complexed Forms Antonio Alberola,† Victoria L. Blair,‡ Luca M. Carrella,§ William Clegg,⊥ Alan R. Kennedy,‡ Jan Klett,‡ Robert E. Mulvey,*,‡ Sean Newton,‡ Eva Rentschler,§ and Luca Russo⊥ WestCHEM, Department of Pure & Applied Chemistry, UniVersity of Strathclyde, Glasgow, Scotland G1 1XL, United Kingdom, Institut fu¨r Anorganische Chemie and Analytische Chemie, Johannes-Gutenberg-UniVersita¨t Mainz, Duesbergweg 10-14, 55128 Mainz, Germany, School of Natural Sciences (Chemistry), Newcastle UniVersity, Newcastle upon Tyne NE1 7RU, United Kingdom, and Departament de Quı´mica Fı´sica i Analı´tica, UniVersitat Jaume I, AVda. Sos Baynat s/n, E-12071 Castello, Spain ReceiVed NoVember 27, 2008
First synthesized in 1976 and recently taking on a new significance as a key precursor to heterobimetallic alkali-metal-manganese(II) complexes, bis[(trimethylsilyl)methyl]manganese has been structurally characterized by X-ray crystallography. It forms a polymeric chain structure of formula [{Mn(CH2SiMe3)2}∞], 1, in which distorted tetrahedral, spiro Mn atoms are linked together via µ2-bonding alkyl ligands. The structure is notable for displaying two distinct categories of Mn-C bond lengths with a mean size differential of 0.225 Å and for being the first fully crystallographically characterized polymeric manganese(II) dialkyl compound. Magnetic measurements of 1 indicate a surprisingly strong spin exchange coupling of J ≈ -45 cm-1 between the manganese ions aligned along the chain. Four Lewis base complexes of bis[(trimethylsilyl)methyl]manganese have also been subjected to X-ray crystallographic studies. Previously known [TMEDA · Mn(CH2SiMe3)2], 2, and [(pyridine)2Mn(CH2SiMe3)2], 3, both adopt a simple monomeric arrangement with C2N2 distorted tetrahedral coordinations of the metal atom. Synthesized by direct addition of the Lewis base to 1, two further, new complexes, [{(dioxane)[Mn(CH2SiMe3)2]2}∞], 4, and [{(dioxane)[Mn(CH2SiMe3)2]}∞], 5, are also reported. Hemisolvate 4 displays dimeric [(Me3SiCH2)Mn(µ-CH2SiMe3)2Mn(CH2SiMe3)] subunits, whereas 1:1 solvate 5 consists of monomeric subunits of [{Mn(CH2SiMe3)2}∞]; in both cases these subunits are linked together via O(CH2CH2)2O bridges to generate one-dimensional polymers. Introduction Endowed with steric bulk, a non-carbon atom in the β-position, and no β-hydrogen atoms, the heteroneopentyl ligand (trimethylsilyl)methyl, Me3SiCH2-, has long been utilized as an entry to stabilized transition-metal-alkyl σ bonds in work chiefly pioneered by Lappert1 and Wilkinson.2 Germane to the present study, the manganese(II) dialkyl compound Mn(CH2SiMe3)2 (1), in its pure form free of Lewis basic ancillary ligands, was first synthesized by Wilkinson in 1976.3 A recent review4 alerted us to the fact that 1 has hitherto never been fully crystallographically characterized. The original paper states through a personal communication that “in the crystal * To whom correspondence should be addressed. E-mail: r.e.mulvey@ strath.ac.uk. † Universitat Jaume I. ‡ University of Strathclyde. § Johannes-Gutenberg-Universita¨t Mainz. ⊥ Newcastle University. (1) Collier, M. R.; Lappert, M. F.; Truelock, M. M. J. Organomet. Chem. 1970, 25, C36. (2) Yagupsky, G.; Mowat, W.; Shortland, A.; Wilkinson, G. Chem. Commun. 1970, 1369. (3) Andersen, R. A.; Carmona-Guzman, E.; Gibson, J. F.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1976, 2204. (4) Sheridan, J. B. In ComprehensiVe Organometallic Chemistry III; Crabtree, R., Mingos, M., Eds.; Elsevier: New York, 2006; Vol. 5, Chapter 11.
the compound is polymeric with alkyl bridges, each Mn(II) atom being four-coordinate”. Other than this comment, no further details are given. Our current interest in 1 is two-fold. First, it is a key component of our heteroleptic alkali-metal alkylamidomanganate reagents that can directly metalate (manganate!) C-H bonds of aromatic and functionalized aromatic substrates.5 In recognition that the lithium or sodium cocomponent is essential for these bimetallic reagents to execute direct manganation(II), we have coined the phrase “alkali-metalmediated manganation” to describe this new methodology. Second, to the best of our knowledge there have been no reported structural determinations of polymeric manganese(II) dialkyl compounds, or indeed of any polymer that propagates through Mn-C-Mn σ-bonded linkages (there is one π-bonded polymer in the ionic metallocene manganocene, which exhibits a Mn-Cp-Mn zigzag chain arrangement6,7). To date the highest oligomeric manganese(II) diorganyl to be crystallographically ´ lvarez, J.; Kennedy, A. R.; Klett, J.; Mulvey, R. E. (5) (a) Garcia-A Angew. Chem., Int. Ed. 2007, 46, 1105. (b) Carella, L. M.; Clegg, W.; Graham, D. V.; Hogg, L. M.; Kennedy, A. R.; Klett, J.; Mulvey, R. E.; Rentschler, E.; Russo, L. Angew. Chem., Int. Ed. 2007, 46, 4662. (c) Blair, V. L.; Clegg, W.; Conway, B.; Hevia, E.; Kennedy, A. R.; Klett, J.; Mulvey, R. E.; Russo, L. Chem.sEur. J. 2008, 14, 65. (6) Bunder, W.; Weiss, E. Z. Naturforsch., B: Chem. Sci. 1978, 33, 1235. (7) For an example of a Mn(II) structure exhibiting both σ- and π-C ligand bonding to the metal see: Layfield, R. A.; Humphrey, S. M. Angew. Chem., Int. Ed. 2004, 43, 3067.
10.1021/om801135d CCC: $40.75 2009 American Chemical Society Publication on Web 03/04/2009
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Organometallics, Vol. 28, No. 7, 2009 2113
Figure 2. Polymeric chain structure of 1. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at 50% probability.
Figure 1. A trinuclear section of 1. Hydrogen atoms are omitted for clarity. Symmetry operator A: -x + 1, -y + 1, -z. Symmetry operator B: -x, -y + 1, -z. Thermal ellipsoids are shown at 50% probability.
characterized is the linear trimer [{Mn(Mes)2}3] (Mes ) 2,4,6C6H2Me3), reported by Floriani.8,9 For these reasons we have carried out a full and accurate structural determination of 1 and report our findings herein. Adding to the previously recorded magnetic moment,3 we have also explored the magnetic properties of 1 using variable-temperature magnetization as well as EPR measurements on a powdered sample. Furthermore, for comparison we include the crystal structures of the TMEDA, pyridine, and dioxane complexes of 1, namely, [TMEDA · Mn(CH2SiMe3)2], 2, [(pyridine)2Mn(CH2SiMe3)2], 3, [{(dioxane)[Mn(CH2SiMe3)2]2}∞], 4, and [{(dioxane)[Mn(CH2SiMe3)2]}∞], 5. The four new structures are discussed in relation to pertinent examples from the literature.
Figure 3. Molecular structure of 2. Hydrogen atoms are omitted for clarity. Symmetry operator A: -x, y, -z + 0.5. Thermal ellipsoids are shown at 50% probability.
Results and Discussion Following the original literature,3 we synthesized 1 by the salt metathesis reaction between the magnesium congener Mg(CH2SiMe3)2, itself freshly prepared from the Grignard reagent (Me3SiCH2)MgCl by manipulation of the Schlenk equilibrium via the dioxane precipitation method, and manganous chloride, MnCl2, in ether solution. Orange needle crystals of 1 were initially obtained by replacing the ether solvent by toluene, but sublimation of these at 150 °C in vacuo produced better quality crystals for X-ray crystallographic study. Simply admitting either TMEDA or pyridine to an n-hexane solution of 1 afforded colorless crystals of the diamine complex 2 or orange crystals of the bispyridine complex 3, respectively, as was previously described by Wilkinson.3 Pink needles of the hemidioxane complex 4 were also obtained by direct addition of the donor solvent to an n-hexane suspension of 1. If this solution was allowed to stand for several days or cooled to -27 °C, colorless blocks of the full dioxane complex 5 formed instead. Figure 1 picks out a defining trinuclear section of 1, while Figure 2 shows a longer section of its polymeric chain structure. Figures 3-8 show the structures of 2-5, respectively. Table 1 lists key metrical parameters for all five structures, with their crystallographic data compiled in Table 2. Polynuclear 1 con(8) Gambarotta, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. Chem. Commun. 1983, 1128. (9) It is mentioned in ref 3 that the structure of [{Mn(CH2CMe3)2}4] is tetrameric as found by an X-ray diffraction study, though no details are given.
Figure 4. Molecular structure of 3. Only one of two independent molecules is shown; hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at 50% probability.
tains one unique spiro manganese atom, Mn1, with its nearest neighbors Mn1A and Mn1B generated by inversion centers. It follows that there are two distinct, but essentially equivalent metal · · · metal separations [Mn1 · · · Mn1A, 2.8874(5) Å; Mn1 · · · Mn1B, 2.8897(5) Å]. Trinuclear Mn1A · · · Mn1 · · · Mn1B sections are nonlinear [161.39(2) Å] and are connected via bridging alkyl (µ2-C) ligands. There are four distinct Mn-C bonds, the lengths of which fall into two categories: short [Mn1-C1, 2.2023(17) Å; Mn1-C5, 2.2039(17) Å] and long
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Figure 8. Extended zigzag chain arrangement of 5. Thermal ellipsoids are shown at 50% probability.
Figure 5. A dimeric section of 4. Hydrogen atoms are omitted for clarity, and only one of two crystallographically independent molecules is shown. Symmetry operator A: -x + 1, -y + 1, -z. Thermal ellipsoids are shown at 50% probability.
Figure 6. A polymeric section of 4. Thermal ellipsoids are shown at 50% probability.
Figure 7. A trimeric section of 5. Hydrogen atoms are omitted for clarity. Symmetry operator A: -x + 1, y, -z + 0.5. Symmetry operator B: -x + 1, -y, -z. Thermal ellipsoids are shown at 50% probability.
[Mn1-C1A, 2.4358(17) Å; Mn1-C5B, 2.4203(17) Å]. The widest bond angle subtended at the metal atom is C1-Mn1-C5 [120.31(7)°], and the narrowest is C1A-Mn1-C5B [99.84(6)°], with the overall mean (109.21°) indicative of a highly distorted tetrahedral coordination. The R-C(H2) atoms of the ligand form acute bridges to the metal (mean Mn-C-Mn, 77.00°) to propagate the chain. Turning to 2, its molecular structure possesses crystallographic C2 symmetry with the rotation axis passing through Mn1 and bisecting TMEDA. Occupying a distorted tetrahedral C2N2 environment, Mn1 forms a short bond to the anionic C atoms [Mn1-C1, 2.1379(15) Å] and a longer dative bond to the TMEDA N atoms [Mn1-N1, 2.3284(12) Å]. Distortion from perfect tetrahedral geometry is most pro-
nounced at C1-Mn1-C1A [140.50(9)°] to minimize steric clashing between the bulky alkyl ligands and is offset by an acute N1-Mn1-N1A bond angle [79.92(6)°] enforced by the restrictive bite angle of TMEDA. The mean bond angle subtended at Mn1 is 106.77°. Pyridine complex 3 has two crystallographically independent units without intermolecular interactions, both showing a distorted tetrahedral C2N2 environment for Mn1 and Mn2. Again the bonds between manganese and the carbon atoms [Mn1-C1, 2.144(4) Å; Mn1-C5, 2.143(4) Å; Mn2-C19, 2.150(4) Å; Mn2-C23, 2.145(4) Å] are shorter than the manganese nitrogen bonds to the coordinating pyridines [Mn1-N1, 2.263(3) Å; Mn1-N2, 2.260(3) Å; Mn2-N3, 2.251(3) Å; Mn2-N4, 2.244(3) Å]. The large C1-Mn-C5 (C19-Mn-C23) bond angle of 138.69(18)° (138.46(17)°) expresses the high steric demand of the two (trimethylsilyl)methyl groups, whereas the two molecules of pyridine with their much less bulky, planar, character allow narrower N1-Mn1-N2 (N3-Mn2-N4) bond angles of 90.74(12)° (90.87(12)°). To the best of our knowledge, 2 and 3 are only the third and fourth monomeric bis[(trimethylsilyl)methyl]manganese complexes to be crystallographically characterized after the chiral diamine analogue [((-)-sparteine)Mn(CH2SiMe3)2], 6,10 and the recently reported bis(benzamide)ligated [{(iPr)2NC(Ph)(dO)}2Mn(CH2SiMe3)2], 7.5c Like 2 and 3, 7 was prepared by direct addition of its Lewis acidic and Lewis basic components, whereas 6 was prepared by first making the ((-)-sparteine)manganese(II) dibromide complex and then treating it with 2 molar equivalents of [(trimethylsilyl)methyl]lithium.11 Hemidioxane complex 4 is a polymer in the crystal, made up of [Mn(CH2SiMe3)2]2 dimers linked together by O(CH2CH2)2O chairs via dative Mn-O bonds. Each Mn center of the dimeric subsections carries one bridging and one terminal alkyl ligand and is thus part of a (MnC)2 ring. There are two crystallographically distinct polymeric chains, though the distinction is limited to small differences in dimensions; both extend along the crystallographic a axis. The internuclear Mn · · · Mn separations within the dimers (mean 2.77 Å) are shorter than those in polymeric 1 (mean 2.89 Å). Terminal Mn-C bonds (mean 2.123 Å) are contracted compared to their bridging counterparts (mean 2.282 Å), consistent with the lower coordination of the terminal ligands. The mean Mn-O bond length is 2.2467 Å. While the Mn-dioxane-Mn propagative coordination is new for a manganese(II) dialkyl complex, or indeed for manganese compounds of any type, such linkages (10) Bart, S. C.; Hawrelak, E. J.; Schmisseur, A. K.; Lobkovsky, E.; Chirik, P. J. Organometallics 2004, 23, 237. (11) Tecle’, B.; Rahman, A. F. M. M.; Oliver, J. P. J. Organomet. Chem. 1986, 317, 267.
Bis[(trimethylsilyl)methyl]manganese
Organometallics, Vol. 28, No. 7, 2009 2115 Table 1. Key Bond Lengths (Å) and Angles (deg) for 1-5
1 Mn-C
2
2.2023(17) 2.2039(17) 2.4203(17) 2.4358(17)
3
2.1379(15)
Mn-N
4
2.144(4) 2.143(4) 2.150(4) 2.145(4) 2.263(3) 2.260(3)
2.3284(12)
Mn-O Mn-Mn C-Mn-C
Mn-C-Mn
2.8874(5) 2.8897(5) 99.84(6) 102.81(5) 103.18(5) 114.33(6) 114.83(6) 120.31(7) 76.82(5) 77.19(5)
C-Mn-N N-Mn-N
140.50(9)
138.69(18) 138.46(17)
100.85(5) 109.26(5) 79.92(6)
101.35(14)108.12(15) 90.74(12) 90.87(12)
2.1385(12)
2.2419(15) 2.2515(15) 2.7715(8) 2.7747(8)
2.2611(9)
105.17(8) 105.37(8) 116.26(10) 117.72(12) 127.13(10) 129.38(14) 74.63(8) 74.83(8)
C-Mn-O Mn · · · Mn · · · Mn
5
2.202(2) 2.218(3) 2.346(2) 2.365(3)
145.46(7)
93.53(8)-109.72(9)
98.34(4) 106.54(5)
161.39(2) Table 2. Crystallographic Data
empirical formula fw cryst syst space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z Dc, g cm-3 µ, mm-1 Τ, Κ θmax no. of reflns measd no. of unique reflns Rint no. of reflns (I > 2σ(I)) no. of refined params R(F, obsd data) Rw(F2, all data) S (F2, all data) max and min residual F, e Å-3
1
2
3
4
5
C8H22MnSi2 229.38 triclinic P1j 5.7011(5) 10.5246(12) 11.0936(13) 106.652(12) 100.101(11) 92.256(12) 625.00(12) 2 1.219 1.199 150 25 7633 2187 0.0231 1971 106 0.0221 0.0553 1.055 +0.28, -0.44
C14H38MnN2Si2 345.58 monoclinic C2/c 18.3438(10) 9.8222(5) 12.5923(7) 90 107.800(3) 90 2160.2(2) 4 1.063 0.715 123 30 16907 3146 0.044 2312 100 0.0330 0.0784 1.053 +0.33, -0.34
C18H32MnN2Si2 387.58 triclinic P1j 12.3232(6) 12.5823(6) 16.7053(6) 101.911(4) 110.874(4) 103.379(4) 2232.51(17) 4 1.153 0.700 123 29.9 43395 11882 0.0334 8658 459 0.0656 0.1930 1.094 +1.49, -0.59
C20H52Mn2O2Si4 546.86 triclinic P1j 8.2431(10) 10.6831(18) 20.141(3) 82.895(16) 88.064(15) 67.505(12) 1625.9(4) 2 1.117 0.936 150 25 18694 5626 0.0418 4225 293 0.0327 0.0695 1.029 +0.26, -0.23
C12H30MnO2Si2 317.48 monoclinic C2/c 18.118(4) 9.849(2) 10.870(2) 90 99.58(3) 90 1912.7(7) 4 1.103 0.81 150 27.5 10466 2185 0.0315 2016 82 0.0229 0.0620 1.098 +0.36, -0.31
are well-known for other metal systems.12 The dioxane ligation completes a distorted tetrahedral environment for the Mn atoms in 4, with bond angles covering the range 93.53(8)° [O1-Mn1-C3A] to 129.38(14)° [C13-Mn11-C17B]. Compound 5, which has a 1:1 dioxane:Mn ratio, contains monomeric Mn(CH2SiMe3)2 units linked together by bridging dioxane molecules, forming zigzag chains along the crystallographic c axis. The manganese atoms, which are situated on the corners of the chain, show a distorted tetrahedral C2O2 (12) (a) Barnes, J. C. Acta Crystallogr., Sect. E: Struct. Rep. Online 2004, E60, m971. (b) Almond, M. J.; Beer, M. P.; Drew, M. G. B.; Rice, D. A. J. Organomet. Chem. 1991, 421, 129. (c) Taube, R.; Windisch, H.; Maiwald, S.; Hemling, H.; Schumann, H. J. Organomet. Chem. 1996, 513, 49. (d) Sanchez-Barba, L. F.; Hughes, D. L.; Humphrey, S. M.; Bochmann, M. Organometallics 2005, 24, 5329. (e) Fokken, S.; Reichwald, F.; Spaniol, T. P.; Okuda, J. J. Organomet. Chem. 2002, 663, 158.
environment. The manganese carbon bonds to the two C2symmetrically equivalent alkyl groups [Mn1-C1/C1A, 2.1386 (12)Å ] separated by a C1-Mn1-C1A bond angle of 145.46(7)°, which is again wider than the angle between the manganese oxygen bonds [Mn1-O1/O1A, 2.2611(9) Å] of 87.27° to the chain-propagating dioxane molecules, expressing the high steric demand of the (trimethylsilyl)methyl groups. Figure 8 shows the extended zigzag chain arrangement of 5. With no previously crystallographically characterized manganese(II) dialkyl polymer available, one can look to its s-block compatriot Mg(II) for a comparison with 1, given the similarities recently discovered in alkali-metal-mediated manganation5 and magnesiation.13 The solvent-free mixed alkyl-halide complex (13) Mulvey, R. E. Organometallics 2006, 25, 1060.
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Table 3. Comparative Metrical Data (Å, deg) from Crystallographically Characterized Manganese(II) Bis[(trimethylsilyl)methyl] Compoundsa compd [Mn(CH2SiMe3)2]∞ (1)
Mn · · · Mn
Mn-C
C-Mn-C
Mn-C-Mn
2.8885m
2.200m 2.428m 2.1379 2.146m
109.2m
77.0m
140.50 138.69 138.46 132.84 127.43 105.3m,e
74.7m,e
145.46 105.5
74.5
103.9
76.1
105.28
75.07
[TMEDA · Mn(CH2SiMe3)2] (2) [(pyridine)2 · Mn(CH2SiMe3)2] (3)
a
[((-)-sparteine)Mn(CH2SiMe3)2] (6) [{(iPr)2NC(Ph)(dO)}2Mn(CH2SiMe3)2] (7) [{(dioxane)[Mn(CH2SiMe3)2]2}∞] (4)
2.7731
[{(dioxane)[Mn(CH2SiMe3)2]}∞] (5) [Mn2(CH2SiMe3)4(PMe3)2] (11)
2.772
[Mn2(CH2SiMe3)4(PMePh2)2] (12)
2.828
[Mn2(CH2SiMe3)4(THF)2] (13)
2.7878
2.162m 2.162m 2.123m,t 2.282m,b 2.1385 2.289m,b 2.111t 2.291m,b 2.117t 2.287m,b 2.129t
Meaning of superscript letters: m, mean; b, bridging bond; t, terminal bond; e, endocyclic bond angle.
[(Np3Mg2Br)∞] provides a pertinent example (Np ) tBuCH2).14 Its polymeric chain is composed of Mg2Np2 and Mg2Br2 spiro rings in a 3:1 ratio, with Mg-C bonds ranging from 2.20(2) to 2.42(2) Å and thus of a length similar to that of the Mn-C bonds in 1. In this structure the tBu-CH2 bonds are inclined at an angle of 28-44° to the respective Mg2C planes (28-31°, ignoring disordered groups, which are less reliable). In 1 the corresponding values for the Me3Si-CH2 bonds are 19.9° and 21.6°. Internuclear Mn · · · Mn separations in 1 can be compared with those in the aforementioned linear trimer [{Mn(Mes)2}3]8 (8) which measure on average 2.8515 Å, just 0.037 Å shorter than in 1. Lack of substitution on the aryl ligand lowers the aggregation to the dimeric state in the contacted ion pair structure of [{Li(OEt2)2}2Mn2Ph6] (9) and its solvent-separated relative [Li(THF)4][Mn2Ph6] (10), with the diminished steric constraints allowing the Mn atoms to approach each other more closely [2.733(1) Å in 9, 2.763(2) Å in 10].15 Table 3 lists comparative metrical data for organomanganese(II) crystal structures specifically containing the Me3SiCH2ligand. Including the new set of crystal structures, Table 3 contains one bona fide alkyl-propagated chain polymer, two dioxane-propagated chain polymers with dimeric or monomeric manganese dialkyl subsections, three discrete dimers, and four discrete monomers. The aforementioned bis(benzamide) solvate 75c is the most sterically crowded monomer with the narrowest inter-alkyl ligand separation of 127.43°. (-)-Sparteine solvation in 6 also forces the alkyl ligands to be pushed together with a C-Mn-C bond angle of 132.84°, but with TMEDA in 2 and two pyridine molecules in 3 there is a lessening of the alkyl-alkyl repulsion (corresponding angles 140.50° and 138.69°/ 138.46°).10 There is a further relaxation of steric crowding at the Mn atom on changing the donor solvent to THF (in 13)16 or a phosphine (in 1117 and 1218) as dimerization now occurs. Dioxane solvate 4 also comes into this category, though its second O donor atom causes its dimeric units to join together, but the existence of monomeric unit 5 shows that two molecules of dioxane are bulky enough to stabilize a monomer if there is (14) Markies, P. R.; Schat, G.; Akkerman, O. S.; Bickelhaupt, F.; Smeets, W. J. J.; Duisenberg, A. J. M.; Spek, A. L. J. Organomet. Chem. 1989, 375, 11. (15) Bartlett, R. A.; Olmstead, M. M.; Power, P. P.; Shoner, S. C. Organometallics 1988, 7, 1801. (16) Crewdson, P.; Gambarotta, S.; Yap, G. P. A.; Thompson, L. K. Inorg. Chem. 2003, 42, 8579. (17) Davies, J. I.; Howard, C. G.; Skapski, A. C.; Wilkinson, G. Chem. Commun. 1982, 1077. (18) Howard, C. G.; Wilkinson, G.; Thornton-Pett, M.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1983, 2025.
a sufficient amount of donor present. All four dimers exhibit a planar CMnCMn ring with remarkably similar obtuse C-Mn-C and acute Mn-C-Mn endocyclic bond angles. Without exception, the terminal Mn-C bonds are shorter than their bridging counterparts. A noteworthy feature not apparent from Table 3 is the marked asymmetry in the bridging Mn-C bond lengths within each individual dimer: in 11, 2.208(3)/2.369(5) Å; in 12, 2.193(5)/2.389(5) Å; in 13, 2.214(3)/2.360(3) Å; in 4, 2.202(2)/2.365(3) and 2.218(3)/2.346(2) Å. This reflects the unsymmetrical coordination about the distorted Mn atoms with three alkyl ligands (two bridging, one terminal) and one nonalkyl ligand. Gambarotta remarked on the curious T-shaped coordination of the bridging C atom in 1316 with one Si and twoMnatoms[Si-C-Mn,175.92(13)°;Si-C-Mn′,106.03(13)°], which including the two “equatorial” H atoms gives a trigonal bipyramidal Siax-C(H,H,Mn)eq-Mn′ax coordination overall. This effect is also evident in both chains of 4 though it is slightly lesspronounced[Si-C-Mn,169.77(15)°;Si-C-Mn′,105.77(12)°; Si-C-Mn, 167.24(15)°; Si-C-Mn′, 104.17(11)°]. Interestingly, both of these features are also apparent in the polymeric structure of 1. The two distinct types of bridging Mn-C bonds have been discussed previously. Bond angles Si1-C1-Mn1A [166.03(9)°], Si1-C1-Mn1 [110.51(8)°] and Si3-C5-Mn1B [167.16(9)°], Si3-C5-Mn1 [110.64(8)°] define the two distinct T-shaped coordinations within 1. On the other hand, the monomers display Si-C-Mn bond angles far removed from linearity [in 2, 123.31(8)°; in 3, 115.9(2)/122.7(2)°; in 5, 117.66(6)°; in 6, 121.15/121.37°; in 7, 113.4(12)/118.77(12)°]. The magnetic properties of compound 1 were studied on a powdered sample by variable-temperature measurements in a temperature range of 2-300 K in an applied field of 1 T. At room temperature the χMT product reaches a value of 0.62 cm3 K mol-1, considerably below the expected value for an uncoupled manganese(II) species in a high spin state. A sharp decrease of the χMT product with decreasing temperature indicates a significant interaction between the manganese ions along the chain. Simulating the high-temperature magnetic data down to 50 K, an exchange coupling constant of -45 cm-1 is obtained. This value is in accordance with an exchange coupling constant of less than -70 cm-1 estimated by Wilkinson et al. on frozen solutions by EPR studies and -42.5 cm-1 as measured forthedinuclearspecies[(Me3SiCH2)(µ-CH2SiMe3)Mn(THF)]2.3,16 As estimated by low-temperature EPR studies on 1 (see below) an exchange interaction of around -55 cm-1 seems to be effective at lower temperatures. We therefore used the Fisher equation to calculate the exchange interaction within a classical
Bis[(trimethylsilyl)methyl]manganese
Organometallics, Vol. 28, No. 7, 2009 2117
atom of the bifunctional cyclic ether to generate a polymer of dimeric or monomeric units.
Experimental Section
Figure 9. χMT vs T for compound 1. Solid line: see the text.
Heisenberg chain model and obtained -51.4 cm-1 as the best fit to the overall experimental data as shown in Figure 9. We propose that an effective through-space interaction between the manganese(II) ions due to the short Mn-Mn distance of about 2.89 Å has to be taken into consideration operating at low temperatures together with the superexchange mechanism via the Me3SiCH2- bridging ligands to explain the antiferromagnetic interaction along the chain. To confirm our findings, additional EPR measurements were performed on a polycrystalline sample of 1. Previous X-band EPR studies by Wilkinson et al.3 on frozen solutions of 1 revealed the presence of a single resonance with g values close to the spin-only value of 2.0. The lack of additional structure can be ascribed to the zero-field splitting being effectively modulated by small changes in the molecular environment throughout the sample, giving rise to an inhomogeneous line broadening. Another factor we should take into account in view of the strong magnetic interaction observed in the magnetic studies is the presence of dipolar line broadening.19 Solid-state EPR on a polycrystalline sample was recorded between 5 and 300 K at the Q-band (∼34 GHz). At room temperature a broad (∆Hpp ≈ 450-600 G) single resonance at g ) 1.97 was observed (Figure 10), with no observable hyperfine coupling. The intensity of the signal clearly decreases with decreasing temperature, indicating strong antiferromagnetic coupling. Since the signal is still visible at 90 K, there is still population of the S ) 1 at 90 K, giving a lower estimate of the exchange interaction of -55 cm-1, in good agreement with the values obtained from the magnetic measurements. The peak to peak line width of the EPR resonance remains approximately constant in the range of temperatures studied, although it becomes increasingly difficult to accurately ascertain its value, since the intensity of the signal decreases quite dramatically. In conclusion, five variations of bis[(trimethylsilyl)methyl]manganese have been structurally characterized by X-ray crystallography. All exhibit distorted tetrahedral Mn(II) atoms, though interestingly each is arrived at in a different way. The pure solvent-free dialkyl compound exists as an infinite chain with lines of spiro Mn atoms held together by µ2-bonding Me2SiCH2- ligands. Bidentate or bismonodentate chelation by TMEDA or pyridine results in simple monomeric arrangements made up of two anionic C-alkyl ligands and one dative (N)2diamine ligand or two dative N-heteroarene ligands, respectively. The weaker donor dioxane does not completely break down the polymeric arrangement but gives an O-solvated dimer or monomer, which self-aggregates through the second O donor
General Procedures. All reactions were carried out under a protective argon atmosphere by using standard Schlenk techniques. n-Hexane was distilled from sodium/benzophenone. Mg(CH2SiMe3)2 was prepared from the Grignard reagent (Me3SiCH2)MgCl by manipulation of the Schlenk equilibrium via the dioxane precipitation method. The resultant off-white solid was purified via sublimation at 175 °C (10-2 Torr) to furnish pure Mg(CH2SiMe3)2. The IR spectra were recorded on a Nicolet Avator 360 FT-IR spectrometer, and elemental analyses were carried out on a PerkinElmer 2400 elemental analyzer. Melting/decomposition points were measured with a Bu¨chi B-545 melting point apparatus. X-ray Crystallography. Data for compounds 1, 2, 4, and 5 were measured on Nonius KappaCCD instruments and for 3 on an Oxford Diffraction Xcalibur S diffractometer, with Mo KR radiation (λ ) 0.71073 Å). The structures were solved by direct methods and refined to convergence on F2 using programs from the SHELX suite.20 Selected crystallographic parameters are given in Table 2, and full details in CIF format are available as Supporting Information. Magnetic Properties. Magnetic susceptibility data for polycrystalline samples of 3 were collected in the temperature range 2-300 K in an applied magnetic field of 1 T with a SQUID magnetometer (MPMS-7, Quantum Design). The temperature-dependent magnetic contribution of the glass holder was experimentally determined and substracted from the measured susceptibility data. The routine JULIUS was used for spin Hamiltonian simulations of the data (C. Krebs, E. Bill, F. Birkelbach, V. Staemmler, unpublished results). Q-band EPR spectra were recorded on a polycrystalline sample with a Bruker E-500 ELEXSYS spectrometer in the temperature range 300-5 K. Synthesis. Synthesis of [{Mn(CH2SiMe3)2}∞] (1). Mg(CH2SiMe3)2 (3.00 g, 15 mmol), MnCl2 (1.91 g, 15 mmol), and 60 mL of dry ether were allowed to stir for 2-3 days at room temperature. All the solvent was removed under vacuum to give a dull orange solid. Subsequently, 160 mL of dried toluene was introduced, and with vigorous heating to boiling an orange solution with a white precipitate (MgCl2) was observed. The solution was filtered hot and the bright orange filtrate allowed to cool to room temperature before storage of the solution in the freezer (-27 °C) where crystallization occurred. Thin orange needle crystals were observed, the solvent was removed via cannula, and the resultant orange crystals were dried under vacuum for 2 h (2.5-2.8 g, 73-81%). To obtain suitable crystals for the X-ray diffraction experiment, a sample of Mn(CH2SiMe3)2 was sublimed in vacuo at 150 °C to give larger orange cubic crystals. IR (Nujol): νC ) 2776.1 cm-1 (C-H). Anal. Calcd for C8H22MnSi2 (229.38): C, 41.89; N, 0.00; H, 9.67. Found: C, 41.89; N, 0.00; H, 9.79. Mp: 152 °C dec. Synthesis of [Mn(CH2SiMe3)2 · TMEDA] (2). Mn(CH2SiMe3)2 (0.23 g, 1 mmol) was suspended in 20 mL of dry n-hexane. TMEDA (0.15 mL, 1 mmol) was added to give an orange-yellow solution which was allowed to stir at room temperature for 12 h. The solution was filtered and concentrated by removing some solvent under vacuum. Storage of the solution at room temperature afforded a crop of orange crystals after two days (0.11 g, 32%). IR (Nujol): νC ) 2799.5 cm-1 (C-H). Anal. Calcd for C10H27N2MnSi2 (345.58): C, 48.66; N, 8.11; H, 11.08. Found: C, 48.66; N, 7.92; H, 10.90. Mp: 93 °C. Synthesis of [(C5H5N)2Mn(CH2SiMe3)2] (3). Mn(CH2SiMe3)2 (0.23 g, 1 mmol) was suspended in 20 mL of dry n-hexane. Pyridine (0.08 mL, 1 mmol) was added and the reaction allowed to stir at room temperature for 2 h. The bright orange solution was filtered
2118 Organometallics, Vol. 28, No. 7, 2009
Alberola et al.
Figure 10. Q-band powder EPR spectrum of 1 at 298 and 130 K. and the solvent volume reduced under vacuum. Storage of the solution in the refrigerator (4 °C) afforded a crop of bright orange plate crystals (0.12 g, 62%). Synthesis of [{(Dioxane)[Mn2(CH2SiMe3)4]}∞] (4). Mn(CH2SiMe3)2 (0.23 g, 1 mmol) was suspended in 20 mL of dry hexane. Dioxane (0.09 mL, 1 mmol) was added to give a light brown solution which was allowed to stir at room temperature for 12 h. The solution was filtered and concentrated by removing some solvent under vacuum. Storage of the solution at room temperature afforded a crop of pink needle crystals after two days (0.13 g, 48%). IR (Nujol): νC ) 2770.6 cm-1 (C-H). Anal. Calcd for C20H52Mn2O2Si4 (546.86): C, 45.39; N, 0.00; H, 9.53. Found: C, 45.39; N, 0.00; H, 9.88. Mp: 156 °C. Synthesis of [{(Dioxane)[Mn(CH2SiMe3)2]}∞] (5). Mn(CH2SiMe3)2 (0.23 g, 1 mmol) was suspended in 20 mL of dry hexane. (19) Soria-Alvarez, S.; Bashall, A.; McInnes, E. J. L.; Layfield, R. A.; Mole, R. A.; McPartlin, M.; Rawson, J. M.; Wood, P. T.; Wright, D. S. Chem.sEur. J. 2006, 12, 3053. (20) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112.
Dioxane (0.09 mL, 1 mmol) was added to give a light brown solution which was allowed to stir at room temperature for 12 h. The solution was filtered and concentrated by removing some solvent under vacuum. Storage of the solution in the freezer (-27 °C) afforded a crop of colorless plate crystals after two days (0.19 g, 60%). Anal. Calcd for C12H30MnO2Si2 (317.48): C, 45.40; N, 0.00; H, 9.52. Found: C, 45.40; N, 0.00; H, 9.99. Mp: 153 °C.
Acknowledgment. We thank the EPSRC (Grant Award Nos. GR/T27228/01 and EP/D076889/1) for generously sponsoring this research. A.A. thanks the Spanish Ministerio de Educacion y Ciencia for a Ramon y Cajal research contract. We also thank Ian MacGee of the University of Strathclyde for carrying out the elemental analyses. Supporting Information Available: Full crystallographic data in CIF format for compounds 1-5. This material is available free of charge via the Internet at http://pubs.acs.org. OM801135D
