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Organometallics 2010, 29, 3672–3675 DOI: 10.1021/om100543t
Synthesis and X-ray Structure of a Diamagnetic Oxo-Bridged Trifluoromethyl-Chromium(V) Complex: Structural and Computational Comparisons between CF3 and CH3 Ligands in Two Different Oxidation States of Chromium Hui Huang, Arnold L. Rheingold, and Russell P. Hughes* Departments of Chemistry, 6128 Burke Laboratory, Dartmouth College, Hanover, New Hampshire 03755, and University of California, San Diego, California 92093-0358 Received June 1, 2010 Summary: A diamagnetic oxo-bridged chromium(V) complex, [Cp*Cr(O)(CF3)]2O, has been synthesized through exposure of a hexane solution of Cp*Cr(CO)3(CF3) to air in the presence of light and characterized by a single-crystal X-ray diffraction study. DFT calculations have been employed to probe its structure and properties and to provide a comparison between CF3 and CH3 ligands in Cr(V) and Cr(II) analogues.
Dinuclear oxo-bridged transition metal complexes are important in catalysis, biological mimicry, multi-electrontransfer reactions, and metal-metal reactions,1-4 while stable oxochromium(V) complexes have applications in chemistry, biological chemistry, and nuclear physics.5-9 There appear to be only two examples of oxo-bridged transition metal compounds with perfluoroalkyl ligands. Exposure of WCp(NO)2(C3F7) to air resulted in the crystallographically characterized diamagnetic d0 complex 1.10 Similarly, oxidation of MnCp(CO)(C4F6) with O2 led to the paramagnetic oxo-bridged d1 complex 2.11 High oxidation state chromium complexes with CH3 ligands, 3 and 4, were reported to result from O2 oxidation of the Cr(II) precursor [Cp*Cr(μ-CH3)]2.12 Here we report the serendipitous
*To whom correspondence should be addressed. E-mail: rph@ dartmouth.edu. (1) Fortin, S.; Beauchamp, A. L. Inorg. Chim. Acta 1998, 279, 159– 164. (2) Alessio, E.; Zangrando, E.; Iengo, E.; Macchi, M.; Marzilli, P. A.; Marzilli, L. G. Inorg. Chem. 2000, 39, 294–303. (3) Fortin, S.; Beauchamp, A. L. Inorg. Chem. 2000, 39, 4886– 4893. (4) Lazzaro, A.; Vertuani, G.; Bergamini, P.; Mantovani, N.; Marchi, A.; Marvelli, L.; Rossi, R.; Bertolasi, V.; Ferretti, V. J. Chem. Soc., Dalton Trans. 2002, 2843–2851. (5) Rocek, J.; Radkowsky, A. E. J. Am. Chem. Soc. 1968, 90, 2986– 2988. (6) Hasan, F.; Rocek, J. J. Am. Chem. Soc. 1976, 98, 6574–6578. (7) Masaike, A.; Glattli, H.; Ezratty, J.; Malinovski, A. Phys. Lett. 1969, 30A, 63–64. (8) Gl€ attli, H.; Odehnal, M.; Ezratty, J.; Malinovski, A.; Abragam, A. Phys. Lett. 1969, 29A, 250–251. (9) Levina, A.; Barr-David, G.; Codd, R.; Lay, P. A.; Dixon, N. E.; Hammershøi, A.; Hendry, P. Chem. Res. Toxicol. 1999, 12, 371–381. (10) Preut, H.; Varbelow, H. G.; Naumann, D. Acta Crystallogr., Sect. C 1990, C46, 2460–2462. (11) Lentz, D.; Akkerman, F.; Kickbusch, R.; Patzschke, M. Z. Anorg. Allg. Chem. 2004, 630, 1363–1366. (12) Heintz, R. A.; Ostrander, R. L.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 1994, 116, 11387–11396. pubs.acs.org/Organometallics
Published on Web 07/22/2010
synthesis and characterization of the trifluoromethyl analogue of 3 by oxidation of a Cr(II) precursor.
The Cr(II)-CF3 compound 5 was originally isolated as a somewhat air-sensitive yellow solid by decarbonylation of the acyl precursor 6.13 Accidental exposure of a hexane solution of 5 to air and ambient light for 4 days afforded a brownish-red solution with an unidentified dark brown precipitate; filtration gave a solution from which complex 7 was obtained as bright red crystals in ∼50% yield. In our hands the previously reported molybdenum analogue Cp*Mo(CO)3(CF3)14 was unreactive under these conditions.
A single-crystal X-ray diffraction study confirmed the structure of 7; Figure 1 shows an ORTEP representation of the structure with selected bond lengths and angles. Compound 7 has crystallographically imposed C2 symmetry with a 2-fold crystallographic axis passing through the O1 atom and contains a dinuclear structure in which two chromium (13) Huang, H.; Hughes, R. P.; Rheingold, A. L. Organometallics 2010, 29, 1948–1955. (14) Koola, J. D.; Roddick, D. M. Organometallics 1991, 10, 591–597. r 2010 American Chemical Society
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Table 1. Selected Bond Lengths (A˚) and Angles (deg) for 3, 3-B3LYP, 7, and 7-B3LYP
Cr-C11 CrdO2 Cr-O1 C11-CrdO2 Cr-O1-Cr O1-CrdO2 C11-Cr-Cr-C110 O2dCr-CrdO20
Figure 1. ORTEP diagram of 7 with ellipsoids drawn at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (A˚) and angles (deg): Cr1-O2, 1.592(2); Cr1-O1, 1.7602(6); Cr1-C11, 2.047(3); Cr1-C5, 2.225(3); Cr1-C4, 2.244(3); Cr1-C1, 2.246(3); Cr1-C2, 2.329(3); Cr1C3, 2.332(3); F1-C11, 1.356(4); F2-C11, 1.374(3); F3-C11, 1.346(4); Cr1-O1-Cr10 , 170.41(16); O2-Cr1-O1, 109.57(8); O2-Cr1-C11, 96.27(14); O1-Cr1-C11, 91.24(12); F3-C11Cr1, 118.4(2); F1-C11-Cr1, 110.0(2); F2-C11-Cr1, 114.4(2).
atoms are bridged by an oxo ligand. Only the rac-diastereomer is observed. Each chromium center has a typical threelegged piano-stool structure. The two Cp* rings and two terminal oxo ligands are each arranged approximately trans to each other, with the two CF3 ligands approximately cis to each other. Similar overall geometry and identical relative rac-stereochemistry have been reported for the CH3 analogue 3, but in this molecule there is no crystallographically imposed symmetry, and two sets of metric parameters are available for the Cr ligand sets at each end of the molecule.12 Solution spectroscopic studies of 7 are consistent with the solid-state structure and the diamagnetic nature of this compound. There is one singlet peak at δ 1.81 ppm in the 1 H NMR spectrum corresponding to the Cp* ring and a broad singlet peak at δ -20.7 ppm (ω1/2 ≈ 130 Hz) in the 19F NMR spectrum corresponding to the CF3 ligand. A tiny impurity peak (∼1%) at δ -24.3 ppm is also invariably observed in the 19F NMR spectrum, which may be due to the other possible meso-diastereomer. With a view to obtaining more details on the electronic structures of these compounds, full-molecule DFT calculations were carried out on 7 and 3 using the popular hybrid B3LYP15-18 functional, together with the triple-ζ LACV3P**þþ basis set,19-22 which uses extended core potentials on heavy atoms and a 6-311G**þþ basis for (15) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (16) Becke, A. D. J. Chem. Phys. 1993, 98, 1372–1377. (17) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (18) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623–11627. (19) Dunning, T. H.; Hay, P. J. In Modern Theoretical Chemistry, Vol. 4: Applications of Electronic Structure Theory; Schaefer, H. F., III, Ed.; Plenum: New York, 1977. (20) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283. (21) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310. (22) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284–298. (23) Jaguar, versions 7.0-7.5; Schr€odinger, LLC: New York, 2007-2009. (24) Siddall, T. L.; Miyaura, N.; Huffman, J. C.; Kochi, J. K. J. Chem. Soc., Chem. Commun. 1983, 1185–1186.
3
3-B3LYP
7
7-B3LYP
2.086(11); 2.055(10) 1.603(7); 1.584(7) 1.774(6); 1.752(6) 98.0(4); 98.7(4) 173.8(4) 108.8(3); 109.3(3) -1.2 159.4
2.07 1.58 1.75 98.2 172.6 107.8 -11.1 148.6
2.047(3) 1.592(2) 1.7602(6) 96.27(14) 170.41(16) 109.6(3) -14.3 148.3
2.07 1.58 1.75 96.0 169.8 107.7 -21.3 142.1
other atoms, as implemented in the Jaguar23 suite of programs. This provided optimized gas-phase structures 3-B3LYP and 7-B3LYP, and comparison of these structures with those computed for the corresponding meso-diastereomers confirms that the rac-isomers have the lower free energies in the gas phase by 4.4 kcal/mol for 3-B3LYP and 5.5 kcal/mol for 7-B3LYP. A comparison of selected geometric parameters for 3, 3-B3LYP, 7, and 7-B3LYP is shown in Table 1. DFT does well in reproducing the solid-state bond lengths and angles in 3 and 7 using this functional and basis set, but less well in reproducing dihedral angles; this is probably not surprising, as dihedral angles will be the most flexible in responding to any crystal packing forces. The bond length of CrdO1 in 7 is 1.592(2) A˚, similar to Cr(V)dO values of other known complexes (Table 2) and understandably shorter than the bond length of the single bond Cr-O1 [1.7602(6) A˚]. The single-bond Cr-O1 distance is similar to those reported for other Cr-O-Cr bonds in 3 [1.774(6) A˚] and 4 [1.817(4) A˚]. The bond angles [Cr-O1-Cr, 170.41(16)°; O1-Cr-O2, 109.6(3)°; O2-Cr-CF3, 96.27(14)°] in 7 are similar to those of 3 [Cr-O1-Cr, 173.8(4)°; O1Cr-O2, 109.3(3)°; O2-Cr-CH3, 98.0(4)°]. The dihedral angle C-Cr-Cr-C in 3 (-1.2°) is slightly smaller than that in 7 (-14.3°), while the dihedral angle OdCr-CrdO in 3 (159.4°) is slightly bigger than that in 7 (148.3°). The Cr(V) dimer complex 7 is diamagnetic due to strong antiferromagnetic coupling between the two chromium atoms.12 The three Cr(V)-Ob-Cr(V) atoms afford three orbitals [two dxy orbitals from two Cr(V) atoms and one py orbital from Ob] and four electrons [two electrons from two d1 Cr(V) atoms and two electrons from Ob] to form three molecular orbitals. The two trans Ot atoms allow the two dxy orbitals of Cr(V) to be parallel with the py orbital of Ob to allow π-bonding.33 The Kohn-Sham HOMO and LUMO are shown in Figure 2 and constitute the two highest energy π-combinations: the HOMO with one node and the LUMO with two. The synthesis of 7 allows for the first comparison between the structures of a high oxidation state CF3 compound and (25) Herberhold, M.; Kremnitz, W.; Razavi, A.; Sch€ ollhorn, H.; Thewalt, U. Angew. Chem., Int. Ed. Engl. 1985, 24, 601–602. (26) Srinivasan, K.; Kochi, J. K. Inorg. Chem. 1985, 24, 4671–4679. (27) Morse, D. B.; Rauchfuss, T. B.; Wilson, S. R. J. Am. Chem. Soc. 1988, 110, 8234–8235. (28) Noh, S.-K.; Heintz, R. A.; Haggerty, B. S.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 1992, 114, 1892–1893. (29) Hess, J. S.; Leelasubcharoen, S.; Rheingold, A. L.; Doren, D. J.; Theopold, K. H. J. Am. Chem. Soc. 2002, 124, 2454–2455. (30) Wistuba, T.; Limberg, C.; Kircher, P. Angew. Chem., Int. Ed. Engl. 1999, 38, 3037–3039. (31) Kapre, R.; Ray, K.; Sylvestre, I.; Weyherm€ uller, T.; George, S. D.; Neese, F.; Wieghardt, K. Inorg. Chem. 2006, 45, 3499–3509. (32) Meier-Callahan, A. E.; Gray, H. B.; Gross, Z. Inorg. Chem. 2000, 39, 3605–3607. (33) Mayer, J. M. Comments Inorg. Chem. 1988, 8, 125–135.
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Table 2. Cr(V)dO Bond Distances in Cr(V) Compounds Cr(V) complex 12
Cr(V)-Ot (A˚)
(Cp*Cr(O)CH3)2(μ-O) [Cp*Cr(O)(μ-O)]225 Cp*CrOBr227 CrO2Cl(NC9H18) dichlorooxo((η1,η1)-spiro(adamantine-2,20 homoadamantan-3-on-4-oate)chromate(V)30
1.603(7); 1.584(7) 1.594(3) 1.58(2) 1.581(7); 1.574(6) 1.561(2)
[(n-Bu)4N][CrO(LMe)2] LMe=4-methyl-1,2-benzenedithiolato dianion31
1.5796(15)
its CH3 analogue; previous comparative studies have been with compounds in relatively low oxidation states and with relatively high d-electron counts.34-36 The Cr-CF3 bond distance in 7 [2.047(3) A˚] is slightly shorter than the two crystallographically independent Cr-CH3 bond distances in ˚ ], consistent with features observed 3 [2.086(11); 2.055(10) A previously in low oxidation state metal complexes.34-36 However it is noteworthy that the two Cr-CH3 distances in 3 are significantly different from each other, and the shorter of the two is not significantly different from the CrCF3 distance in 7. In the DFT structures the Cr-CH3 and Cr-CF3 distances are calculated to be identical to 0.01 A˚, so it may be an overinterpretation to attribute too much significance to a shorter bond to the fluorinated ligand in this particular comparison. In the Cr(II) complex 5 the CrCF3 distance is 2.129(2) A˚, but while the Cr(II)-CH3 complex CrCp*(CO)3(CH3) (8) is known,37 it does not appear to have been characterized crystallographically. Using the same functional and basis set described above, DFT calculates the Cr-CH3 distance in 8 to be 2.27 A˚ and the Cr-CF3 distance in 5 to be 2.17 A˚, i.e., a significantly shorter value for the fluorinated compound. An NBO38,39 analysis of 3-B3LYP, 7-B3LYP, 5-B3LYP, and 8-B3LYP was also carried out. As expected, the reference resonance structures found by NBO for 5-B3LYP and 8-B3LYP are consistent with d4 Cr(II), with two lone pairs of electrons on Cr. Second-order perturbative NBO analysis for estimating metal-ligand back-bonding interaction energies (ΔEbb), obtained from the off-diagonal Fock matrix element expressed in the NBO basis, is now well established,40-42 but examination of the NBO output indicates that there are no delocalizations above threshold that would be consistent with back-bonding from Cr(II) into the C-H or C-F σ* orbitals in these two compounds; similar studies on CF3 (34) Hughes, R. P. Eur. J. Inorg. Chem. 2009, 31, 4591–4606. (35) Hughes, R. P.; Sweetser, J. T.; Tawa, M. D.; Williamson, A.; Incarvito, C. D.; Rhatigan, B.; Rheingold, A. L.; Rossi, G. Organometallics 2001, 20, 3800–3810. (36) Hughes, R. P. Adv. Organomet. Chem. 1990, 31, 183–267. (37) Mahmoud, K. A.; Rest, A. J.; Alt, H. G.; Eichner, M. E.; Jansen, B. M. J. Chem. Soc., Dalton Trans. 1984, 175–186. (38) Weinhold, F.; Landis, C. R. Valency and Bonding: A Natural Bond Orbital Donor-Acceptor Perspective; Cambridge University Press: Cambridge, 2005. (39) Glendening, E. D.; Badenhoop, J. K.; Reed, A. K.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0; Theoretical Chemistry Institute, University of Wisconsin: Madison, WI, 2001. (40) Leyssens, T.; Peeters, D.; Orpen, A. G.; Harvey, J. N. Organometallics 2007, 26, 2637–2645. (41) Leyssens, T.; Peeters, D. J. Org. Chem. 2008, 73, 2725–2730. (42) Leyssens, T.; Peeters, D.; Orpen, A. G.; Harvey, J. N. New J. Chem. 2005, 29, 1424–1430. (43) Huang, H.; Hurubeanu, N. R.; Bourgeois, C. J.; Cheah, S.-M.; Yuan, J.; Rheingold, A. L.; Hughes, R. P. Can. J. Chem. 2009, 87, 151– 160.
Cr(V) complex þ
OdCr(salen) PF6-24 þ -26
OdCr(salen) OTf Cp*Cr(O)(CH3)228 Cp*Cr(O)Cl229 [(Ph)4As][CrO(LBu)2] 3 2CH3CN31 LBu = 3,5-di-tert-butyl-1,2benzenedithiolato dianion (TpFPC)Cr(O) TpFPC = (triarylcorrolato)32
Cr(V)-Ot (A˚) 1.49(4) 1.545(2) 1.579(3) 1.578(4) 1.572(7) 1.5700(17)
Figure 2. Computed (DFT/B3LYP/LACV3P**þþ) KohnSham LUMO and HOMO for the Cr-O-Cr complex 7 (light blue, Cr; red, O; gray, C; green, F).
compounds with d6 configurations have indicated small but significant back-bonding contributions of this type.40,43 Consequently any real shortening of the Cr-CF3 bond in 7 compared to the Cr-CH3 distance in 3 cannot be due to π-effects. Not unexpectedly, analogous observations pertain to the higher oxidation state analogues 3-B3LYP and 7-B3LYP. NBO analysis also allows a comparison of natural charges on atoms and ligands to be made; these are summarized in Table 3, along with the Cr-C bond hybridizations and ionicities38 for each of these compounds. This analysis illustrates a dramatically larger positive charge on Cr in the formally Cr(V) complexes compared to the Cr(II) analogues; this is clearly a result of the presence of negatively charged oxo ligands in the Cr(V) compounds compared to the net positively charged CO ligands in the Cr(II) compounds. Interestingly, this increased positive charge at the metal center results in significantly greater positive charge on the Cp* ligands in the Cr(V) compounds, but the group charge on the CH3 or CF3 ligands does not change significantly on oxidation from Cr(II) to Cr(V). In each case the CF3 ligand group charge is more negative than that for CH3, and the ionic character of the Cr-CF3 bond is higher than the corresponding Cr-CH3 bond, but in each case the ionic
Note
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Table 3. Natural Charges (NBO) Computed for Chromium and Ligands, and Cr-C Bond Ionicities for Compounds 3-B3LYP, 5-B3LYP, 7-B3LYP, and 8-B3LYP
charge on Cr
group charge on CX3
8-B3LYP
-0.84
-0.11
5-B3LYP
-0.87
-0.23
3-B3LYP
þ0.61
-0.13
7-B3LYP
þ0.55
-0.20
a
charge terminal oxo
-0.36 -0.36 -0.34 -0.34
charge μ-oxo
-0.44 -0.46
group charge on CO
group charge on Cpa
Cr-C bond occupancy
Cr-CX3 bond ionicity iCrCb
þ0.26 þ0.26 þ0.24a þ0.29 þ0.29 þ0.26a
þ0.19
1.75
0.087
þ0.26
1.84
0.189
þ0.46
1.95
0.015
þ0.56
1.87
0.125
CO trans to CX3. b Calculated from the NBO polarization coefficients38 for the Cr-CX3 bond: iCrC = |(cCr2 - cC2)/(cCr2 þ cC2)|.
character is strongly diminished in the higher oxidation state compound. In fact the Cr-C σ-bond in the Cr(V) compound 3-B3LYP is very slightly polarized toward Cr.
Experimental Section General Consideration. Air-sensitive reactions were performed in oven-dried glassware, using standard Schlenk techniques, under an atmosphere of nitrogen, which was deoxygenated over BASF catalyst and dried over Aquasorb, or in a Braun drybox. Methylene chloride, hexanes, and diethyl ether were dried over an alumina column under nitrogen.44 NMR spectra were recorded on a Varian Unity Plus 300 or 500 FT spectrometer. 1H NMR spectra were referenced to the protio impurity in the solvent: C6D6 (δ 7.16 ppm), CD2Cl2 (δ 5.32 ppm). 19F NMR spectra were referenced to external CFCl3 (δ 0.00 ppm). Coupling constants are reported in units of hertz and are absolute values. Elemental analyses were performed by Schwartzkopf (Woodside, NY) and X-ray crystallographic analyses at the University of California, San Diego. Cp*Cr(CO)3(CF3) (5) was prepared according to the previously reported method.13 [Cp*Cr(O)(CF3)]2(μ-O) (7). A yellow hexane (100 mL) solution of Cp*Cr(CO)3(CF3) (170 mg, 0.500 mmol) was exposed to light in air. The solution slowly changed to red. The solution was exposed for 4 days in air to give a red solution with some dark brown precipitate. During this period, the mixture was filtered each time the dark brown precipitate appeared. At last, the (44) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518–1520.
solution was filtered and the solvent was removed by rotary evaporator and dried in vacuo to give a red solid, which was chromatographed on deactivated neutral Al2O3 in a 2 6 cm column, eluting with hexane to give two bands. The first, yellow band was collected and dried to give a yellow solid (30 mg), shown to be unreacted Cp*Cr(CO)3(CF3). The second, dark red band was collected and dried to give 7 as a dark red solid (30 mg, 26%). Crystals suitable for X-ray structure determination were obtained from an ether/hexane solution at -30 °C. Anal. Calcd for C22H30Cr2F6O3: C, 47.14; H, 5.40. Found: C, 47.64; H, 5.99. 1 H NMR (C6D6, 300 MHz, 25 °C): δ 1.81 (15H, s, C5(CH3)5). 19 F NMR (C6D6, 282 MHz, 25 °C): δ -20.72 (3F, brs, CF3). DFT Calculations. Full-molecule calculations were carried out using the hybrid B3LYP15-18 functional, together with the triple-ζ LACV3P**þþ basis set,19-22 which uses extended core potentials on heavy atoms and a 6-311G**þþ basis for other atoms, as implemented in the Jaguar23 suite of programs. All computed structures were confirmed as energy minima by calculating the vibrational frequencies using second derivative analytic methods and confirming the absence of imaginary frequencies. Thermodynamic quantities were calculated assuming an ideal gas and are zero point energy corrected.
Acknowledgment. R.P.H. is grateful to the National Science Foundation for generous financial support. Supporting Information Available: A CIF file giving full crystallographic data for 7 and text and tables giving all optimized DFT structures. This material is available free of charge via the Internet at http://pubs.acs.org.
