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Organometallics 2010, 29, 4756–4758 DOI: 10.1021/om100327w
Synthesis and Structure of a Potassium Potassiochromate: A Bis-Chromium(II) Molecule Held Together by Near-Square-Planar Potassium-Ligand Bridges† Luca M. Carrella,‡ Christoph F€ orster,§ Alan R. Kennedy,§ Jan Klett,§ Robert E. Mulvey,*,§ and Eva Rentschler‡ ‡
Institut f€ ur Anorganische Chemie und Analytische Chemie, Johannes-Gutenberg-Universit€ at Mainz, Duesbergweg 10-14, 55128 Mainz, Germany, and §WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, UK G1 1XL Received April 21, 2010 Summary: No Cr-Cr bonding is found in a new type of mixedmetal ate complex having two coordinatively unsaturated but sterically saturated bisamido-monoalkyl Cr(II) groups linked via an unusual near-square-planar-coordinated K atom in the anionic moiety of the ate, while the cationic moiety is a separated tris-tmeda solvated second potassium atom. Alkali-metal organochromates have attracted attention since the beginning of the 1970s, in particular through Krause’s intriguing report of [Me8Cr2Li4(thf)4].1 Like the parent alkyllithium structure of [{(MeLi)4}¥] determined a few years earlier,2 the methylchromate structure marked a milestone in that it revealed a supershort Cr-Cr distance (1.968(2) A˚)3 initially thought to represent a quadruple Cr-Cr bond without supporting bridging interactions. Gambarotta has since presented compelling evidence4 suggesting that in fact the small separation distance between the two Cr centers is an unavoidable artifact of a network of Cr-Me 3 3 3 Li 3 3 3 Me-Cr agostic interactions as opposed to an indicator of significant intermetallic bonding. Interestingly, the unique properties of methyllithium (for example, lack of solubility in hydrocarbon solvents) can also be attributed to a network of Li-Me 3 3 3 Li agostic interactions between tetrameric (MeLi)4 subunits.5 Given this critical relationship between the structures of alkali-metal organochromates and alkali-metal 3 3 3 H-C agostic interactions,6 and the fact that the nature of the latter depends on the individuality of the alkali metal (Li vs Na vs K etc.), it is surprising that only a small number of such chromates have been investigated. Heteroleptic organochromates are of special interest to us as part of our continuing development of “alkali-metal-mediated metalation”,7,8 and thus we have recently reported the † Part of the Dietmar Seyferth Festschrift. Dedicated to Dietmar Seyferth, a true “giant” of organometallic chemistry. *To whom correspondence should be addressed. E-mail: r.e.mulvey@ strath.ac.uk. (1) Krausse, J.; Mark, G.; Schaedl, G. J. Organomet. Chem. 1970, 21, 159. (2) Weiss, E. Angew. Chem., Int. Ed. 1993, 32, 1501. (3) For a more accurate redetermination of the structure: Haa, S.; Song, J. I.; Berna, P.; Gambarotta, S. Organometallics 1994, 13, 1326. (4) Haa, S.; Gambarotta, S.; Bensimon, C. J. Am. Chem. Soc. 1992, 114, 3556. (5) Stey, T.; Stalke, D. In The Chemistry of Organolithium Compounds; ed. Rappoport, Z., Marek, I., Eds.; Wiley: London, 2004; Chapter 2. (6) Klinkhammer, K. W. Chem. Eur. J. 1997, 3, 1418. (7) Mulvey, R. E. Acc. Chem. Res. 2009, 42, 743. (8) Mulvey, R. E.; Mongin, F.; Uchiyama, M.; Kondo, Y. Angew. Chem., Int. Ed. 2007, 46, 3802.
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Published on Web 07/09/2010
monoalkyl-bisamido chromate [(TMEDA)Na(TMP)(CH2SiMe3)Cr(TMP)] (TMEDA = N,N,N0 ,N0 -tetramethylethylenediamine; TMP = 2,2,6,6-tetramethylpiperidide) and the sodium chromate inverse crown [Na4(1,4-Cr2C6H4)(μ-TMP)6].9 Eager to expand the underdeveloped area of potassium mediation, toward this goal, we report herein what is, to the best of our knowledge, the first potassium heteroleptic organochromate in [{K(TMEDA)3}þ{(Me3Si)2NCr{μ-N(SiMe3)2}(μ-CH2SiMe3)K(μ-CH2SiMe3){μ-N(SiMe3)2}CrN(SiMe3)2}-] (1).10 This remarkable complex adopts a structure unprecedented in potassium ate chemistry and alkalimetal ate chemistry generally, which though dinuclear in Cr(II) atoms is devoid of any Cr-Cr interaction. In previous potassium zincate research11 we have synthesized heteroleptic formulations via a simple cocomplexation procedure in which a potassium amide complex is mixed with a zinc bis(alkyl) complex in the presence of a donor solvent. Here in the synthesis of 1 we similarly mixed together equimolar proportions of the potassium alkyl12 KCH2SiMe3,11 the donor solvent TMEDA, and the chromium amide [Cr{N(SiMe3)2}2 3 2THF]13 in hexane solution (Scheme 1). From the formula of 1, it can be deduced that cocomplexation of (9) Albores, P.; Carrella, L. M.; Clegg, W.; Garcı´ a Alvarez, P.; Kennedy, A. R.; Klett, J.; Mulvey, R. E.; Rentschler, E.; Russo, L. Angew. Chem., Int. Ed. 2009, 48, 3317. (10) Synthesis of [{K(TMEDA)3}þ{(Me3Si)2NCr{μ-N(SiMe3)2}(μCH2SiMe3)K(μ-CH2SiMe3){μ-N(SiMe3)2}CrN(SiMe3)2}-] (1): TMEDA (0.15 mL, 1.0 mmol) was added to a suspension of KCH2SiMe3 (0.13 g, 1.0 mmol) in n-hexane (20 mL). Then (thf)2Cr[N(SiMe3)2]2 (0.52 g, 1.0 mmol) was added; the resulting green solution was stirred for 1.5 h and then filtered. Removing most of the solvent under vacuum and storing the solution at -27 C afforded a crop of green crystals (0.26 g, 38.6%). 1H NMR (400.13 Hz, d6-benzene, 300 K): δ 1.8, 35.8 ppm; UV-vis (hexane): λmax 700 nm. Anal. Calcd for C50H142Cr2K2N10Si10: C, 44.59; H, 10.63; N, 10.40. Found: C, 44.08; H, 10.92; N, 10.56. Crystal data for 1: C50H142Cr2K2N10Si10, Mr = 1346.84, monoclinic, space group I2/a, a = 23.2373(18) A˚, b = 17.7035(14) A˚, c = 21.5073(17) A˚, β = 108.206(9), V = 8404.8(11) A˚3, Z = 4, λ = 0.710 73 A˚, μ = 0.534 mm-1, T = 123 K; 43 417 measured reflections, 8116 unique reflections, Rint = 0.1180; final refinement to convergence on F2 gave R = 0.0570 (F, 2648 observed data only) and Rw = 0.1112 (F2, all unique data), GOF = 0.895. (11) Conway, B.; Graham, D. V.; Hevia, E.; Kennedy, A. R.; Klett, J.; Mulvey, R. E. Chem. Commun. 2008, 2638. (12) (a) Seyferth, D.; Freyer, W. J. Org. Chem. 1961, 26, 2604. (b) Robison, J. L.; Davis, W. M.; Seyferth, D. Organometallics 1991, 10, 3385. (c) Seyferth, D.; Lang, H. Organometallics 1991, 10, 551. (13) (a) Bradley, D. C.; Hursthouse, M. B.; Newing, C. W.; Welch, A. J. Chem. Commun. 1972, 567. (b) Horvath, B.; Strutz, J.; Horvath, E. G. Z. Anorg. Allg. Chem. 1979, 475, 38. r 2010 American Chemical Society
Communication
Organometallics, Vol. 29, No. 21, 2010
Figure 1. Molecular structure of the anion of 1, with hydrogen atoms omitted for clarity. Selected bond lengths (A˚) and bond angles (deg): Cr(1)-C(1) = 2.112(4), Cr(1)-N(1) = 2.009(3), Cr(1)-N(2) = 2.068(3), K(1)-C(1) = 3.050(4), K(1)-N(2) = 2.853(3), K(1)-C(11)=3.420(4), K(1)-C(16)=3.456(4), C(1)Si(1)=1.848(4); C(1)-Cr(1)-N(1)=117.61(16), N(1)-Cr(1)N(2) = 137.07(12), N(2)-Cr(1)-N(1) = 104.84(15), Cr(1)C(1)-Si(1) = 116.7(2). Symmetry operator A: -x þ 1/2, -y þ 1/2, -z þ 1/2.
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Figure 2. (a) Molecular structure of the cation of 1, with hydrogen atoms omitted for clarity. (b) Octahedral coordination of K1 by six carbons, with two nitrogens capping opposite faces. Scheme 2. Classical “Weiss” Motif of a Trinuclear Ate Compound
Scheme 1. Formation of 1
two units each of the potassium alkyl and the chromium amide has taken place though incompletely, as one Kþ cation has been sequestered by three TMEDA ligands to generate a solvent-separated ion pair arrangement. On the basis of TMEDA, the isolated yield of green crystalline 1 was 38.6%. The identity of 1 was established by an X-ray crystallographic study. While the [K(TMEDA)3]þ cation is a simple distorted octahedron which merits no further discussion,14 the molecular structure of the anion (Figure 1) is unknown, unexpected, and unconventional (for full structural details see the Supporting Information). Located at an inversion center, K1 possesses an extraordinary near-square-planar coordination with respect to the attached two N (from HMDS) and two C (from CH2SiMe3) anionic ligands, the like atoms of which lie trans to each other as imposed by the centrosymmetry (HMDS = 1,1,1,3,3,3hexamethyldisilazide=(Me3Si)2N). Thus, the N(2)-K(1)N(2*) and C(1)-K(1)-C(1*) bond angles are exactly 180. The trinuclear Cr(1) 3 3 3 K(1) 3 3 3 Cr(1*) arrangement is also linear. Pertinent to the intrigue surrounding [Me8Cr2Li4(thf)4], 1 also features close contacts which could be interpreted as agostic alkali-metal 3 3 3 H-C interactions involving K(1) and two C atoms (C(11) and C(16)) from distinct Me groups of the HMDS ligands. On the basis of their lengths (K(1)-C(11) = 3.420(4) A˚; K(1)-C(16) = 3.456(4) A˚), these contacts are considerably longer than those bonds involving the C anions (K(1)-C(1) = 3.050(4) A˚), though (14) Disorder in one of the TMEDA molecules prevents a discussion of the metrical parameters of this cationic moiety.
the shortest bonds are to the N anions (K(1)-N(2) = 2.853(3) A˚). An alternative description of the coordination polyhedron surrounding K(1) when including agostic interactions is of a C6 octahedron bicapped by trans-oriented N atoms (see Figure 2). Cr(1) adopts a distorted-trigonalplanar geometry (sum of bond angles 359.52) with distortion most pronounced at N(1)-Cr(1)-N(2) (137.07(12)) to avoid steric clashing of the bridging and terminal, sterically demanding HMDS ligands. To construct the bridge, the N(2)-Cr(1)-C(1) bond angle is the smallest of the three at 104.84(15), some 12.77 smaller than the N(1)-Cr(1)-C(1) (117.61(16)) junction spanning bridging and terminal positions. Highlighting the size differential between K and Cr(II), the N(2)-K(1)-C(1) bond angle is extremely acute at 68.19(9), some 36.65 less than the transannular N(2)-Cr(1)-C(1) bond angle, while the bimetallic, four-element (K(1)N(2)Cr(1)C(1)) ring is puckered overall (sum of endocyclic bond angles 347.14). The classical “Weiss” motif2 for an alkali-metal trinuclear ate compound of general formula [L 3 AM(μ-R)2M(μ-R)2AM 3 L] (where AM = alkali metal; M = divalent metal; L = Lewis base ligand; R = anionic ligand) possesses a 2:1, AM: M stoichiometry and accommodates the alkali metal in the terminal positions of the trimetallic chain (Scheme 2). In the context of the earlier discussion, a relevant example would be [TMEDA 3 Li(μ-Me)2Cr(μ-Me)2Li 3 TMEDA],4 a key piece of evidence in disfavoring strong Cr-Cr bonding in the THF analogue [Me8Cr2Li4(thf)4]. In such Weiss motifs the alkali metal almost always adopts a tetrahedral geometry. Departing markedly from these conventions, the anion of 1 possesses a reversed 1:2 AM:M stoichiometry and the potassium occupies a central Cr 3 3 3 K 3 3 3 Cr position and has a near-square-planar coordination. Emphasizing the structural novelty of 1, a search of the Cambridge Structural Database (CSD)15 for a “M(μ-R)2AM(μ-R)2M” motif (where M = any metal; R = any ligand) revealed no hits both for AM = K specifically and AM = Li, Na generally, in which AM is in a square-planar coordination. Although it is still (15) Allen, F. H. Acta Crystallogr. 2002, B58, 380.
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rare, a few complexes exhibit a “Al(μ-R)2Li(μ-R)2Al” motif, as for example in [(LiAl(NHiPr)4)¥];16 however, without exception the Li coordination in these complexes is tetrahedral. Interestingly we found no hits for bimetallic potassiumchromium complexes containing either the Me3SiCH2- or (Me3Si)2N- ligand. There are several reports of KHMDSbased crystal structures with K-N bond lengths spanning 2.746-3.050 A˚ and a mean value of 2.839 A˚. From the context of bimetallic chemistry, comparisons can be drawn with Williard’s landmark structures [(thf)AM{μ-N(SiMe3)2}2K(thf)2]17 (AM = Li, Na), which established that bimetallic modifications of important alkali-metal utility bases could be easily prepared (K-N bond lengths 2.832(3) and 2.810(3) A˚, respectively; cf. 2.853(3) A˚ in 1). Only one Cr(II) HMDS structure13 is available for comparison, the simple transsquare-planar monomer [Cr{N(SiMe3)2}2 3 2THF], which has a Cr-N bond length of 2.089(10) A˚ (cf. mean of 2.038 A˚ in 1). Predominately Cr(III) examples, there are many published structures featuring Cr-CH2SiMe3 linkages18 (the lack of β-H atoms on the alkyl chain eliminates the possibility of a β-hydride decomposition pathway common to other transition-metal-alkyl bonds) with Cr-C bond lengths spanning the range 1.953-2.501 A˚. Measuring 2.112(4) A˚, the Cr1-C1 bond length in 1 is toward the middle of this range. The paramagnetic nature of 1 (note that Cr(II) is a d4 system) was confirmed by a 1H NMR spectrum recorded in [D6]benzene solution. Two broad resonances were revealed at 35.8 and 1.8 ppm. The former is tentatively assigned to the N(SiMe3)2 group and the latter to overlapping Me2NCH2CH2NMe2 and CH2SiMe3 resonances (next to the paramagnetic center, the CH2SiMe3 resonance is not observed). Additional investigations of the magnetic properties have been performed using a SQUID magnetometer. The temperature dependence of the magnetic moment indicates no significant interaction between the two chromium centers. A g value of 1.9 was observed for the Cr(II) sites, confirming the d4 configuration.
Bonded to only three ligands as opposed to four in [Me8Cr2Li4(thf)4], the Cr atom in 1 can be considered coordinatively unsaturated. In theory this coordinative unsaturation could encourage significant Cr-Cr bonding within the bis-chromium anionic moiety if such bonding is structurally important. However, steric saturation by the three bulky ligands appears to inhibit the two Cr atoms from closely approaching each other (the Cr 3 3 3 Cr internuclear separation is 7.0506(12) A˚) and, as in the former structure, it is alkali-metal-ligand bridges in the main which account for the bis-chromium nuclearity of 1. At the simplest level, 1 clearly fits the description of a potassium chromate. More specifically, it could be described as a potassium potassiochromate or in full, counting the ligands, as a potassium bisalkyl-tetraamido-potassiobischromate. While there are other ate complexes that bear a degree of resemblance to 1, for example the potassium potassiate [(thf)1.5K{cyclo-NC(Ph)NC(Bun)(But)NC(Ph)}],19 the sodium sodate [Na{Na[N(SiMe3)2AlMe3]2}],20 and the sodium sodiumdizincate [(3-Me-C6H4CN)2Na(TMEDA)2]þ[{6-Zn(But)2-3-Me-C6H3CN}2Na(TMEDA)2]-,21 1 clearly represents a novel new category of ate. Studies are underway to determine whether the unique packaging of the distinct metal atoms and Brønsted basic ligands within an anionic framework will have an influence on the reactivity of 1 vis- a-vis conventional alkali-metal chromate complexes.
(16) Eisler, D. J.; Chivers, T. Can. J. Chem. 2006, 84, 443. (17) Williard, P. G.; Nichols, M. A. J. Am. Chem. Soc. 1991, 113, 9671. (18) (a) Kuzeika, J.; Legzdins, P.; Rettig, S. J.; Smith, K. M. Organometallics 1997, 16, 3569. (b) Schulzke, C.; Enright, D.; Sugiyama, H.; LeBlanc, G.; Gambarotta, S.; Yap, G. P. A. Organometallics 2002, 21, 3810.
(19) Clegg, W.; Horsburgh, L.; Mulvey, R. E.; Ross, M. J. Chem. Commun. 1994, 2393. (20) Niemeyer, M.; Power, P. P. Organometallics 1996, 15, 4107. (21) Clegg, W.; Dale, S. H.; Hevia, E.; Hogg, L. M.; Honeyman, G. W.; Mulvey, R. E.; O’Hara, C. T.; Russo, L. Angew. Chem., Int. Ed. 2008, 47, 731.
Acknowledgment. We are grateful to the EPSRC (Grant Award Nos. EP/F063733/1 and EP/D076889/1) and the Royal Society/Wolfson Foundation (research merit award to R.E.M.) for their generous support of this research. Supporting Information Available: Text and figures giving experimental details, spectroscopic data, and magnetic susceptibility data and a CIF file giving full crystallographic data for compound 1. This material is available free of charge via the Internet at http://pubs.acs.org.
