Organometallics 2010, 29, 867–875 DOI: 10.1021/om901018n
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Ground-State Electronic Asymmetry in Cp*W(NO)(η1-isonitrile)2 Complexes Scott P. Semproni,† W. Stephen McNeil,*,§ Rhett A. Baillie,† Brian O. Patrick,† Charles F. Campana,‡ and Peter Legzdins*,† †
Department of Chemistry, The University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1, §Department of Chemistry, The University of British Columbia Okanagan, Kelowna, British Columbia, Canada V1 V 1 V7, and ‡Bruker AXS Inc., Madison, Wisconsin 12345 Received November 24, 2009
The three Cp*W(NO)(η1-isonitrile)2 complexes [Cp*=η5-C5Me5; isonitrile=CNCMe3 (1), CN-2, 6-Me2C6H3 (2), CN-n-C4H9 (3)] possess asymmetrical “piano-stool” molecular structures in the solid state in which their two isonitrile ligands exhibit distinctly different degrees of Wfisonitrile backbonding. Thus, an X-ray crystallographic analysis of 1 as a benzene hemisolvate has established that it has one essentially linear isonitrile ligand having a C-N bond length of 1.159(5) A˚ and a C-N-C bond angle of 171.4(5)° and one very bent isonitrile ligand (indicative of considerable WfCNCMe3 π-backbonding) having C-N = 1.200(5) A˚ and C-N-C = 135.1(4)°. Complex 2 crystallizes with two crystallographically independent molecules in the asymmetric unit, and the isonitrile ligands in each of these molecules exhibit metrical parameters similar to those extant in 1. The presence of the inequivalent isonitrile ligands is also manifested in the IR spectra of the three compounds both in the solid state and in solution. For instance, the Nujol mull IR spectrum of 1 contains absorptions at 2038 and 1853 cm-1, attributable to νCN of the linear and bent isonitrile ligands, respectively. An Et2O solution of 1 exhibits these bands at 2051 and 1855 cm-1. However, their 1H and 13C{1H} NMR spectra indicate that on the NMR time scale all three isonitrile complexes are fluxional in solution. Thus, at room temperature the 1H and 13C{1H} NMR spectra of 1 in C6D6 and C6D5CD3 indicate the equivalence of the two isonitrile carbon atoms attached to the tungsten center and the slight inequivalence of the CMe3 components of the two isonitrile ligands, and these spectra remain invariant to -80 °C. The results of DFT calculations on the model complex CpW(NO)(CNMe)2 reproduce the asymmetric distortion as the optimized geometry from various starting points even though it has a relatively shallow potential well. Interestingly, optimization of the related TpW(NO)(CNMe)2 [Tp = [HB(C3N2H3)3]-] results in a symmetric geometry with isonitrile CNC angles of 177.0° and 177.8°. Hence, it is clearly the interaction of the Cp ligand with the orbitals of the W(NO)(CNMe)2 fragment that allows the system to get to lower energy by skewing the isonitrile bending to one side. Introduction Cyclopentadienyl groups such as Cp (η5-C5H5) or Cp* (η5-C5Me5) are ubiquitous in transition-metal organometallic chemistry and are probably the most important of the polyenyl (i.e., unsaturated hydrocarbon) ligands because they bind strongly to many metals and are relatively inert to electrophilic and nucleophilic reagents.1 Consequently, they form a large number of complexes Cp0 MLn [Cp0 =Cp or Cp*; M = transition metal; L=Lewis base (ligand); n=2, 3, or 4], which are frequently referred to as “two-, three-, or four-legged piano stools” for obvious reasons. The vast majority of three-legged, piano-stool molecules having the composition Cp0 M(L0 )(L)2 (Cp0 = Cp or Cp*; L0 , L = ligands) are relatively symmetrical entities in the solid state that have identical ligands (L) attached to the metal centers
in the Cp0 M(L0 ) fragments in a virtually identical manner. Indeed, our search of the more than one-quarter million crystal structures in the Cambridge Structural Database2 fails to turn up any Cp0 M(L0 )(L)2 compounds having angular differences involving the L ligands greater than about 15°, and differences that large are rare. We now wish to report our recent discovery that some solid Cp*W(NO)(η1-isonitrile)2 complexes are notable exceptions to this generalization since their molecular structures do not possess the expected planes of symmetry and their two isonitrile ligands exhibit metrical parameters indicative of distinctly different degrees of Wf isonitrile back-bonding.3
Results and Discussion Synthesis and Properties of the Three Cp*W(NO)(η1-isonitrile)2 Complexes. Like related CpM(NO)(L)2 complexes
*Corresponding authors. E-mail:
[email protected]; legzdins@ chem.ubc.ca. (1) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 4th ed.; Wiley & Sons: New York, 2005; Chapter 5.
(2) Allen, F. H. Acta Crystallogr. 2002, B58, 380. (3) Rabinovich, D.; Parkin, G. Inorg. Chem. 1995, 34, 6341, and references therein.
r 2010 American Chemical Society
Published on Web 01/28/2010
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Figure 1. Solid-state molecular structure of Cp*W(NO)(η1CNCMe3)2 3 1/2(C6H6) with the solvate omitted and 50% probability thermal ellipsoids shown. Selected interatomic distances (A˚) and angles (deg): C(26)-W(2) = 1.977(4), C(26)-N(6) = 1.200(5), C(27)-N(6) = 1.482(5), C(26)-N(6)-C(27) = 135.1(4), C(21)-W(2) = 2.034(4), C(21)-N(5) = 1.159(5), C(22)-N(5) = 1.447(5), C(21)-N(5)-C(22) = 171.4(5). 4
1
[M = Mo or W], the new Cp*W(NO)(η -isonitrile)2 compounds may be conveniently synthesized in good yields by the reduction of Cp*W(NO)Cl2 in the presence of the isonitrile, and they are all isolable as orange to red solids (eq 1). These compounds are formally 18e species; hence, it is not surprising that they are relatively stable toward dioxygen and moisture in the solid state. They are also soluble in most common organic solvents, and they have been fully characterized by conventional methods including single-crystal X-ray crystallographic analyses of two of the complexes.
CpWðNOÞCl2 þ xs Na=Hg THF
þ 2RNC sf CpWðNOÞðη1 -CNRÞ2 þ 2NaCl ð1Þ Solid-State Molecular Structures. The solid-state molecular structure of Cp*W(NO)(η1-CNCMe3)2 (1) is shown in Figure 1, and its packing with a molecule of benzene of solvation in the asymmetric unit of the lattice is shown in Figure 2. The most striking feature of the “piano-stool” molecular structure of 1 in the solid state is the fact that the two isonitrile ligands are attached to the tungsten center in a distinctly different manner. Thus, one isonitrile ligand is essentially linear and has a C-N bond length of 1.159(5) A˚ and a C-N-C bond angle of 171.4(5)°. It may thus be described by the resonance form W--CtNþ-CMe3. In contrast, the other isonitrile ligand is very bent (indicative of considerable WfCNCMe3 π-back-bonding) and has C-N = 1.200(5) A˚ and C-N-C = 135.1(4)°. It is thus best described by the resonance form WdCdN-CMe3 with the nitrogen atom having a lone pair of electrons. Consistently, the bent isonitrile also has the shorter W-C bond of 1.977(4) A˚.3 Complex 2 does not cocrystallize with a molecule of solvent, but its lattice does contain two crystallographically independent molecules in the asymmetric unit (Figure 3). It is readily evident that each of these molecules also contains linear and bent isonitrile ligands whose metrical parameters are similar to those extant in 1. Consequently, it is clear that this isonitrile ligand asymmetry is an (4) Hunter, A. D.; Legzdins, P. Organometallics 1986, 5, 1001.
Figure 2. The asymmetric unit in the lattice contains two identical Cp*W(NO)(η1-CNCMe3)2 molecules as well as a disordered benzene molecule. As shown, the nearest contact between the nitrogen on the bent isonitrile ligand and the benzene ring is 2.887 A˚.
inherent property of these Cp*W(NO)(η1-isonitrile)2 complexes and is not simply a manifestation of steric factors in the lattice of 1 involving the benzene of solvation (Figure 2). Complex 3 has not been subjected to a crystallographic analysis, but its spectroscopic properties (vide infra) are consistent with it also possessing different W-isonitrile linkages. For comparison, we have also established the solid-state molecular structure of Cp*W(NO)(CO)(η1-CNCMe3) (4), which is conveniently synthesized by the treatment of Cp*W(CO)2(NO) with sodium amalgam in the presence of the isonitrile. As is apparent from the structure shown in Figure 4, complex 4 possesses a normal three-legged “pianostool” molecular structure in which the three monodentate ligands are essentially linear. Specifically, the isonitrile ligand has a C-N bond length of 1.164(4) A˚ and a C-N-C bond angle of 162.5(3)°, metrical parameters that resemble those of the linear CNCMe3 ligand in 1 (vide supra). The conversion of 4 into 1 involves the formal replacement of the CO ligand by the CNCMe3 ligand, which is a significantly better electron donor than CO.5 Consequently, it is not surprising that the tungsten center in 1 is more electron rich than that in 4. What is surprising is that this extra electron density in 1 is back-donated unequally to the two isonitrile ligands rather than being equipartitioned between them. While complexes containing both linear and bent isonitrile ligands are known, they have these isonitrile ligands at fundamentally different sites in the metal’s coordination sphere. For instance, there are two types of CNCMe3 ligands present in the distorted octahedral W(PMe3)(CNCMe3)4(η2Te2) molecule, namely, (i) two mutually trans CNCMe3 ligands that are essentially linear with C-N-C angles of 167(2)° and (ii) two mutually cis CNCMe3 ligands that are markedly bent at nitrogen with C-N-C angles of 138(1)° and 146(2)°.3 Similarly, the solid-state molecular structure of Cp*W(SeCMe3)(CNCMe3)3 has two nearly linear CNCMe3 ligands [C-N-C = 175.8(8)° and 164.3(7)°] and one very bent one [C-N-C = 128.7(6)°].6 In our Cp*W(NO)(η1-isonitrile)2 systems, however, the isonitrile ligands are in fundamentally similar coordination sites and would (5) Reference 1, Chapter 4. (6) Kawaguchi, H.; Tatsumi, K. Chem. Commun. 2000, 1299.
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Figure 3. Solid-state molecular structures of the two molecules of Cp*W(NO)(η1-CN-2,6-Me2C6H3)2 (2) in the asymmetric unit with 50% probability thermal ellipsoids shown. Selected interatomic distances (A˚) and angles (deg): C(11a)-W(1) = 1.937(15), C(11a)-N(2a) = 1.223(18), C(12a)-N(2a) = 1.440(19), C(11a)-N(2a)-C(12a) = 145.4(18), C(20a)-W(1) = 1.948(17), C(20a)-N(3a) = 1.21(2), C(21a)-N(3a) = 1.37(2), C(20a)-N(3a)-C(21a) = 166.3(17), C(11b)-W(2) = 1.992(15), C(11b)-N(2b) = 1.195(19), C(12b)N(2b) = 1.443(18), C(11b)-N(2b)-C(12b) = 139.9(19), C(20b)-W(2) = 2.090(17), C(20b)-N(3b) = 1.120(19), C(21b)-N(3b) = 1.43(2), C(20b)-N(3b)-C(21b) = 161.9(17).
Figure 5. DFT-optimized structure of the model complex CpW(NO)(CNMe)2. The four atoms defining the CWNC dihedral-type angle are labeled. The view on the left shows the CNC angles in the linear and bent isonitrile groups, and the view on the right is down the bent CNMe W-C-N bond axis, showing the CWNC dihedral-type angle.
thus be expected to attach to the tungsten center in a similar manner. In that connection, it may be noted that the recently reported two-coordinate monomeric complex Pd(CNArDipp2)2 [C-Pd-C = 169.8(2)°] contains two sterically encumbering m-terphenyl isonitrile ligands whose C-N-C angles are 174.1(4)° and 163.6(4)°,7 i.e., a markedly smaller difference than that displayed by our Cp*W(NO)(η1isonitrile)2 compounds. Spectroscopic Properties. The spectroscopic data for the three Cp*W(NO)(η1-isonitrile)2 complexes are very similar
and indicate that they retain their unusual solid-state molecular structures in solutions. For instance, the Nujol mull IR spectrum of 1 contains absorptions at 2038 and 1853 cm-1, attributable to νCN of the linear and bent isonitrile ligands, respectively, and an Et2O solution of 1 exhibits these bands at 2051 and 1855 cm-1. Similarly, the Nujol mull spectrum of 3 has these spectral features at 2089 and 1866 cm-1. Interestingly, the IR spectra of 2 both as a mull and in Et2O solution contain three absorptions in this region, e.g., 2114, 2043, and 1905 cm-1 in the Nujol mull spectrum, that possibly reflect the existence of conformational isomers. Similar features have been observed previously in the IR spectra of Fe(PMe3)2(η1-CN-2,6-Me2C6H3)3 and Fe(PMe3)(η1-CN-2,6-Me2C6H3)4 as KBr pellets.8 In any event, the 1H and 13C{1H} NMR spectra of 1-3 indicate that on the NMR time scale all three isonitrile complexes are fluxional in solution. Thus, at room temperature the 1H and 13C{1H} NMR spectra of 1 in C6D6 and C6D5CD3 indicate the
(7) Labios, L. A.; Millard, M. D.; Rheingold, A. L.; Figueroa, J. S. J. Am. Chem. Soc. 2009, 131, 11318.
(8) Jones, W. D.; Foster, G. P.; Putinas, J. M. Inorg. Chem. 1987, 26, 2120.
Figure 4. Solid-state molecular structure of Cp*W(NO)(CO)(η1CNCMe3) (4) with 50% probability thermal ellipsoids shown. Selected interatomic distances (A˚) and angles (deg): C(1)-W(1) = 2.031(3), C(1)-N(2) = 1.164(4), N(2)-CMe3 = 1.455(4), C(1)N(2)-CMe3 = 162.5(3), N(1)-O(1) = 1.197(4), N(1)-W(1) = 1.839(3), O(1)-N(1)-W(1) = 175.8(3), C(6)-W(1) = 1.893(3), C(6)-O(2) = 1.180(4), W(1)-C(6)-O(2) = 173.6(3).
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equivalence of the two isonitrile carbon atoms attached to the tungsten center and the slight inequivalence of the CMe3 components of the two isonitrile ligands, and these spectra remain invariant to -80 °C. The corresponding spectra of 2 and 3 are also temperature independent and indicate that on average their two isonitrile ligands experience magnetically identical environments probably via a rearrangement such as that shown below. Such fluxionality involving linear and bent isonitrile ligands has been documented previously.3
Theoretical Investigations. In an attempt to determine the cause of the surprising asymmetry exhibited by these bis(isonitrile) complexes, DFT calculations were performed on
Figure 6. Energy of CpW(NO)(CNMe)2 with variation of the bent CNC angle. Values below 180° orient the CH3 group away from the linear isonitrile, while those above 180° orient it toward that group.
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CpW(NO)(CNMe)2 as a model compound using the B3LYP method and LANL2DZ basis set. The resulting optimized geometry reproduces the notable asymmetry of the two isonitrile ligands, albeit to a lesser extent than that observed in the experimental structure of Cp*W(NO)(CNCMe3)2 (vide supra), with optimized bond angles and C-N distances of 163.1°/1.204 A˚ and 175.2°/1.198 A˚. The bending of the isonitrile group is approximately trans to the position of the linear CNMe ligand, with a dihedral-type angle formed among the linear CNMe C atom, the W atom, the bent CNMe N atom, and the bent CNMe methyl C atom being approximately 166° (Figure 5). Attempts to determine lowest-energy structures for CpW(NO)(CNMe)2 with the bent CNC angle constrained to other values did not converge, but single-point calculations on the optimized structure with only the CNC angle changed revealed the energy profile associated with the bending of the isonitrile group. Results are shown in Figure 6 and demonstrate a shallow energy profile for variation of the CNC angle, with a further decrease of 20° being only 5 kJ/ mol uphill, and altering of the bend orientation from distal to proximal with regard to the second isonitrile costs less than 2 kJ/mol. The energy of the complex is also insensitive to the dihedral orientation of the bent CH3 group, as shown in Figure 7. Rotation of the methyl group orientation keeping a constant CNC angle of 163° demonstrates that the minimum energy positions are with the CH3 group pointed either away from the second isonitrile (a CWNC dihedral angle of 166° or a relative angle of 0° in Figure 7) or toward it (180°). Energy maxima occur when the dihedral angle is rotated 90°. However, even at its highest-energy position, the compound is never more than 7 kJ/mol above its calculated ground-state structure, again confirming that the energy well associated with distortion of the bent CNC angle is a fairly shallow one. This fact suggests that the driving force to selectively bend a single isonitrile group, attributable to genuine electronic factors within the complex and not merely to crystal packing forces, is not large and accounts for the observed symmetric equivalence of the groups observed by solution NMR spectroscopy. The nitrosyl group is very nearly;but not strictly; linear in both the observed solid-state structure of
Figure 7. Energy of CpW(NO)(CNMe)2 with variation of the CWNC dihedral angle in the bent CNMe ligand at a constant CNC angle of 163°. In this diagram, 0° is taken as the minimum energy orientation of the bent CNMe methyl group, with a CWNC dihedral angle of 166.3° as described in Figure 5.
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Table 1. WNO and CNC Angles in Constrained and Unconstrained CpW(NO)(CNMe)2 complex
WNO angle
CNC angles
CpW(NO)(CNMe)2 (calcd) CpW(NO)(CNMe)2 (calcd) CpW(NO)(CNMe)2 (calcd)
179.95° (constrained) 176.6° 160° (constrained)
162.2°, 176.2° 163.1°, 175.2° 165.8°, 178.0°
Figure 8. HOMO of CpW(NO)(CNMe)2.
Cp*W(NO)(CNCMe3)2 (175.8°) and the DFT-optimized structure of CpW(NO)(CNMe)2 (176.6°). To assess the possibility that the mild bending of the NO ligand might serve as the origin of the isonitrile asymmetry, lowest-energy structures for CpW(NO)(CNMe)2 were determined with the WNO angle constrained at both linear (179.95°) and strongly bent (160°) angles. The results of these changes on the isonitrile CNC angles are shown in Table 1, along with comparisons to the unconstrained calculation. The large variation of the WNO angle does have a small effect on the calculated angles of the isonitrile groups, in that decreasing the WNO angle serves to increase slightly the CNC angle in the bent isonitrile entities. There is less resulting variation in the CNC angle of the more linear isonitrile and no consistent trend. More importantly, the variation in the WNO angle does nothing to remove the calculated asymmetry of the isonitrile ligands. We therefore conclude that the mild bending of the WNO group cannot be principally responsible for the observed different binding modes of the isonitrile ligands. However, an examination of the frontier orbitals of CpW(NO)(CNMe)2 offers a rationale for the observed distortion. The HOMO of the complex lies principally in the plane of the two isonitrile ligands and arises from the bonding overlap (i.e., in-phase combination) of the in-plane isonitrile π-symmetry orbitals with a tungsten d-orbital (Figure 8). This molecular orbital is strongly C-N π*-antibonding. Bending of an isonitrile group at the nitrogen atom renders the nitrogen more “sp2-like”, including a greater nitrogen s-orbital character into the HOMO and effectively generating an orbital with greater “lone pair” character on the nitrogen, while also reducing its C-N antibonding nature as the extent of C-N π-overlap is diminished. Both effects lower the energy of the HOMO as the bend angle is increased (Figure 9). The HOMO-1 and HOMO-2 of CpW(NO)(CNMe)2 (Figure 10) are primarily W-NO π-bonding in character, but each has a smaller W-C π-interaction as well, stemming from the isonitrile π-orbitals orthogonal to the CWC plane. These two orbitals are also slightly stabilized with a bending of the isonitrile group (Figure 9), but to a lesser extent than the HOMO, because the bending angle is nearly at right angles to the orientation of the π-interactions.
Figure 9. Variation of frontier-orbital energies of CpW(NO)(CNMe)2 with change in isonitrile CNC angle. Open circles (O) represent energies of the nonsymmetric complex with one strongly bent isonitrile group, while closed circles (b) indicate the energies in a minimum-energy symmetric complex with both CNC angles being 171.0°.
Figure 10. HOMO-1 and HOMO-2 of CpW(NO)(CNMe)2.
The HOMO-5 and HOMO-8 of the complex again lie principally in the plane of the two isonitrile ligands, and they arise from the out-of-phase and in-phase combinations of the in-plane isonitrile C-N π-bonding orbitals, with a slight bonding overlap to a tungsten d-orbital (Figure 8). There is negligible contribution from the Cp ring. In contrast to the higher-energy orbitals, these orbitals are destabilized as the isonitrile groups bends due to the loss of effective C-N π-bonding overlap that results. Overall, the extent of the isonitrile distortion appears to depend on a balance of these orbital energies, trading a loss of C-N π-bonding and increase in energy of the HOMO-5 and HOMO-8 in return for a stabilization of the HOMO, HOMO-1, and HOMO-2, due to a partial loss of C-N π*-antibonding and an increased “lone pair” character of these orbitals. Although this argument offers a rationale for why a partial bending of the isonitrile occurs, it does not explain why that bending is asymmetric. The calculated lowest-energy structure of CpW(NO)(CNMe)2 with enforced Cs symmetry has two CNC angles of 171.0°, and although it exhibits an electronic energy that is above that of the unsymmetric complex by an essentially negligible amount ( 2σ(I)); wR2 = [ ( (Fo - Fc ) )/ w(Fo ) ] (all data); w = [σ Fo ] . GOF = [ (w(|Fo| - |Fc|)2)/degrees of freedom ]1/2.
Anal. Calcd for C20H33N3OW: C, 46.61; H, 6.45; N, 8.15. Found: C, 46.24; H, 6.97; N, 8.23. HRMS-EI m/z: [M]þ calcd for 184WC20H33N3O 515.21384; found 515.21332. IR (Nujol, cm-1): 1575 (s, νNO), 1866 (m, νCN), 2089 (m, νCN). 1H NMR (400 MHz, C6D6): δ 0.72 (m, 6H, butyl Me), 1.24 (m, 8H, butyl CH2), 2.04 (s, 15H, C5Me5), 3.31 (m, 4H, NCH2). 13C{1H} NMR (150 MHz, C6D6): δ 11.4 (C5Me5), 13.9 (butyl Me), 20.4 (butyl CH2), 33.5 (butyl CH2), 47.5 (butyl CH2), 103.0 (C5Me5), 199.7 (CdN). Preparation of Cp*(W)(NO)(CO)(η1-CNCMe3) (4). In a glovebox, a sample of CNCMe3 (30.0 mg, 0.361 mmol) was added to Cp*W(CO)2(NO) (45 mg, 0.111 mmol), and both solids were dissolved in THF (ca. 5 mL) in a 4-dram vial to obtain a redorange solution. In a separate 4-dram vial a stir bar and Na/Hg amalgam (764 mg, 0.332 mmol of Na) were suspended in THF (ca. 5 mL). The red-orange solution was added to the stirred Na/Hg amalgam suspension. After 1 h, the supernatant solution was dark red, and it was filtered through a Celite column (3 0.5 cm) to remove any excess mercury. The filtrate was taken to dryness in vacuo, the residue was redissolved in a minimum of pentane, and the solution was transferred to the top of an alumina column (3 0.5 cm). The column was eluted with Et2O, the red band that developed was collected, and the solvent was removed from the eluate in vacuo. The remaining residue was dissolved in THF (ca. 2 mL), and the solution was stored overnight at -30 °C to induce deposition of 4 as cubic red crystals of X-ray quality (36 mg, 70% yield). Anal. Calcd for C16H24N2O2W: C, 41.76; H, 5.26; N, 6.09. Found: C, 42.07; H, 5.28; N, 6.15. IR (Nujol, cm-1): 1620 (s, νNO), 1899 (s, νCN), 1993 (w, νCO), 2092 (w, νCN). MS (LREI, m/z, probe temperature 150 °C): 460 [Pþ, 184W]. 1H NMR (400 MHz, C6D6): δ 0.95 (s, 9H, CNMe3), 1.90 (s, 15H, C5Me5). 13C{1H} NMR (150 MHz, C6D6): δ 11.1 (C5Me5), 31.3 (CNCMe3), 58.7 (CNCMe3), 103.4 (C5Me5), 202.5 (WCN), 235.6 (WCO). X-ray Crystallography. Data collection for each compound was carried out at -100 ( 1 °C on a Bruker X8 APEX diffractometer, using graphite-monochromated Mo KR radiation.
Data for 1 were collected to a maximum 2θ value of 55.00° in 0.5° oscillations. The structure was solved by direct methods12 and expanded using Fourier techniques. The asymmetric unit contained two molecules of 1 and one molecule of benzene. The benzene molecule was disordered and was modeled in two orientations (one with 66% occupancy and one with 33% occupancy). All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were included in fixed positions. The final cycle of full-matrix least-squares analysis was based on 10 847 observed reflections and 581 variable parameters. Data for 2 were collected to a maximum 2θ value of 50.7° in 0.5° oscillations. The structure was solved by direct methods12 and expanded using Fourier techniques. The material solves in space group Pbca; however anisotropic refinements result in a large number of NPD displacement parameters. It was ultimately determined that the material crystallizes as a two-component twin in space group P21/c (with two crystallographically independent molecules in the asymmetric unit), related by a 180° rotation about the [100] direct lattice direction and a refined twin ratio of 0.54:0.46 between the major and minor twin components, with pseudo-orthorhombic symmetry. ISOR, DELU, and SIMU commands were employed to maintain reasonable ADPs. All non-hydrogen atoms were refined anisotropically. All C-H hydrogen atoms were placed in calculated positions but were not refined. The final cycle of full-matrix least-squares analysis was based on 8734 observed reflections and 422 variable parameters. Data for 4 were collected to a maximum 2θ value of 55.00° in 0.5° oscillations. The structure was solved by direct methods12 and expanded using Fourier techniques. All non-hydrogen atoms were refined anisotropically, and all hydrogen atoms were included in fixed positions. The final cycle of full-matrix least-squares analysis was based on 3974 observed reflections and 198 variable parameters. For each structure neutral-atom scattering factors were taken from Cromer and Waber.13 Anomalous dispersion effects were included in Fcalc;14 the values for Δf0 and Δf00 were those of
(12) SIR97. Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115.
(13) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystallography; The Kynoch Press: Birmingham, 1974; Vol. IV, Table 2.2 A. (14) Ibers, J. A.; Hamilton, W. C. Acta Crystallogr. 1964, 17, 781.
Article Creagh and McAuley.15 The values for mass attenuation coefficients are those of Creagh and Hubbell.16 All calculations were performed using SHELXL-9717 via the WinGX18 interface. X-ray crystallographic data for the three structures are presented in Table 2 and in the cif files provided as Supporting Information. (15) Creagh, D. C.; McAuley, W. J. International Tables for X-ray Crystallography; Kluwer Academic Publishers: Boston, 1992; Vol. C, Table 4.2.6.8. (16) Creagh, D. C.; Hubbell, J. H. International Tables for X-ray Crystallography; Kluwer Academic Publishers: Boston, 1992; Vol. C, Table 4.2.4.3. (17) Sheldrick, G. M. SHELXTL Version 5.1; Bruker AXS Inc.: Madison, WI, 1997. (18) Farrugia, L. J. WinGX-V1.70. J. Appl. Crystallogr. 1999, 32, 837. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Know, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, J,; Cammi, R.; Pomelli, C,; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Dalvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dappich, S.; Daniels, A, D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision B.05; Gaussian, Inc.: Pittsburgh, PA, 2003.
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DFT Calculations. All theoretical calculations were performed using Gaussian03W19 utilizing the LANL2DZ basis set and a DFT method using the three-parameter exchange functional of Becke20 and the correlation functional of Lee, Yang, and Parr (B3LYP).21 The LANL2DZ basis set included both Dunning and Hay’s D95 sets for H, C, N, and O22 and the relativistic electron core potential (ECP) sets of Hay and Wadt for W.23 Geometry optimizations were performed in C1 symmetry unless otherwise noted, and frequency calculations on optimized geometries established the presence of either no imaginary frequencies or frequencies of negligible energy (
