Inorg. Chem. 2008, 47, 5293-5303
Synthesis, Characterization, and Electronic Structure of Diimine Complexes of Chromium Kevin A. Kreisel,† Glenn P. A. Yap, and Klaus H. Theopold* Department of Chemistry and Biochemistry, Center for Catalytic Science and Technology, UniVersity of Delaware, Newark, Delaware 19716 Received February 18, 2008
We have prepared and structurally characterized several complexes of chromium coordinated by diimine (or 1,4diazadiene) ligands, that is, ArsNdC(R)s(R)CdNsAr (RLAr) (where Ar ) 2,6-diisopropylphenyl (“iPr”) or 2,6dimethylphenyl (“Me”) and R ) H or Me). The reaction of CrCl2 with HLiPr gave dinuclear [(HLiPr)Cr]2(µ-Cl)3(Cl)(THF) when isolated from Et2O; in THF solution, however, the product exists as mononuclear (HLiPr)CrCl2(THF)2. Two isostructural derivatives, (MeLMe)CrCl2(THF)2 and (HLMe)CrCl2(THF)2, have also been prepared. Furthermore, the bis-ligand complex, (HLiPr)2Cr, has been prepared along with its reduction product, Li(THF)4[(HLiPr)2Cr]. We have also synthesized the tetracarbonyl complex, (HLiPr)Cr(CO)4, by addition of HLiPr to Cr(CO)4(NCCH3)2. The structure and variable temperature magnetic susceptibility of the previously reported Cr halide dimer, [(HLiPr)Cr(µ-Cl)]2, is also discussed in detail. All of the diimine complexes have been characterized structurally, spectroscopically, and magnetically, and their electronic structures are discussed with the aid of density-functional theory calculations.
Chart 1. Possible Electronic Structures of Diimine Complexes
Introduction As part of our investigation of chromium alkyls as homogeneous models for commercially used heterogeneous chromium catalysts for the polymerization of ethylene and R-olefins,1,2 we required a class of neutral, bidentate nitrogen ligands to compare the reactivity of their chromium complexes with that of the structurally related “nacnac” (i.e., R-diketiminate) derivatives. Inspired by the extensive use of diimine complexes of late transition metals in polymer-
* To whom correspondence should be addressed. E-mail: theopold@ udel.edu. † Current address: University of Wisconsin-Madison, Department of Chemistry, 1101 University Ave. Madison, WI 53706. (1) (a) Richeson, D. S.; Hsu, S. W.; Fredd, N. H.; Van Duyne, G.; Theopold, K. H. J. Am. Chem. Soc. 1986, 108, 8273. (b) Thomas, B. J.; Noh, S. K.; Schulte, G. K.; Sendlinger, S. C.; Theopold, K. H. J. Am. Chem. Soc. 1991, 113, 893. (c) Thomas, B. J.; Theopold, K. H. J. Am. Chem. Soc. 1988, 110, 5902. (d) Thomas, B. J.; Mitchell, J. F.; Leary, J. A.; Theopold, K. H. J. Organomet. Chem. 1988, 348, 333. (e) White, P. A.; Calabrese, J.; Theopold, K. H. Organometallics 1996, 15, 5473. (f) Liang, Y.; Yap, G. P. A.; Rheingold, A. L.; Theopold, K. H. Organometallics 1996, 15, 5284. (2) (a) Kim, W.; Fevola, M. J.; Liable-Sands, L. M.; Rheingold, A. L.; Theopold, K. H. Organometallics 1998, 17, 4541. (b) MacAdams, L. A.; Kim, W.; Laible-Sands, L. M.; Guzei, I. A.; Rheingold, A. L.; Theopold, K. H. Organometallics 2002, 21, 952. (c) MacAdams, L. A.; Buffone, G. P.; Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 2005, 127, 1082.
10.1021/ic800302w CCC: $40.75 Published on Web 04/29/2008
2008 American Chemical Society
ization catalysis,3 we chose to initiate a study of diimines (or 1,4-diazadienes) as ancillary ligands in organochromium chemistry. Diimine ligands are popular because of the ease of varying the steric and electronic effects that they exert upon the metal in their complexes.4 Furthermore, the redox properties of these ligands provide an interesting electronic relationship between the ligand and the metal (see Chart 1). Diimine complexes can be classified as having electronic structures A, B, or C in Chart 1.5 In the case of A, the diimine ligand is neutral and structurally essentially unperturbed; that is, it (3) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV 2000, 100, 1169; and references therein. (4) van Koten, G.; Vrieze, K. AdV. Organomet. Chem. 1982, 21, 151. (5) (a) Milione, S.; Cavallo, C.; Tedesco, C.; Grassi, A. J. Chem. Soc., Dalton Trans. 2002, 1839. (b) Hannant, M. D.; Schormann, M.; Bochmann, M. J. Chem. Soc., Dalton Trans. 2002, 4071. (c) Gibson, V. C.; Tomov, A.; Wass, D. F.; White, A. J. P.; Williams, D. J. J. Chem. Soc., Dalton Trans. 2002, 2261. (d) Schleis, T.; Spaniol, T. P.; Okuda, J.; Heinemann, J.; Mulhaupt, R. J. Organomet. Chem. 1998, 569, 159.
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Kreisel et al. shows C-N bond distances that are in accord with double bonds and a central C-C single bond distance. Transfer of one electron from the metal to the ligand gives B, which can best be described as featuring a delocalized ligandcentered radical, typically with very strong antiferromagnetic coupling between any unpaired electrons on the metal and the ligand centered spin.6 Structurally, B would show lengthened C-N bond distances and a shortened C-C distance compared to A.7 Transfer of a second electron from the metal to the diimine ligand results in the electronic structure of C, that is, a coordinated enediamide.8 C would show longer C-N distances yet and a short C-C distance that approaches typical double bond length. However, bond distances alone are too blunt a tool to accurately assess the degree of reduction of the ligand(s) and hence the formal oxidation state of the metal.9 Furthermore, the truth may lie somewhere between the idealized structures of A, B, and C, whichsafter allsare but limited valence bond descriptions of particular points of a continuum. Work on nickel diimine complexes by Wieghardt and others has greatly improved our understanding of the unique interplay between these ligands and transition metals.10 Specifically, it has been shown that a combination of structural, spectroscopic, and computational techniques is profitably employed to fully understand the complex electronic structure of these systems, and in some cases there remains some room for argument.11 In any event, from a reactivity viewpoint one of the attractive features of complexes of this sort is their electronic malleability; in other words, depending on the nature of the other ligands and the overall charge of the complex, the diimine ligand may relieve the metal of excess electron density or supply it when required. Functioning as a sort of “electronic buffer”, coordinated diimines may thus facilitate transformations that would otherwise strain the metal’s tolerance for oxidation or reduction.12 With this idea in mind, we are exploring the reactivity of chromium coordinated by aryl-substituted diimines. (6) Tsukahara, Y.; Kamatani, T.; Iino, A.; Suzuki, T.; Kaizaki, S. Inorg. Chem. 2002, 41, 4363. (7) (a) Bart, S. C.; Hawrelak, E. J.; Schmisseur, A. K.; Lobkovsky, E.; Chirik, P. J. Organometallics 2004, 23, 237. (b) Ionkin, A. S.; Marshall, W. J. J. Organomet. Chem. 2004, 689, 1057. (c) Gardiner, M. G.; Hanson, G. R.; Henderson, M. J.; Lee, F. C.; Raston, C. L. Inorg. Chem. 1994, 33, 2456. (8) (a) Richter, B.; Scholz, J.; Neumu¨ller, B.; Weimann, R.; Schumann, H. Z. Anorg. Allg. Chem. 1995, 621, 365. (b) Fedushkin, I. L.; Skatova, A. A.; Chudakova, V. A.; Fukin, G. K. Angew. Chem., Int. Ed. 2003, 42, 3294. (9) (a) Rindorf, G.; Thorup, N.; Bjornhom, T.; Bechgaard, K. Acta. Cryst., Sect. C 1990, C46, 1437. (b) Ray, K.; Weyhermu¨ller, T.; Goossens, A.; Craje´, M. W. J.; Wieghardt, K. Inorg. Chem. 2003, 42, 4082. (10) (a) Balch, A.; Holm, R. H. J. Am. Chem. Soc. 1966, 88, 5201. (b) Walther, D. Z. Anorg. Allg. Chem. 1974, 405, 8. (c) Khusniyarov, M. M.; Harms, K.; Burhaus, O.; Sundermeyer, J. Eur. J. Inorg. Chem. 2006, 2985. (d) Herebain, D.; Bothe, E.; Neese, F.; Weyhermu¨ller, T.; Wieghardt, K. J. Am. Chem. Soc. 2003, 125, 9116. (e) Herebain, D.; Wieghardt, K.; Neese, F. J. Am. Chem. Soc. 2003, 125, 10997. (f) Chlopek, K.; Bothe, E.; Neese, F.; Weyhermu¨ller, T.; Wieghardt, K. Inorg. Chem. 2006, 45, 6298. (g) Muresan, N.; Chlopek, K.; Weyhermu¨ller, T.; Neese, F.; Wieghardt, K. Inorg. Chem. 2007, 46, 5327. (h) Muresan, N.; Weyhermu¨ller, T.; Wieghardt, K. Dalton Trans. 2007, 4390. (11) Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky, E.; Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 13901.
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To complement our work with nacnac chromium complexes,2we have recently taken an interest in the organometallic chemistry of chromium supported by neutral ligands.13 One aim of this undertaking was to synthesize cationic Cr(II) alkyl complexes to compare their reactivity with that of structurally related cationic nacnacCr(III) alkyls. Such a direct comparison might shed light on the longstanding conundrum concerning the formal oxidation state of chromium during these catalytic processes when activated by alkyl aluminum cocatalysts.14 Diimine chromium complexes have been synthesized and studied in the past,15 but there have been no examples of N-Aryl substituted ligands coordinated to chromium(II) except for a recent report,16 which we believe to be in error. Herein we summarize the coordination chemistry of the diimine ligands ArsNdC(R)s (R)CdNsAr (RLAr) (where Ar ) 2,6-diisopropylphenyl (“iPr”) or 2,6-dimethylphenyl (“Me”) and R ) H or Me) when bound to a variety of low-valent chromium fragments. Organometallic derivatives of these compounds will be the focus of a separate report. Results and Discussion Syntheses and Structures of (RLAr)Cr Complexes. Our initial foray into chromium diimine chemistry employed the H iPr L ligand and CrCl2 (see Scheme 1). Thus, to a slurry of CrCl2 in tetrahydrofuran (THF) was added one equivalent of HLiPr, to give a dark brown solution. After stirring overnight, solvent removal, extraction with, and crystallization from Et2O produced dark green crystals of [(HLiPr)Cr]2(µ-Cl)3(Cl)(THF) (1′), which were structurally characterized by X-ray diffraction. The molecular structure of 1′ is shown in Figure 1, and selected interatomic distances and angles are listed in Table 1. The asymmetric unit of the crystal contains two independent, but chemically equivalent mol(12) (a) Blackmore, K. J.; Ziller, J. W.; Heyduk, A. F. Inorg. Chem. 2005, 44, 5559. (b) Haneline, M. R.; Heyduk, A. F. J. Am. Chem. Soc. 2006, 128, 8410. (c) Blackmore, K. J.; Ziller, J. W.; Heyduk, A. F. Inorg. Chem. 2008, 47, 265. (d) Stanciu, C.; Jones, M. E.; Fanwick, P. E.; Abu-Omar, M. M. J. Am. Chem. Soc. 2007, 129, 12400. (13) (a) Kreisel, K. A.; Yap, G. P. A.; Theopold, K. H. Organometallics 2006, 25, 4670. (b) Kreisel, K. A.; Yap, G. P. A.; Theopold, K. H. Chem. Commun. 2007, 1510. (14) (a) Budzelaar, P. H. M.; van Oort, A. B.; Orpen, A. G. Eur. J. Inorg. Chem. 1998, 1485. (b) Rahim, M.; Taylor, N. J.; Xin, S. X.; Collins, S. Organometallics 1998, 17, 1315. (c) Lee, L. W. M.; Piers, W. E.; Elsegood, M. R. J.; Clegg, W.; Parvez, M. Organometallics 1999, 18, 2947. (d) McGuinness, D. S.; Brown, D. B.; Tooze, R. P.; Hess, F. M.; Dixon, J. T.; Slawin, A. M. Z. Organometallics 2006, 25, 3605. (e) Jabri, A.; Temple, C.; Crewdson, P.; Gambarotta, S.; Korobkov, I.; Duchateau, R. J. Am. Chem. Soc. 2006, 128, 9238. (15) (a) Franz, K. D.; tom Dieck, H.; Krynitz, U.; Renk, I. W. J. Organomet. Chem. 1974, 64, 361. (b) tom Dieck, H.; Kinzel, A. Angew. Chem., Int. Ed. Engl. 1979, 18, 324. (c) Balk, R. W.; Snoeck, T.; Stufkens, D. J.; Oskam, A. Inorg. Chem. 1980, 19, 3015. (d) Bell, A.; Walton, R. A. Polyhedron 1986, 5, 951. (e) Bell, A.; Walton, R. A. Polyhedron 1986, 5, 1321. (f) Holzmeier, P.; Goerner, H.; Knoch, F.; Kisch, H. Chem. Ber. 1989, 122, 1457. (g) Grevels, F.-W.; Kayran, C.; Ozkar, S. Organometallics 1994, 13, 2937. (h) Bildstein, B.; Malaun, M.; Kopacka, H.; Fontani, M.; Zanello, P. Inorg. Chim. Acta 2000, 300-302, 16. (16) Turki, T.; Guerfel, T.; Bouachir, F. Inorg. Chem. Commun. 2006, 9, 1023; We believe that the diamagnetism of the tetrahedral (diimine)MCl2 complexes as well as the crystal structure of 2b2 reported in this paper are more consistent with zinc complexesszinc was used as a reducing agent. We have communicated our view to the authors.
Diimine Complexes of Chromium Scheme 1. Synthesis of Diimine Cr Halide Complexes 1, 2, 3, and 1′
ecules; only one of these is shown in Figure 1. In the solid state, 1′ is an unsymmetric, dinuclear complex with both chromium atoms exhibiting distorted octahedral coordination. Each chromium atom is coordinated by a diimine ligand and three bridging chlorides; the dissymmetry is introduced by a THF molecule coordinated to Cr(1), while Cr(2) is bonded to a terminal chloride ligand. The Cr-Cl distances to the bridging chlorides range from 2.350(1) to 2.452(1) Å and fall within the range of other bridging chloride complexes of chromium (2.293-2.813 Å; avg ) 2.397 Å).17 The shorter Cr-Cl distances of the terminal chlorides are also unexceptional at 2.279(1) and 2.291(1) Å. The Cr-N distances range from 1.964(4) to 1.993(4) Å and are slightly shorter than most other reported Cr(II)-N bond lengths (avg ) 2.079 Å).18 More interesting are the C-N and C-C bond distances of the diimine ligands, which deviate substantially from those of the free ligand (C-N, 1.2632(2) Å; C-C, 1.468(2) Å).7c The average C-N distance in 1′ is 1.333(5) Å, whereas the average C-C distance is only marginally longer at 1.391(6) Å. These distances suggest some reduction of the diimine ligand, possibly amounting to the formation of a ligand centered radical (i.e., B in Chart 1).10g,h Consequently, the Cr atoms in 1′ may be best described as adopting the Cr(III) oxidation state. The room temperature magnetic moment of 1′ (µeff ) 2.8(1) µB per Cr) is consistent either with quartet Cr(III) strongly antiferromagnetically coupled to a ligand centered radical (S ) 1/2) or, however improbable, with low spin octahedral Cr(II) (d4, S ) 1). The distance between the two chromium atoms (Cr(1)-Cr(2) ) 3.090(4) Å) militates against metal-metal bonding, and the agreement between measured and expected magnetic moment renders significant antiferromagnetic coupling between the Cr atoms unlikely. Indeed, the effective magnetic moment of 1′ is similar to that of the monomeric species 3 (vide infra), strongly suggesting that each Cr atom in 1′ is acting as an isolated paramagnet. We suggest that the most appropriate description of the electronic structure of 1′ is as a dinuclear complex containing two noninteracting Cr(III) ions, each coordinated by, and antiferromagnetically coupled to a diimine radical anion. Green CD2Cl2 solutions of 1′ showed very broad and isotropically shifted resonances in the 1H NMR spectrum, (17) A search of the Cambridge Structural Database for chloride ligands bridging two chromium atoms resulted in 116 hits. Allen, F. H. Acta Crystallogr. 2002, B58, 380. (18) A search of Cambridge Structural Database for Cr(II) complexes with single Cr-N bonds resulted in 832 hits. Allen, F. H. Acta Crystallogr. 2002, B58, 380.
Figure 1. ORTEP plot of one of the two independent molecules of [(HLiPr)Cr]2(µ-Cl)3(Cl)(THF) (1′) at the 30% probability level. Hydrogen atoms have been omitted for clarity.
with overlapping peaks at 16.1, 0.51, -1.9, -7.4, and -21.2 ppm. In contrast, solutions of 1′ in THF-d8 were brown and revealed a dramatically different spectrum with resolved resonances at 20.0, 0.27, -1.9, and -10.2 ppm. These 1H NMR data, as well as the structures of the related complexes 2 and 3 suggest that in THF solution 1′ dissociates into mononuclear (HLiPr)CrCl2(THF)2 (1). Unfortunately, the high solubility of 1 in THF precluded its crystallization and structural characterization. Reaction of MeLMe or HLMe with CrCl2 in THF gave similar brown solutions (see Scheme 1). Crystallization of these products from THF afforded (MeLMe)CrCl2(THF)2 (2) and (HLMe)CrCl2(THF)2 (3). The molecular structure of 2, as determined by X-ray diffraction, is shown in Figure 2; its interatomic distances and angles are listed in Table 2. 2 features slightly distorted octahedral coordination, with cis bond angles ranging from 79.68(6)° to 96.48(5)° and trans angles ranging from 170.87(2)° to 174.95(6)°. The terminal chloride distances in 2 (2.309(1) and 2.340(1) Å) are similar to those in 1′, as are the Cr-N distances (1.983(2) and 1.969(2) Å). The Cr-OTHF distances of 2.148(2) and 2.154(2) Å are also on par with other reported THF complexes of chromium.19 Like 1′, 2 shows evidence of diimine reduction with C-N distances of 1.347(2) and 1.343(2) Å and a C-C distance of 1.407(2) Å. Once again, this suggests the existence of a radical centered on the diimine ligand. The molecular structure and metric data of 3 are similar to those of 2 and therefore the full structural details have been relegated to the Supporting Information. 3, and by analogy 2 (see Experimental Section), has a magnetic moment of 2.9(1) µB which is consistent with an S ) 1 spin state. The same options as described above for 1′ exist, and akin to the latter, the electronic structures of 2 and 3 are most straightforwardly rationalized by invoking strong antiferro(19) (a) Bazan, G. C.; Rogers, J. S.; Fang, C. C. Organometallics 2001, 20, 2059. (b) Bhandari, G.; Kim, Y.; McFarland, J. M.; Rheingold, A. L.; Theopold, K. H. Organometallics 1995, 14, 738. (c) Dionne, M.; Jubb, J.; Jenkins, H.; Wong, S.; Gambarotta, S. Inorg. Chem. 1996, 35, 1874.
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Kreisel et al. Table 1. Selected Interatomic Distances (Å) and Angles (°) for One of the Two Independent Molecules of Cr(1)-Cl(1) Cr(1)-N(2) Cr(2)-Cl(2) Cr(2)-N(4) N(4)-C(40) N(1)-Cr(1)-N(2) N(1)-Cr(1)-Cl(1) O(1)-Cr(1)-Cl(1) N(3)-Cr(2)-N(4) N(3)-Cr(2)-Cl(3)
2.350(1) 1.986(4) 2.406(1) 1.972(4) 1.334(5)
Cr(1)-Cl(2) Cr(1)-O(1) Cr(2)-Cl(3) N(1)-C(13) C(13)-C(14) 81.0(2) 95.9(1) 165.80(9) 81.1(2) 179.4(1)
Distances 2.381(1) 2.065(3) 2.452(1) 1.328(5) 1.386(6)
(Å) Cr(1)-Cl(3) Cr(1)-Cr(2) Cr(2)-Cl(4) N(2)-C(14) C(39)-C(40)
Angles (°) N(1)-Cr(1)-O(1) N(1)-Cr(1)-Cl(2) O(1)-Cr(1)-Cl(2) N(3)-Cr(2)-Cl(1) N(3)-Cr(2)-Cl(4)
94.1(1) 96.0(1) 87.19(9) 99.5(1) 91.2(1)
[(HLiPr)Cr]2(µ-Cl)3(Cl)(THF)
2.373(1) 3.090(4) 2.279(1) 1.343(5) 1.396(6)
Cr(1)-N(1) Cr(2)-Cl(1) Cr(2)-N(3) N(3)-C(39)
N(2)-Cr(1)-O(1) N(1)-Cr(1)-Cl(3) O(1)-Cr(1)-Cl(3) N(3)-Cr(2)-Cl(2)
(1′) 1.987(4) 2.432(1) 1.964(4) 1.328(5)
95.1(1) 177.4(1) 86.05(9) 95.9(1)
magnetic coupling between a Cr(III) ion and an adjacent diimine ligand radical anion. In an effort to access lower oxidation state complexes of chromium, we began metalating with the one electron reduced form of the diimine ligand, Na[HLiPr], which was formed in situ by reduction with sodium metal (see Scheme 2).20 Addition of dark red THF solutions of Na[HLiPr] to a slurry of CrCl2 in THF did not produce the expected [(HLiPr)Cr(µ-Cl)]2 (4, vide infra), but rather the red-brown bis-ligand complex (HLiPr)2Cr (5) in
