Communication pubs.acs.org/Organometallics
Mild Reduction Route to a Redox-Active Silicon Complex: Structure and Properties of (IP2−)2Si and (IP−)2Mg(THF) (IP = α-Iminopyridine) Owen T. Summerscales, Thomas W. Myers, and Louise A. Berben* Department of Chemistry, University of California, Davis, California 95616, United States S Supporting Information *
ABSTRACT: Use of SiCl4 as an organometallic reagent can be complicated by access to Si3+, Si2+, and unwanted sigmatropic rearrangements. Herein we report a mild reduction route, using (IP−)2Mg(THF) (1) and Mg metal, to cleanly access (IP2−)2Si (2). Electrochemical measurements show that IP2− is stabilized by Si4+ > Al3+ > Mg2+.
he α-iminopyridine ligand set, specifically 2,6-bis(1methylethyl)-N-(2-pyridinylmethylene)phenylamine (IP), has been used successfully in transition-metal-based systems as a hybrid of the well-known α-diimine and 2,2′bipyridine (bpy) classes of chelating ligands,1 enjoying success especially in the field of olefin polymerization.2 On ligation to classically redox-inactive metal centers, it has been shown that it is possible to engender redox chemistry that emulates d-blockmetal complexes.3 We have recently demonstrated this for a series of Al(III) complexes (IPn−)2Al(X) (n = 1, 2; X = monodentate ligand), for which the coordinatively unsaturated Lewis acidic metal combined with a moderately strong redox couple (E°(IP2−/−) = −1.36 V vs SCE for [(IP2−)2Al][Na(DME)3])4 has allowed high-energy processes such as aliphatic C−H activation.5 To explore variations in these systems based on earthabundant main-group elements, we proposed that silicon may be a valuable alternative to allow fine tuning of redox potential and electrophilicity. Coordination of small molecules to generate six-coordinate Si species is well-documented,6 and Si4+ centers have been previously demonstrated to stabilize noninnocent ligands in their highly reduced forms: cf. Si(bpy2−)2,7 Si(bpy2−)(bpy−)2,8 Si(IP2−)(Mes)2,9 and spirocyclic Si(dbdab2−)2,10 among others11 (bpy = 2,2′-bipyridine, Mes = 2,4,6-trimethylphenyl; dbdab = 1,4-di-tert-butyl-1,4-diazabutadiene). A variety of synthetic routes for these compounds have been reported, including metathesis of alkali-metal and Si4+ salts, cycloaddition with Si2+ intermediates generated photochemically, and thermal rearrangement with ligand redistribution or isomerism. In many cases multiple products are obtained, owing to the harsh conditions employed and the susceptibility of the conjugated heterocycles to electrophilic and/or radical attack by reduced silicon species. Reduced αiminopyridine silicon derivatives are known,9,11 but we wished to access homoleptic (IP2−)2Si (2), desirable for its potential for four-electron redox chemistry. Herein, we report mild conditions for the synthesis of 2 that employ Mg both as a reductant and as a ligand transfer salt.
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© 2012 American Chemical Society
Reaction of reduced forms of the IP ligand, (IP2−)2Na4(EtO)4, {(IP−)K(THF)2}2, and {(IP−)K(Et2O)}2,1b with SiCl4 in various hydrocarbon solvents, with or without prior cooling, did not reliably yield pure samples of 2. Due to the highly reducing nature of these species, it was supposed that unwanted side reactions such as the reduction of SiCl4 to the dichlorosilylene had occurred. Therefore, a milder reducing agent was chosen. Magnesium metal is insufficiently reducing to afford (IP2−)Mg(solv) (E°(Mg0/2+) = −2.13 V vs SCE), and emerald green solutions of 1 were obtained after 24 h of reaction between Mg and 2 equiv of IP, in a manner similar to that reported for (bpy−)2Mg(THF)x (x = 1, 3)12 (Scheme 1). Scheme 1. Synthesis of 1 and 2 from IP
The magnetic moment of 2.4 μB is consistent with the formulation of 1 with two monoanionic IP− ligands, as in previously reported complexes of the form (IP−)2Al(X).5 Temperature-dependent susceptibility measurements indicate a drop to 0.21 μB upon cooling to 5 K, consistent with a singlet ground state and thermal population of an excited triplet state (Figure 1a). Quantification of the singlet−triplet gap was achieved by fitting the data using MAGFIT3.0,13 assuming g = 2.0 and the spin Hamiltonian Ĥ = −2JŜL(1)·ŜL(2), to afford J = −50 cm−1 and TIP = 0.31 × 103 emu. Notably, the singlet− triplet energy gap is less than half as large as that observed in complexes of the form (IP−)2MX (M = Al3+, Ga3+), where J = Received: March 22, 2012 Published: May 1, 2012 3463
dx.doi.org/10.1021/om300242q | Organometallics 2012, 31, 3463−3465
Organometallics
Communication
Similar highly reduced species [(IP2−)2Al][Na(DME)3] and Si(bpy2−)2 give intensely colored, dark solutions,4,7 but 2 is found as a translucent red-orange color with ε = 21 830 L mol−1 cm−1 as the highest absorbance at 218 nm. Presumably the interaction of the 4+ charge on Si with IP2− shifts the intense band associated with the ligand-based π → π* transitions on IP into the UV region, in comparison with Al3+ and Mg2+ complexes, where π → π* transitions afford intense colors due to visible absorption bands. All absorption bands observed in the visible region for 2 have ε < 2000 L mol−1 cm−1. Diamagnetic 2 was analyzed by 1H and 13C{1H} NMR spectroscopy and afforded diagnostic peaks consistent with the IP2− redox state,5 and a single 29Si{1H} resonance was observed at δ −48.67 ppm, similar to resonances for other Si4+ species in four-coordinate N4 coordination enivironments.10,17 The solid-state structure of 2, as determined by single-crystal X-ray analysis, shows a four-coordinate Si in a pseudotetrahedral environment (ligand bite angle Npy−Si−Nim = 90.39(6)°) with overall geometrical parameters very similar to those of [(IP2−)2Al][Na(DME)3] indicative of two-electron-reduced, dianionic IP ligands (Figure 2a and Tables S1 and S2
Figure 1. (a) Magnetic susceptibility measurement for 1 from 5 to 300 K. Fit parameters for Ĥ = −2JŜL(1)·ŜL(2): g = 2.00, J = −50 cm−1, TIP = 0.31 × 103 emu. (b) Solid-state structure of 1 (40% probability thermal ellipsoids; H atoms and toluene solvent omitted).
−94 to −250 cm−1 over four examples.4,5,14 The ionic radii of Mg2+, Al3+, and Ga3+ are 86, 67.5, and 76 pm, respectively, and so the orbitals of Mg2+ would be expected to provide better overlap with IP−-based orbitals to facilitate exchange: this is not observed. We propose that the larger J couplings observed through Al3+ and Ga3+ are a result of a better energy match between metal and ligand orbitals. Indeed, UV−vis spectra indicate that the charge on the metal center has a large effect on the energy of ligand-based π → π* transitions (vide infra). Continuous wave (CW) X-band EPR spectroscopy measurements show a distinct four-line pattern centered at g = 2.00 characteristic of the triplet spin state and a half-field transition characteristic of the integer spin state (Figure S1, Supporting Information).1a,4 Single crystals of 1 were analyzed by X-ray diffraction and revealed a five-coordinate Mg center between trigonal bipyramidal and square planar (τ = 0.5365),15 with the overall formulation (IP−)2Mg(THF)·(toluene) (Figure 1b). The bond lengths within the conjugated heterocyclic ligand are consistent with the radical anion formin particular, diagnostic bonds Cim−Cpy 1.410(2) Å and Cim−Nim 1.338(1) Å are found to be intermediate between the values of neutral and dianionic forms.1,5 The average M−Npy and M−Nim distances of 2.150(1) and 2.085(1) Å are slightly elongated in 1 compared with those in (IP−)2Al(X) analogues (2.026(14) and 1.921(12) Å, respectively),4 as expected. A Mg dimer has been synthesized that contains a pair of two-electron-reduced trialkylsilylsubstituted IP groups, [(IP-SiiPr32−)Mg]2, with bridging Nim donors and terminal Npy donors bound slightly closer than is found in 1 at Mg−Npy = 2.035(4) Å.16 As has been observed exclusively with five-coordinate species of the type (IP−)2M(X), the Npy donor atoms occupy axial positions with an Npy−M− Npy′ angle that is close to linear (1, 172.02(4)°).1a,e,4 We found that Mg reduction of IP would not proceed in nondonor solvents such as toluene and that 1 could not be desolvated using elevated temperatures under dynamic vacuum. Synthesis of (IP2−)2Si (2) was achieved by preparing 1 in situ, as described above, with the addition of a molar equivalent excess of Mg metal, followed by controlled addition of a SiCl4 solution at −78 °C (Scheme 1). After workup, bright redorange crystals of 2 were obtained in 74% yield. No reaction was observed between Mg metal and 1, and it must be assumed that the consequent reductive steps proceed after metathesis of 1 with SiCl4, possibly via an intermediate species of the type (IP−)2SiCl2. Attempts to isolate such a compound, either through stoichiometric reaction of 1 with SiCl4 or by oxidation of 2 with I2, did not yield tractable products.
Figure 2. (a) Solid-state structure of 2 (40% probability thermal ellipsoids; H atoms omitted). (b) CV of 1 (black), 2 (green), and [Na(DME)3][(IP2−)2Al]4 (red) in 0.3 M Bu4NPF6/THF solution (GC electrode, 100 mV s−1).
(Supporting Information)).4 In particular, Cim−Cpy and Cim− Nim (1.349(2), 1.425(2) Å) and bond distances within the pyridyl heterocycle are found to be consistent with dearomatization. The iminopyridine moieties within each ligand are planar, and the two IP ligand planes are orthogonal. Average Si−N distances (Si−Npy = 1.740(1), Si−Nim = 1.726(1) Å) are contracted with respect to 1 and [(IP2−)2Al][Na(DME)3] and are found to be within the range of distances Si−Npy = 1.720(9)−1.766(3) Å and Si−Nim = 1.725(1)− 1.762(3) Å found in various Si4+ complexes of bpy2−, dbdab2−, and IP2−.7−10,17 The Npy−M−Npy′ parameter in four-coordinate 2 is 106.59(6)°, similar to that found in [(IP2−)2Al][Na(DME)3] at 117.1(4)°.4 Cyclic voltammetry measurements were analyzed to gauge the relative stabilization of the IP ligand charges imparted by the acidic metal centers (Table 1, Figure 2b). Altogether, upon comparison with [(IP2−)2Al][Na(DME)3],5 these measurements demonstrate that Mg2+ and Si4+ congeners 1 and 2 do not support the same range of stable redox states under similar conditions, evidenced by the lack of reversibility.4 In addition, the trend observed for the E°(IP−,−/2−,2−) couples suggests a relative stabilization of dianionic IP2− in the order Si4+ > Al3+ > Mg2+. The predicted lower stability of multiple redox states for 1 and 2 was borne out by experiments which have so far failed to 3464
dx.doi.org/10.1021/om300242q | Organometallics 2012, 31, 3463−3465
Organometallics
Communication
(2) (a) Vrieze, K. J. Organomet. Chem. 1986, 300, 307. (b) Gibson, V. C.; O’Reilly, R. K.; Wass, D. F.; White, A. J. P.; Williams, D. J. Dalton Trans. 2003, 2824. (c) Irrgang, T.; Keller, S.; Maisel, H.; Kretschmer, W.; Kempe, R. Eur. J. Inorg. Chem. 2007, 4221. (d) Tagata, T.; Nishida, M. Adv. Synth. Catal. 2004, 346, 1655. (e) Bianchini, C.; Giambastiani, G.; Luconi, L.; Meli, A. Coord. Chem. Rev. 2010, 254, 431. (f) Wu, J. W.; Moreau, B.; Ritter, T. J. Am. Chem. Soc. 2009, 131, 12915. (3) Gastel, M. V.; Lu, C. C.; Wieghardt, K.; Lubitz, W. Inorg. Chem. 2009, 48, 2626. (4) Myers, T. W.; Kazem, N.; Stoll, S.; Britt, R. D.; Shanmugam, M.; Berben, L. A. J. Am. Chem. Soc. 2011, 133, 8662. (5) Myers, T. W.; Berben, L. A. J. Am. Chem. Soc. 2011, 133, 11865. (6) (a) Seiler, O.; Bertermann, R.; Buggisch, N.; Burschka, C.; Penka, M.; Tebbe, D.; Tacke, R. Z. Anorg. Allg. Chem. 2003, 629, 1403. (b) Fleischer, H. Eur. J. Inorg. Chem. 2001, 393 and references therein. (c) Fester, G. W.; Eckstein, J.; Gerlach, D.; Wagler, J.; Brendler, E.; Kroke, E. Inorg. Chem. 2010, 49, 2667. (7) Morancho, R.; Pouvreau, P.; Constant, G.; Joud, J.; Galy, J. J. Organomet. Chem. 1979, 166, 329. (8) Herzog, S.; Krebs, F. Naturwissenschaften 1963, 50, 300. (9) Weidenbruch, M.; Piel, H.; Peters, K.; von Schnering, H. G. Organometallics 1993, 12, 2881. (10) Haaf, M.; Schmiedl, A.; Schmedake, T. A.; Powell, D. R.; Millevolte, A. J.; Denk, M.; West, R. J. Am. Chem. Soc. 1998, 120, 12714. (11) (a) Weidenbruch, M.; Piel, H.; Peters, K.; von Schnering, H. G. Organometallics 1994, 13, 3990. (b) Weidenbruch, M.; Piel, H.; Lesch, A.; Peters, K.; von Schnering, H. G. J. Organomet. Chem. 1993, 454, 35. (12) (a) Herzog, S.; Taube, R. Z. Chem. 1962, 2, 208. (b) Horiba, T.; Hara, K.; Motomichi, I.; Kubo, M. Bull. Chem. Soc. Jpn. 1974, 47, 1624. (13) Schmitt, E. A. Ph.D. Thesis, University of Illinois UrbanaChampaign, Urbana, IL. 1995. (14) Kowolik, K.; Shanmugam, M.; Myers, T. W.; Cates, C. D.; Berben, L. A. Dalton Trans. 2012, Advance Article. (15) Addison, A. W.; Rao, T. N.; Van Rijn, J. J.; Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349. (16) Westerhausen, M.; Bollwein, T.; Makropoulos, N.; Schneiderbauer, S.; Suter, M.; Nöth, H.; Mayer, P.; Piotrowski, H.; Polborn, K.; Pfitzner, A. Eur. J. Inorg. Chem. 2002, 389. (17) (a) Tomasik, A. C.; Mitra, A.; West, R. Organometallics 2009, 28, 378. (b) Hill, N. J.; Moser, D. F.; Guzei, I. A.; West, R. Organometallics 2005, 24, 3346.
Table 1. Quasi-Reversible Redox Events for 1 and 2, with [Na(DME)3][(IP2−)2Al] for Comparison4 (V vs SCE); IP0,0/−,− −
(IP )2Mg(THF) (1) [Na(DME)3][(IP2−)2Al] (IP2−)2Si (2) a
−0.97 −0.40 0.17a
IP−,−/−,2−
IP−,2−/2−,2−
−1.17 −0.53
−2.08b −1.36 −0.99
a
Irreversible redox event. bSimultaneous events.
yield chemically oxidized or reduced forms of 1 or 2 in the same facile manner previously demonstrated for aluminum.5 In preliminary reactivity studies, 2 was found to lack the necessary Lewis acidity and/or steric properties to bind additional donor molecules (e.g., THF, DME, dioxane, triethylamine, CO2, CS2), which may be attributable to a rigid molecular geometry.6b Indeed 2 is found to be remarkably stablesolid samples take hours to decompose in airand has been found so far to react in solution only with strong oxidizing agents such as pyridine N-oxide or N-[(tolylsulfonyl)imino]phenyliodinane (PhINTs), which both lead to overoxidation and quantitatively yielded free ligand, as observed by 1H NMR. Reaction with S8 afforded no change to 2. These results suggest that future work should focus on oxidants milder than PhIO or PhINTs and on outersphere oxidation chemistry. In conclusion, IP complexes of Mg2+ and Si4+ have been characterized in overall two- and four-electron-reduced states, respectively. (IP−)2Mg(THF) (1) has been used successfully in combination with Mg metal as a mildly reductive method to introduce dianionic IP2− to an electrophilic element center without concomitant heterocycle degradation. Future work will explore the broader synthetic utility of this mild method and the ability of 1 and 2 to effect redox transformations.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
* Supporting Information S
CIF files, text, tables, and a figure giving crystal data, details of the preparation of compounds, CV’s, and EPR data. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the University of California Davis for support of this work, the National Science Foundation (Grant 0840444) for the dual-source X-ray diffractometer, and Prof. R. D. Britt for use of an EPR spectrometer.
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REFERENCES
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