Organometallics 1995, 14,3783-3790
3783
Infrared Spectrum of Cyclopentadienyltrimethyltitanium(IV)and Investigation of the Methyl Group Geometry through Partial Deuteration Studies G. Sean McGrady,” Anthony J. Downs, and Janette M. Hamblin Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K.
Donald C . McKean Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, U.K. Received March 1, 1995@ Infrared spectra have been recorded between 4000 and 200 cm-l for cyclopentadienyltrimethyltitanium(IV) in the forms CpTi(CH&, CpTi(CH2D)3, CpTi(CHD& and CpTi(CD& (Cp = q5-C5H5);measurements have been made (i) on the molecules isolated in solid Nz matrices at 14 K, (ii) on solutions of the compounds at room temperature, and (iii) on the annealed solid condensates at 77 K. Vibrational fundamentals of the TiMe3 fragment have been identified with particular reference to the 4 C H ) and 4CD) modes, which have been analyzed in terms of a harmonic local mode force field. The spectroscopic evidence is t h a t the methyl groups, while unexceptional in their gross geometry, are asymmetric: with the molecule in the matrix-isolated or solution states, they feature one strong and two weak C-H bonds, but the pattern changes to one weak and two strong C-H bonds for the solid. The results offer a basis for comparison with the properties of other methyltitanium compounds, a s well as a test of the utility of “isolated” C-H and C-D stretching frequencies. On the basis of the analysis, a n a-“agostic” Ti. .H-C interaction is proposed.
Introduction Cyclopentadenyltrimethyltitanium(IV), CpTiMe3, where Cp = r,75-C5H5,I, has been known since 1960.’
The compound is not only extremely sensitive to attack by air and moisture but also thermally fragile, decomposing at temperatures above 0 “C. However, the appreciable volatility of the material, which sublimes i n vacuo a t ambient temperatures, affords a convenient means of transfer and purification. CpTiMes appears to have high catalytic activity, with reports of its involvement in Ziegler-Natta polymerization of propene,2 cycloolefin^,^ and norbornene4 and in the formation of highly syndiotactic polystyrene of controlled molecular eight.^ Furthermore, species such as NO and SO2 are reported to insert into the Ti-CH3 bonds.6 This reactivity of the molecule can be ascribed to steric Abstract published in Advance ACS Abstracts, July 1, 1995. (1)Giannini, U.;Cesca, S. Tetrahedron Lett. 1960,14, 19. (2) Chien, J. C. W.; Hsieh, J. T. T. Coord. Polym. 1975,305. (3)Okamoto, T.; Matsumoto, J.; Watanabe, M.; Maezawa, K. PCT Int. Appl. WO 92 06,123,1992. (4)Petasis, N.A.; Fu, D.-K. J . A m . Chem. SOC.1993,115, 7208. (5)Takeuchi, T.;Tomotsu, N. Jpn. Kokai Tokkyo Koho JP 05,271,337,1993.
and electronic unsaturation at the titanium center (with a formal electron count of 12)and to the relatively high energy of the empty d orbitals on this center. Rather surprisingly, in view of its catalytic behavior, there exists very little structural information about CpTiMe3, although the related molecule Cp*TiMe3 (Cp* = v5-CsMes) has been the subject of an electrondiffraction study.’ The lH NMR spectrum of a sample in toluene-ds solution shows two singlet resonances with relative intensities of 5:9 at temperatures as low as -60 “C. An early study included limited measurements of the vibrational properties in the low-frequencyregion, leading to the assignment of a few skeletal modes,8but as the measurements relate to a THF solution of the compound, there must be some doubts regarding the interpretation of the results, given the high Lewis acidity of the titanium center. McKean et al. have shown that analysis of the vibrational frequencies associated with CH3, CHD2, and CD3 derivatives can yield information about the geometry of the methyl group, including both C-H bond lengths and H-C-H angle^.^ Such results are derived from correlations between the “isolated” C-H stretching frequency, vis(CH),displayed by CHD2 groups, with bond length or H-C-H angle, aided by harmonic local mode (energy-factored) force field calculations.1° In addition,
@
(6)Clark, R. J. H.; Stockwell, J. A,; Wilkins, J. D. J . Chem. SOC., Dalton Trans. 1976,120. (7)Blom, R.; Rypdal, K.; Mena, M.; Royo, P.; Serrano, R. J . Organomet. Chem. 1990,391, 47. ( 8 ) Samuel, E.; Ferner, R.; Bigorgne, M. Inorg. Chem. 1973,12,881. (9)McKean, D.C. Chem. SOC.Rev. 1978,7,399; Croat. Chem. Acta 1988,61,447.
Q276-7333/95/2314-3783$Q9.00/0 0 1995 American Chemical Society
McGrady et al.
3784 Organometallics, Vol. 14, No. 8, 1995 empirical correlations linking vis(CH) with the C-H bond dissociation energy, D0m,l1and the mean M-C bond energy, Dhlc,12 allow an assessment of the C-H and M-C bond strengths. A recent infrared study showed the value of this approach for defining the geometry of the methyl group in MeTiC13.13 Since part of the interest in these titanium compounds focuses on identifying different C-H bonds within the same methyl group, it is important to note that the time scale of the infrared experiment means that free rotation effects are not evident in the IR spectrum until the barrier to internal rotation is less than about 4 k J mol-l. For barriers greater than this, the presence of two or three types of C-H bonds within the same methyl group is plain from the occurrence of two or three “isolated” C-H stretching bands in the spectrum of the CHDz d e r i ~ a t i v e The . ~ range of methyl compounds in which such nonequivalent C-H bonds may be identified is therefore greatly enlarged with respect to that accessible to NMR studies, and the IR method is well suited to systems such as CpTiMe3, where the high catalytic activity suggests that C-H* *M interactions, described as “agostic”,14may be important. In a recent NMR study of solutions containing CpTi(CHzD)s,Green et al. measured the magnitude and sign of the coupling constant WH-D) and concluded that there was no NMR evidence of such an intera~ti0n.l~ The authors decided, however, that this technique is not well suited to the identification of agostic interactions in general. Here we describe a detailed study of the IR spectra of the isotopomers CpTi(CHd3, CpTi(CH2D13, CpTi(CHD&, and CpTi(CDd3 and draw on the results for the partially deuterated species to assess the geometry of the methyl groups. The availability of both CHD2and CHzD-labeled species invites a subsidiary exploration t o determine whether Y’YCD)data from the latter constitute a reliable supplement to, or even a substitute for, vis(CH)data from the former. vis(CD) data have been used in earlier studies involving, for example, amine@ and other species17 but without any critical assessment of their quantitative significance. A parallel study of similarly substituted versions of MezTiClz will be reported elsewhere.ls
Experimental Section Preparation. CpTiMe3 was prepared by the reaction of CpTiCla with methyllithium in diethyl ether a t -78 t o 0 “C.’ On completion of the reaction, the ether was evaporated in vacuo a t -45 “C. The resulting yellow solid was purified by fractional sublimation in vacuo; CpTiMe3 was retained in a trap held a t -45 “C. (10)McKean, D. C. J . Mol. Struct. 1984,113, 251; 1976,34, 181. McKean, D. C. Spectrochim. Acta 1973,29A,1559. (11)McKean, D. C.Int. J . Chem. Kinet. 1989,21,445. (12) McKean, D. C.;McQuillan, G. P.; Thompson, D. W. Spectrochim. Acta 1980,36A,1009. (13)McKean, D. C.;McQuillan, G. P.; Torto, I.; Bednall, N. C.; Downs, A. J.; Dickinson, J. M. J . Mol. Struct. 1991,247, 73. (14) Brookhart, M.;Green, M. L. H. J . Organomet. Chem. 1983,250, 395. Brookhart, M.; Green, M. L. H.; Wong, L. L. Prog. Inorg. Chem. 1988,36,1. (15)Green, M. L. H.; Hughes, A. K., Popham, N. A,; Stevens A. H. H.; Wong, L.-L. J . Chem. SOC.,Dalton Trans. 1992,3077. (16) Krueger, P.J.;Jan, J. Can. J . Chem. 1970,48, 3229, 3236. (17)Schultz, A. J.;Williams, J. M.; Schrock, R. R.; Rupprecht, G. A.; Fellmann, J. D. J . Am. Chem. SOC.1979,101, 1593. (18)McGrady, G. S.;Downs, A. J.; McKean, D. C. To be published.
The partially deuterated species CHzDLi and CHDzLi were synthesized from the reaction of lithium metal with the corresponding methyl chloride, itself obtained from the reaction of Bu”3SnD with CHzBrCl or CHBrZCl, re~pectively.’~ CD3Li was prepared from CD31, which was derived from the reaction of CD30D with P13.19 Use of the appropriate methyllithium allowed the preparation of each of the isotopomers CpTi(CH& CpTi(CH2D13, CpTi(CHDz)3,and CpTi(CD313. The purity of the product was checked by reference t o the ‘H NMR spectrum of a solution in toluene-&.’ Spectroscopic Measurements. Infrared spectra were recorded in the region 4000-400 cm-I for solid films of the compound formed by condensation of the vapor a t 77 K and for solutions in CC14 a t ambient temperatures (0.5 mm path length), using a Mattson “Galaxy” FT-IR spectrometer at a resolution of 2 and 1cm-’, respectively. Solutions in CCl4 were opaque to IR radiation between 700 and 820 cm-’. The spectrum of a sample of each isotopomer isolated in an Nz , matrix a t 14 K was recorded over the range 4000-200 cm-’ using a Perkin-Elmer 580A dispersive spectrophotometer, a t a n optimum resolution of 2.8 cm-I. Because of the thermal frailty of the compound, freshly sublimed samples were used for each experiment. The difficulties apparent in handling such a reactive and thermally sensitive material for periods in excess of minutes a t ambient temperatures meant that weak, extraneous absorptions probably arising from decomposition products could not be excluded from all of the spectra recorded for CpTiMes. Force Field Calculations. Harmonic local-mode force field calculations were carried out on the internal vibrational modes of the methyl groups in CpTiMe3 using the program
ASYM20.20
Results Infrared spectra were recorded for the following isotopomers of cyclopentadienyltrimethyltitanium(IV): (a) CpTi(CH313, (b) CpTi(CH2D13, (c) CpTi(CHDd3, and (d) CpTi(CD&. In each case the spectrum of the isotopomer was measured (i) for the annealed solid condensate on a CsI window at 77 K, (ii) for a CC14 solution at ca. 298 K, and (iii) for a solid Nz matrix at 14 K. Figure 1shows the spectra of each of the matrixisolated isotopomers in the region 4000-200 cm-l, whereas Figure 2 shows the spectra of the isotopomers for each of the experimental conditions (i)-(iii) in the region 3200-2000 cm-l. Comparison of the spectra with one another and with those of related molecules, such as CpzMMe2 (M = Ti, Zr, or Hf),21CpTiCls, or NaCp, has allowed us to assign unambiguously the bands arising from the internal modes of the y5-cyclopentadienyl ligand, and the relevant wavenumbers occurring above 700 cm-l are collected in Table 1. Table 2 then lists the modes associated with the TiMe3 moiety, also at wavenumbers exceeding 700 cm-l. Some weak absorptions in the spectra indicate the presence of impurities o r decomposition products. In the case of the CD3 species, the band near 2080 cm-l observed in all three phases is close to a weak Cp combination frequency but is too intense to come from this source. The same is true of the band a t 1444 cm-l displayed by the solid. Other likely impurity bands occur at 2873, 2576, 2113, 1145, 1134,1086, and 914 cm-l (the wavenumbers being those appropriate to the solid). Very weak bands at 3010 cm-l (19)Douglass, I. B. Int. J . Sulfur Chem. 1973,8 , 441. (20) Hedberg, L.;Mills, I. M. J . Mol. Spectrosc. 1993,160,117. (21) McQuillan, G.P;McKean, D. C.; Torto, I . J . Organomet. Chem. 1986,312,183.
Cyclopentadienyltrimethyltitanium(N)
I
I
3000
I
2000
,
/
,
I600
,
1200
Organometallics, Vol. 14,No. 8, 1995 3785
,
,
800
.
,
400
Wavenumber
Figure 1. IR spectra (4000-200 cm-l) of (a) CpTi(CH&, (b) CpTi(CHzD)3, (c) CpTi(CHDz13, and (d) CpTi(CD& isolated in an Nz matrix at 14 K. (CH2D)and 3015 and 2975 cm-' (CHD2)suggest traces of hydrocarbon impurity. The usual features due t o atmospheric H2O and CO2 are also present in the spectra of the solid condensates and carbon tetrachloride solutions; weak absorptions from these species are also evident in the matrix spectra. Analysis of the Spectra. By reference to the results presented in Figures 1 and 2 and Table 2, the spectra of the molecules CpTi(CH,D3-,)3 (n = 0-3) lend themselves to the following analysis. Methyl Deformation and Other Bending Modes, Their Overtones and Combinations, and Lower Frequency Modes. It is helpful to assign the methyl deformation and other bending modes before tackling the v(CH) and 4CD) regions of the spectrum. The bas(CH3) and 6,(CH3) bands are readily located near 1390 cm-l and in the region 1130-1160 cm-l, respectively. As found previously for other methyltitanium compounds, these both occur a t unusually low waveThe mode das(CD3)is expected to occur at ca. 1020 cm-l, but here it is obscured by strong absorption associated with the cyclopentadienyl group; the fundamental is visible only in the spectrum of an (22) Robertson, A. H. J. Ph.D. Thesis, University ofAberdeen, 1990. (23) Williamson, R. L.; Hall, M. B. J . Am. Chem. SOC. 1988,110, 4428.
N2 matrix, appearing as a very weak band at ca. 1005 cm-l. Nevertheless, the overtone of this fundamental near 2000 cm-l is clearly in evidence in the spectra of all three phases. The 6,(CD3) band appears at ca. 870 cm-l. We note a 13 cm-' splitting of the 6,(CH3) band, attributed to A1 and E components and evident in the matrix and solution spectra; this is only slightly smaller than the 15 cm-l splitting found in M ~ s P On .~~ the other hand, no similar splitting is displayed by the 6,(CD3) band. For the CHzD species, the mode 6,(CH2) is identified with a band of medium intensity at ca. 1360 cm-l, the CH2 wagging mode with a band of similar intensity near 1200 cm-l, and the remaining deformation mode with a weak feature at ca. 975 cm-l. One of the deformation modes &CHI in the CHD2 isotopomer is located at ca. 1236 cm-l and the other at ca. 1089 cm-l, while the 6,(CD2) mode appears a t ca. 930 cm-l. The overtone of das(CH3) is found near 2740 cm-l, where an anharmonicity deficit 2 x das(CH3)- 26as(CH3) of 10-15 cm-l indicates a Fermi resonance shift of 2621 cm-l due to interaction with v,(CH) at higher wavenumber. The intensity distribution between vs(CH3) and 26as(CH3),particularly in the solution spectrum, is compatible with a shift of this magnitude. By contrast, v,(CD3) in the perdeuterated compound, which occurs near 2060 cm-', appears to be much less affected by resonance with 2daS(CD3)t o lower wavenumber. With regard t o the spectrum of the CHzD compound, assignment of the methyl bending modes near 970 and 1200 cm-l makes it likely that a combination will appear a t ca. 2170 cm-l. Moreover, the overtone of the CH2 scissors mode near 1360 cm-l, though not observed, will also resonate with CH stretching fundamentals to higher wavenumber. For the CHD2 species, we must expect a combination 1243 932 = 2175 cm-' and an overtone 2 x 1103cm-l, both of which may interact with v(CD) levels. Assignment of vibrational features due to the CH3 moiety below 700 cm-l is rendered difficult by the complexity of the CpTiMes molecule and the presence in this region of numerous bands originating in the cyclopentadienyl ligand. The band associated with e(CH3) is placed in the region of 480 cm-l, an energy lower than those of the corresponding features of Cp2TiMez21and C p T i M e c l ~but ~ ~not as low as those found for e(CH3)of MeTiC1313and Me~TiC12.l~ As with 6(CH3), e(CH3) modes occur at substantially lower energies in methyltitanium compounds than in most other methylmetal derivative^.^^ A band attributable to e(CD3) is observed at 377 cm-l in the spectrum of the matrixisolated CD3 derivative, whereas modes corresponding to e(CH2D) and e(CHD2) are believed t o occur near 470 and 430 cm-l, respectively. The observation in the spectra of CpTiMe3 of two bands near 570 and 520 cm-l whose wavenumbers fall gradually with increasing deuteration of the methyl groups suggests that these arise from what are predominantly the skeletal fundamentals va,(TiC) and y s (Tic),respectively. Assignment of the latter mode is in agreement with the conclusions drawn from an earlier Raman study, which placed this mode at 517 cm-1.8 A fundamental approximating t o the Ti-Cp stretching mode is expected to occur at ca. 350 cm-l,
+
(24) McKean, D. C.; McQuillan, G. P.; Murphy, W. F.; Zerbetto, F. J . Phys. Chem. 1990,94,4820.
3786
McGrady et al.
Organometallics, Vol. 14, No. 8, 1995
~~
3200
2800
2400
2000 3200
Wavenumber
2800
2400
2000
Wovenumber
Figure 2. IR spectra (3200-2000 cm-l) of (a)-(d) (see Figure 1): (i) solid condensate, 77 K, (b) CCl4 solution, 298 K (iii) N2 matrix, 14 K. Table 1. Wavenumbers (in cm-'1 of the Absorptions in the Range 4000-700 cm-' Assigned to the Cyclopentadienyl Ligand in the CpTiMe, IsotopomersaTb CpTi(CH& CpTi(CHzD13 CpTi(CHD2)3 CpTi(CDd3 3105w 3092w 2735vw n.0. 2430vw 2284vw 2208vw 2085vw 1845 w, br 1761 w, br 1669 w, br 1438 ms 1366 w, br 1266 vw 1238vw 1127vw 1067vw 1019 s 973 w, br 904vw 844 mw, sh 825 vs
3105w 3091 w 2727vw 2504 vw 2432vw 2284vw 2209vw 2085vw 1845 w, br 1759 w, br 1666 w, br 1438 ms 1366 w 1266 vw 1237vw 1125vw 1066vw 1019 s 973 w, br 903vw 844 m, sh 825 vs
3105 w 3097 w 2726 vw 2505 vw 2429 vw 2282 vw obs 2081 vw 1844 w ,br 1753 w, br 1660 w, br 1440 ms 1366 w 1266 vw obs 1128 vw 1067 vw 1018 s 973 w, br 925 vw 845 mw, sh 822 vs
3104 w 3088w 2727vw 2508 vw 2432 vw 2282 vw obs 2077 vw 1824 vw 1735 vw,br 1643 w, br 1437 ms 1366 w 1265 vw n.0. 1127 W,sh 1066 vw 1018 s n.0. 914 vw 846 mw 815 vs
assgntC u(CH) 2 x 1366 1438 1067 1366 + 1067 1266 + 1019 1366 + 844 1067 + 1019 1019 825 904 + 825? 844 825 v(CC) (El) v(CC)(Ez) dip(CH)(A21
+
+ +
?
u(CC) (AI) dOp(CH)(Ez) dip(CH)(El) ? dip(CC)(E21 d,,(CH) (El) dop(CH)(AI)
a Abbreviations: s, strong; m, medium; w, weak; v, very; sh, shoulder; br, broad; no., not observed; obs, obscured; ip, in-plane; op, out-of-plane. Values quoted refer to the solid. Based on an (q5-C5H5)Timoiety assumed to have local C5" symmetry.
but it was not possible to fx this feature with any degree of certainty in any of the four isotopomers. The dearth of information available at wavenumbers lower than 400 cm-', allied to the relative complexity of the CpTiMe3 molecule, militated against any attempt at a more detailed analysis of the low-energy regions of the spectra. C-H and C-D Stretching Regions. The coupling between the C-H stretching modes of individual methyl
groups in Me& or Me3X compounds has always proved
to be negligible, as reflected, for example, by the observation that the spectra of CH3(CD3)2Xand (CH313X species are imperceptibly different in the C-H stretching region of the IR spectra.24 There is no reason to suppose that the methyl groups in CpTiMes will behave differently. Additional complexity might arise, however, if the three groups were nonequivalent. Counteracting this would be a signal-averaging effect if the barrier to internal rotation about the Ti-CH3 bonds were less, say, than 100 cm-l, but this seems unlikely if the C-Ti-C angle in CpTiMe3 is similar to that in Cp"TiMe3, namely 110.00.7 With these considerations in mind, the identification and interpretation of the symmetric and antisymmetric 4CH) and v(CD) modes poses few problems, the assignments being set out in Table 2. Most noteworthy is t,he finding that the relevant bands form patterns which in all but two cases are in keeping with what would be expected of an asymmetric methyl group having just one plane of symmetry, i.e. belonging to the point group C,, with two C-H bonds of one kind and one C-H bond of another kind. Thus, split Yas(CH) and Yas(CD)bands are seen in the solid and matrix spectra of the CH3 and CD3 versions of the compound, respectively, and the CHzD and CHD2 versions each give rise to a pair of vis bands. The two exceptions are the Yas(CH) and Yas(CD) bands in the solution spectra of the CH3 and CD3 compounds; both are single features, although broad enough to accommodate splittings of the magnitude observed in the other phases. Pending the further discussion of their contours (q.v.), we will assume that the breadth does indeed arise from such unresolved splitting. Analysis: Application of the Frequency Sum Rule. The first stage in the analysis of the splittings displayed by the v(CH) bands is to determine whether
Organometallics, Vol. 14, No.8,1995 3787
Cyclopentadienyltrimethyltitanium(N)
Table 2. Wavenumbers (in cm-'1 of the IR Absorptions in the Range 4000-700 cm-' Assigned to the TiMe3 Moiety in the CpTiMes IsotopomersD molecule
modeb
CpTi(CH3h
vas(CH3) vs(CH3) 2baACH3) das(CH3)
solid film 77 K
CC4 soln 298 K
2959 m 2934 m} 2859 mw 2735 w 1385 mst
~
N2 matrix 14 K
2935 br, mst
2931 mst 2862 m 2740 w
2856 m 2738 w 1387 mst
1389 mst bs(CH3) CpTi(CHzD)3
1126 mw
Vas(CH2)
2950 mw, br
1128 w (E) 2958 mw, br
vs(CH2)
2874 mw
2893 mw, br
2136 mw 1352 w, sh 1196 mw 973 w, br
2150 vw 2178 1358 m 1200 m 969 vw
2873 2924 mw
2891 mw, br
vas(CD2) vs(CD2) &CHI (A') b(CH) (A')
2201 mw 2096 w 1232 mw 1095 mw, br
2207 mw 2105 w 1236 mw 1086 w
bACD2)
923 w, br
934 mw 2201 mst
"I
vLS(CD)
CpTi(CHD&
ds(CH2D) CH2 wag b(CH2D) viS(CH)
~as(CD3) 2186 m 2055 w 2005 w obs 867 m
vs(CD3) 26dCD3) bas(CD3) ddCD3)
1149 mw (E) 2964 m 2939 sh (2926 mw?) 2888 m 2185 w 2146 w 1358 m 1199 m 974 w, br 2938 mw 2900 w 2887 sh 2216 mw 2111 w 1089 mw 932 mw 2192 m 2065 mw 2010 vw 1005 vw 872 mst
2057 mst 2011 w obs 871 m
a Abbreviations: st, strong; m, medium; w, weak; v, very; sh, shoulder; br, broad; s, symmetric; as, antisymmetric; no., not observed; obs, obscured. The split vas bands are designated v1 and v2 in Table 4. vs similarly becomes v3. v1 and v3 are thus A species; v2 is A . A description of v3 as a symmetric stretching mode is therefore a slight misnomer.
Table 3. Isolated CH and CD Stretching Frequencies (cm-'1, Ratios, and Sum Rule Checks for CpTiMes param vIs(CH)(1) vlYCH) (2) WYCH) (2 x 1) ZvYCH) (2 x 2) Zv(CHda vYCD) (1) vYCD) (2) v(CH)/v(CD)( l ) b v(CH)/v(CD)( 2 ) b AV(CH)lAV(CD)b
N2 matrix 2938 2900 8771 8738 8720 2185 2146 1.3446 1.3514
Av
139 0.974
CCld soln 2928 2891 8747 8710 8701 2178 2150 1.3444 1.3447
Av
Izs 1.321
solid 2924 2873 8721 8670 8726 2176 2136 1.3438 1.3450
Av
140 1.275
a After applying a Fermi resonance correction of 25 cm-' to v ~ ( C H(see ~ ) Table 4). Value expected from [g(CH)/g(CD)]1'2= 1.347 15, where g denotes the relevant G matrix element.
they arise from one strong bond and two weak bonds or vice versa. This is normally evident when the approximate sum rule for CH stretching frequencies (eq 1)is applied. Table 3 shows the results of the calcula-
tions. In summing the CH3 frequencies, we deduct 25 cm-l from each vs(CH3) value t o allow for the likely Fermi resonance with two daS(CH3)levels. The evidence from the matrix results indicates unequivocally the presence of one strong and two weak bonds. The same is true for the carbon tetrachloride solution results if we assume that the wavenumber of the vas(CH3)band (2935 cm-') is the mean of two values. According to the solid spectra, however, the reverse has to be the
case, with one weak and two strong bonds present. Although the bands here are broad, it is impossible to account for the 44 cm-' discrepancy which results when one strong and two weak bonds are assumed. In some circumstances, the relative intensities of the visbands can help to identify the numbers of strong and weak bonds (since the proportion of conformers with Ha present should be roughly twice that of the conformers with H, present, where a and s indicate atoms out of or in the plane of symmetry, respectively). However, infrared intensities are notoriously variable, and the present results are in any case unsuitable for estimating their relative values. Table 3 also illustrates a worrying feature of the vis(CH) and vis(CD)values and of the shifts which result. In the diatomic approximation, the ratio vis(CH)/vis(CD)
McGrady et al.
3788 Organometallics, Vol. 14, No. 8, 1995 should be given by the ratio of the square roots of the corresponding G elements, [g(CH)/g(CD)Im,viz. 1.347 15. The effect of the small couplings involved in the CHzD and CHD2 groups should be to make the observed ratios slightly larger than this figure. In the event, only one of the observed ratios-that for the lower frequency vis(CH) for the N2 matrix sample-fulfils this prediction. More disturbingly, the AviS splittings should also change in this way with the switch from C-H t o C-D bonds, whereas the ratios shown in the table are all well below the expected values; this is particularly evident for the N2 matrix results (Avis(CH)= 38 cm-l; Avis(CD)= 39 cm-l). In part, the anomaly may derive from experimental error due to the widths of the bands involved, but the most likely source of the trouble is the intervention in the v(CD) region of the CH2D species by the combination 1200 970 cm-'. This could both overlap with one of the vis(CD)bands and also be in resonance with either. The picture is further complicated by problems associated with the v(CH) region of the CHD2 species. For we note that the lower energy band a t 2900 cm-l is accompanied by a shoulder at ca. 2887 cm-' and the higher energy one a t 2938 cm-' is noticeably broad. The appearance of weak bands at 3015 and 2975 cm-l suggests the presence of hydrocarbon impurity, and the latter may well be responsible for the above features associated with the viS(CH)bands. Harmonic Local Mode Force Fields. The problems involved with assignment are further highlighted when we pass t o the second stage of the analysis which involves a global treatment of all the C-H and C-D stretching fundamentals together, in isolation from all other vibrations in the molecule, and seeks refinement of a harmonic local mode force field. Such a treatment produces an "energy-factored))force field, more familiarly encountered in the analysis of carbonyl stretching vibrations in organometallic compounds.25 The treatment has the advantage of yielding both diagonal and interaction stretch-stretch force constants and is particularly inportant when the methyl groups in question contain C-H bonds of differing strength. In the present case the local symmetry of the CH3 moieties is reduced from C3" to C,, resulting in two A and one A" stretching vibrations instead of the vas and vs combinations. A diagnosis of the number of weak and strong bonds in the methyl group can then be made simply by identifying the symmetries of the split ~as(CH3)modes.26 Table 4 shows the results of such calculations based on the results of the matrix and solution spectra. For the former spectra, nine frequencies appeared to be reasonably well defined and mutually compatible. These were assigned uncertainties in the range 2-5 cm-l. The remaining ones were unweighted. The fit to v1(CHsD2) is poor (ev = 8.1 cm-l), and that to v3(CH2Ds),still worse (ev = 17.3 cm-'1. While the former may reflect overlap with a weak impurity band, the latter is so wide of the mark as to cause one to query whether the band seen is due in fact not to vis(CH) but to the expected combination. A further matter for concern is the failure t o reproduce v1 in the CH2Da species, which is the more abundant of the two. By contrast, the value of e,, of 22.3
Table 4. Harmonic Local Mode Refinements for the Methyl Grouw in CpTiMea N2 matrix, 14 K group CHx
Vobsa
Uyb
Eyc
Vobsa
VI
2952 2931 2837d 2211 2192 2060d 2964 2888 206od 2926 2888 2185 2900 2216 2111 2938 2216 2111
2 2 5 2 2 3
0.3 2.4 0.6 -0.1 -2.3 -2.9 15.9 10.7 -2.9 -2.4 22.4 17.3 1.4 8.3 2.1 8.1 21.6 12.0
2935 2935 2831d 2201 2201 2052d 2958 2893 2052d 2958 2893 2178 2891 2207 2105 2928 2207 2105
V2
CD3
v3 v1
+
~~
(25) See, for example: Nakamoto, K. Infrared and Raman Spectra oflnorganic and Coordination Compounds, 4th ed.; Wiley: New York, 1986. (26)McKean, D.C.;Torto, I. J. Mol. Struct. 1982,81, 51.
CCl4 soln, 298 K
mode
3 2 2
Oyb
10
5 5
2 2
EVc
(2947.7) (2919.9) -0.5 (2211.2) (2191.2) 0.2 14.1 21.2 0.2 38.1 34.0 12.2 0.0 -0.2 3.7 0.0 15.8 15.1
param
N2 matrix
CC14 soln
H-C-H ("1 fs ( m d y d h
107 4.6927(75) 4.5924(36) -0.0017(35)
110 4.6874(4) 4.5689(4) 0.0191(6)
fa
(mdyd&
f' ( m d y d h
v(CD) data are divided by 1.011 before input, and output values multiplied by 1.011 for the listing (see ref 21). Uncertainty in v ; unweighted datum. = Vobs - vcalc. Values in parentheses are vcale. Fermi resonance corrections applied: -25 cm-l for CH3; -5 cm-' for CD3. e The subscripts s and a denote C-H or C-D bonds lying respectively in or out of the plane of symmetry of the asymmetric methyl group.
cm-' for v2 for the less abundant CH2Ds conformer probably indicates that the frequency listed, viz. 2888 cm-', applies only to the CH2Da conformer. With respect to VI, the model cannot account for any CHzD frequency which exceeds the highest CH3 one, so that the observed breach of this condition again suggests intervention from impurity. In making each refinement, an H-C-H angle has to be assumed. Exploration of a range of likely angles showed only a slight sensitivity of the fit to the angle and the value finally selected, 107", carries an uncertainty of 2-3". If the wavenumber ratio of the antisymmetric stretching modes of the CH3 and CD3 groups is used as an indicator of the angle (only v2 is appropriate for this purpose), the estimated H-C-H angle comes out to be 103.4". However, the force field refinement gives a fit which is markedly worse with this value and, in addition, the value of the interaction force constant f ' determined on this basis is anomalously negative (-0.027 m d y d h . Accordingly this source of information must be disregarded. That the value o f f ' for H-C-H = 107" is -0.002 mdyn/A suggests a greater likelihood of an angle which actually exceeds, rather than falls below, 107". In the attempt t o fit the carbon tetrachloride solution results, only four band centers were sufficiently well defined to be utilized, and two of these, vas(CH3)and vas(CD3),required Fermi resonance corrections of 25 and 5 cm-', respectively. With the assumption of one strong and two weak bond, the mean values of the two vas(CH3) and ~as(CD3)frequencies predicted were very close to the centers of the broad bands seen in the spectra.
Organometallics, Vol. 14, No. 8, 1995 3789
Cyclopentadienyltrimethyltitanium(N)
Table 5. Comparison of viS(CH)and ro(CH)Values of CpTiMes with Corresponding Parameters Reported for other Methyl-Metal Compounds molecule CpTiMe3a CpTiMeClzb CpzTiMezb MeTiClf MezTiClz' CpzZrMezb CpzHfMezb MeMn(C0)g MeRe(C0)s' Me2Znc MezCd' MezHgC Me3Gac Me3Tlc Me4Gec Me4Snc MedPb'
vLS(CH) (cm-')
r,(CH) (@
ref
2943 2905 x 2 2958 x 2 2918 2932 x 2 2915 2952 2938 2904 2900 2955 2935 2935 2948 2954 2940 2967 2954 2960 2978
1.098 1.102 1.096 1.100 1.098
this work 22 21
1.100 1.096 1.098 1.101 1.102 1.096 1.098 1.098 1.097 1.096 1.097 1.094 1.096 1.095 1.093
13 18 21 21 40 40 12 12 12 41 41 42 42 42
a Values relate to a n Nz matrix containing the compound with the addition of 5 cm-' to give an empirical correlation with gas phase values. Values relate to CCld solution of the compound with the addition of 10 cm-l t o give an empirical correlation with gas phase values. Values relate to the gaseous molecule. Calculated from eq 2.
However, the calculations gave a poor account of all the CH2D bands, including both the viS(CD)modes. The H-C-H angle chosen, viz. 110", optimized the quality of the fit, but since this fit depended rather critically on the Fermi resonance shifts assumed, an uncertainty in the bond angle of &3"is likely. No difference in the angle for the matrix-isolated and solution species can therefore be inferred. The marginally larger angle implied by the solution results does, however, illustrate the dependence on H-C-H of the interaction force constant f '. We were unable to account for the spectra of the solid samples using a harmonic local mode analysis in any sensible manner. Whether this implies that the methyl groups are subject to significant additional interactions under these conditions must, however, remain a matter for speculation.
Discussion Despite difficulties in the detailed analysis outlined above, it is plain that we have in the molecule CpTiMes a modest difference in C-H bond strength, in which a 40-50 cm-l change in vis(CH)may be translated via the correlation with ro(CH)(eq 2) into a 0.004-0.005 A difference in bond length.g ro(CH) (A) = 1.3982 - 0.0001023viS(CH)(cm-l) (2) Comparison of the results obtained from this study with the data obtained previously for the species CpT i M e c l ~and ~ ~Cp~TiMe2,~l along with other methylmetal derivatives (Table 5), is instructive for the light it sheds on the behavior of the CH3 moiety when bonded to a metal center. The following points are evident. 1. All three of the cyclopentadienyltitanium derivatives CpTiMe3, CpTiMeCl2, and CpzTiMe2 exhibit asymmetric methyl groups. However, whereas the asymmetry for CpTiMeCl2 and CpaTiMe2, as well as other
methyl-metal compounds, involves one weak and two strong C-H bonds, CpTiMe3 is unique among the compounds studied to date in displaying one strong and two weak C-H bonds. A second unusual feature is the apparent switch from two weak to one weak bond accompanying the move from the matrix or solution t o the solid phase. Comparison of the values of viS(CH) associated with the CHDz versions of the cyclopentadienyl derivatives and of MeTiC1313 and MezTiClP (each of which appears to be characterized by a single value of vis(CH))suggests that the stronger bonds comply with normal behavior, with viS(CH)and ro(CH) parameters generally in line with the values deduced from other methyl derivatives of titanium, the later transition metals, and the post-transition metals. Hence it is the weaker bonds that are anomalous. We have sought to correlate the source of the asymmetry with the groups lying ,9 to the C-H bonds. In the methyltitanium compounds, these can be q5-C5H6,other methyl groups, or chlorine. It is hard to see any regularities underlying the ,9 effects, such as may be found, for example, in conventional organic compound^.^ From the point of view of such organic compounds, it is curious that no distinction can be found between the C-H bonds in Me2TiC12;18 slight asymmetries are certainly observed for M ~ z C and H ~M~e ~ s i H 2 . ~ ~ We considered two other explanations for the weakening of the C-H bonds in the methyl groups of the cyclopentadienyl derivatives. The first invokes steric crowding in the coordination sphere of the small titanium atom, while the second focuses on the electron deficiency of this center, leading to an agostic interaction with the electrons in the C-H bonds. In the series CpTiMes (l),CpTiMeCl2 (21, and CpsTiMe2 (3), steric crowding will decrease in the order 3 > 2 > 1,29whereas electron deficiency might reasonably be expected to follow the order 1 > 2 > 3. Table 6 details salient parameters for the species under consideration. There is no immediate correlation of the mean value, viS(CH,,), or of AviS(CH)(the difference between the two values) either with the degree of steric crowding about titanium or with the electron deficiency of the metal center. However, if we take into account the total weakening of C-H bonds in each molecule, a clearer picture emerges. Multiplying AvYCH) for each compound by the number of C-H bonds responsible for the lower value of vis(CH),XCH, shows the effect to be much more marked in the 12-electron compound CpTiMe3, which has no ability to reduce its electron deficiency by classical n-donation from electron-rich substituents. Correlation of viS(CH) with the dissociation energy D"cH'~implies a difference in strength between the stronger and weaker C-H bonds in CpTiMes of about 14 k J mol-'. Indeed, the spectroscopic properties bring the weaker bonds within, or close to, the regime of C-H bonds in organic molecules which are subject to significant weakening by electronic interactions, for example, with antiperiplanar lone pairs of electrons, as in MeaNH or Me20.9 It is not unreasonable therefore to suggest that an electronic interaction similar in magnitude occurs also in this cyclopentadienylmethyltitanium derivative, with removal of electron density from the C-H bonds into the empty d orbitals located mainly ~
(27) McKean, D. C. Unpublished results. (281Kromer, R.; Thiel, W. Chem. Phys. Lett. 1992, 189, 105. (BgjTolman, C. A. Chem. Rev. 1977, 77, 313.
3790 Organometallics, Vol. 14, No. 8, 1995
McGrady et al.
Table 6. Comparison of vi*(CH)and Related Parameters for Cyclopentadienyl Methyl Derivatives of Titanium(IWa molecule
electron count on Ti
vYCH,,)
CpTiMes (1) CpTiMeClz (2) CpzTiMez (3)
12 12 16
2918 2945 2926
wavenumber (cm-') AvYCH)~ 38 40 17
Av'YCH) x 228 40 34
XCH~
ref this work 22 21
P ( C H ) values taken from Table 5. AviS(CH)= vlS(CHhlgher) viS(CHiower) for an asymmetric methyl group. XCH is the number of C-H bonds exhibiting V~~(CH~,,,,).
on the metal. The very existence of such an interaction isolated CHD2Mn(C0)5 revealed only very small splitin CpTiMe3 may also give a clue to the absence of tings of the vis band, it may be concluded that the source asymmetry in the methyl groups of MezTiClz (vide of the broadening here is exclusively due to free internal supra): extended Huckel MO calculations have indirotation of the methyl In molecules where cated that such Me OH-C agostic interactions are much there is both free internal rotation and also variation more likely to occur in octahedrally, as opposed to of vis with torsional angle, two kinds of behavior are tetrahedrally, ligated titanium system^.^" The bonding found. In t ~ l u e n eand ~ ~n, i~t r~~ m e t h a n e solutions, ~~-~~ in CpTiMe3, although formally involving a four-coordi~as(CH3) splittings can be seen, but the spectrum of the nate metal center, is much better described in terms of gaseous molecule is dominated by a band whose frepseudo-octahedral coordination, with the 6-electron quency represents the average of a cyclical variation and cyclopentadienide ligand straddling three vertices.31 whose intensity derives from the dipole moment change parallel to the top axis. This again cannot correspond 2. The cyclopentadienyl methyl congeners of the heavier Group 4 metals, viz. Cp2ZrMe2 and Cp~HfMe2,~l t o the CpTiMes situation. Finally, we note that compounds of the type MeBX2 show splitting in the vas and each exhibit only one value of vis(CH),yet this occurs also the vis bands, the intensity of which appears to at lower wavenumber than the vis(CH)values for the derive solely from a dipole change perpendicular t o the weaker C-H bonds in any of the titanium derivatives. top axis;39the last feature is reflected in the marked This finding manifests the general tendency for C-H and unusual weakness of the vs(CH3) band. With bonds in any family of methyl-transition metal comCpTiMe3, however, the v&, intensity distribution is pounds t o become progressively weaker as the atomic normal. All that can be said at the present time, number of the transition metal increases [cf. MeMntherefore, is that the behavior of the IR bands due to (co)5and MeRe(CO)5,Table 51. CpTiMe3 in carbon tetrachloride solution does not fall 3. The correlation between vis(CH) and DOCHfor into any previously known category. Good quality a b methyl derivatives has been noted previously.ll The initio calculations of the methyl torsional frequencies difference between the highest and lowest reported vismay yet help t o throw light on the situation. (CHI values in Table 5 represents a difference of 6-7% 6. It will be evident from the difficulties experienced in the C-H bond dissociation energy. This conclusion here in attempting simultaneously to fit vis(CH)and visserves also t o emphasize the comparative insensitivity (CD) results that this investigation casts some doubt of r,(CH) to variations in the strength and character of on the usefulness of vis(CD)results, at least for quantithe C-H bond. tative purposes. In extenuation, it may be said with 4. H-C-H angles in the range 107-110" are comsome justification that the present molecule is not the patible with what is known about other organotitanium most suitable one that could be chosen for this purpose compounds, the best estimates coming from an NMR and that comparative failure with CpTiMe3 should not study15or from a b initio calculation^.^^^^^ The electronbe taken to be symptomatic of the general inutility of diffraction study of Cp*TiMe3 yielded a Ti-C-H angle viS(CD) studies. We are encouraged in this belief by the of 103.8";7in terms of the model assumed (C3" local excellent consistency which has been found in our paralsymmetry for the methyl group), this implies an H-C-H lel study of another methyltitanium compound, Mezangle of 114.5'. We are confident that such an angle TiC12;18 such success may depend in part on the ability can be ruled out for CpTiMe3 on the basis of our IR t o study the spectra of the compound in the vapor phase. spectra, although these do not of course contain any direct information about the Ti-C-H angles. Acknowledgment. We thank Jesus College, Oxford, 5. The failure t o resolve the ~as(CH3)and ~as(CD3) U.K., for the award of a Junior Research Fellowship (to bands in the spectra of carbon tetrachloride solutions G.S.M.) and the SERC (now ESPRC) for funding which is puzzling. The appearance of these bands resembles has enabled the purchase of equipment. that of the similar band displayed by MeMn(C0)s in OM950165U solution.33 For the manganese compound, however, the (34) Dempster, A. B.; Powell, D. B.; Sheppard, N. Spectrochim.Acta vlYCH) band of the CHD2 version is also a singlet, 1972,28A, 373; 1975,31A, 245. although broad, behavior which contrasts with the (35) Cavagnat, D.; Lascombe, J. J. Mol. Spectrosc. 1982, 92, 141. splitting seen in the spectrum of the corresponding (36)Jones, W. J.; Sheppard, N. Proc. R.SOC. London 1968,A30,135. (37) McKean, D. C.; Watt, R. A. J . Mol. Spectrosc. 1976, 61, 184. CpTiMe3 derivative. Since the spectra of matrix(38) Cavagnat, D.; Brom, H.; Nugteren, P. R. J . Chem. Phys. 1987,
(30) Eisenstein, 0.;Jean, Y. J. Am. Chem. SOC. 1985, 107, 1177. (31)See, for example: Elschenbroich, Ch.; Salzer, A. Organometallics: A Concise Introduction, 2nd ed.;VCH: Weinheim, Germany, 1992. (32)Jonas, V. Private communication. (33)Firth, S.; Horton-Mastin, A,; Poliakoff, M.; Turner, J. J.; McKean, D. C.; McQuillan, G. P.; Robertson, J. Organometallics 1989, 8, 2876.
87, 801.
(39) McKean, D. C.; Becher, H.J.; Bramsiepe, F. Spectrochim.Acta 1977,33A, 951. (40) McKean, D. C.; McQuillan, G. P.; Thompson, D. W. Spectrochim. Acta 1980,36A, 1009. (41) McKean, D. C.; McQuillan, G. P.; Torto, I.; Morrison, A. R. J . Mol. Struct. 1986, 141, 457. (42) Burger, H.; Biedermann, S. Spectrochim. Acta 1972,28A, 2283.
