3116
Organometallics 1995,14, 3116-3119
Molecular Structure of Monomeric (Pentamethylcyclopentadienyl)aluminum(I)by Gas-Phase Electron Diffraction Arne Haaland,*tt Kjell-Gunnar Martinsen,t Sergey A. Shlykov,t>§ Hans Vidar Volden,? Carsten Dohmeier,$and Hansgeorg Schnockel*vS Department of Chemistry, University of Oslo, Box 1033, Blindern, N-0315 Oslo, Norway, and Institute for Inorganic Chemistry, University of Karlsruhe, Box 6980, 0-76128 Karlsruhe, Germany Received February 27, 1995@ Summary Gas-phase electron diffractiondata of Cp*Al = (C&fe5)Al,Me = CH3, recorded with reservoir and nozzle temperatures of 139 f 4 "C, show that the gas consists of monomeric (q5-Cp*)Al units. Least-squares refinements of a molecular model of C5, symmetry yield the bond distances AI-C = 238.8(7), C-C(endocyc1ic) = 141.4(5), and C-C(exocyc1ic) = 152.9(5) p m and a perpendicular metal-to-ring distance of 206.3(9) p m . The synthesis of the first molecular compound of monovalent aluminium which is stable at normal temperatures, (pentamethylcyclopentadieny1)aluminumor Cp*Al, was reported by Schnockel et al. in 1991.' The compound is tetrameric in the solid state. Room temperature X-ray crystallography shows that the tetramer consists of a tetrahedral A l 4 core. Each Al atom is q5bonded to a terminal Cp* ring in such a manner that the ring plane is parallel t o the opposite A l 3 face of the tetrahedron.' The 27Al NMR spectrum of Cp*Al in toluene at temperatures ranging from -80 to 25 "C consists of one sharp line at -80.7 ppm.2 This line was assigned to the tetramer [Cp*&.lJ by comparison with the shifts obtained for monomeric Cp*Al, CpAl, and [ C p A d by ab initio calculations with the gauge-including atomic orbital method at at the dzp MP2 level.3 Increasing the temperature above 30 "C led to reversible changes in the 27AlNMR spectrum: A second line appears at - 149 ppm. This line, which was assigned to monomeric Cp*Al, became more intense as the temperature increased, while the peak assigned to the tetramer became less intense. Analyses of the variation of the intensities of the two lines with temperature yielded an estimate of 150 f 20 k J molw1for the dissociation enthalpy of the tetramer in Cp*Al appears t o be thermally stable for infinite periods at room temperature but decomposes at temperatures above 100 "C. When a solution of Cp*Al in University of Oslo. University of Karlsruhe. e Present address: Ivanovo State Academy of Chemistry and Technology, Department of Physics, Engelsa Ave., Ivanovo 153460, Russia. @Abstractpublished in Advance ACS Abstracts, May 15, 1995. (1)Dohmeier, C.; Robl, C.; Tacke, M.; Schnockel, H.Angew. Chem., Int. Ed. Engl. 1991, 30,564-565. (2)Dohmeier, C. Ph.D. Thesis, Ludwig-Maximillians-Universitat Munchen, Munchen, Germany, 1994. (3) Gauss, J.; Schneider, U.; Ahlrichs, R.; Dohmeier, C.; Schnockel, H. J . Am. Chem. SOC.1993,115, 2402-2408. (4) Roesky and co-workers, who prepared Cp*Al by another route, found only one line, corresponding to the tetramer, in the 27Al NMR spectrum in deuterated benzene in the temperature range 40-78 "C: Schulz. S.; Roesky, H. W.; Koch, H. J.; Sheldrick, G . M.; Stalke, D.; Kuhn, A. Angew. Chem., Int. Ed. Engl. 1993,32,1729-1731. +
i
toluene was kept a t 100 "C, finely divided alumini m metal became visible after some 24 h.2 After 20 days a t 100 "C the sample was completely decomposed. The decomposition products were pentamethylcyclopentadiene or Cp*H, metallic aluminum, and nonvolatile organometallic Al(II1) compounds which were not characterized.2 Cp*Al may be sublimed with insignificant decomposition at about 140 "C, but a t higher temperatures the rate of decomposition becomes prohibitive.2 On the basis of the mass spectra and the intensity of the gasphase electron diffraction pattern we estimate that the vapor pressure at 140 "C is about 0.05 Torr. This pressure is 2 or 3 orders of magnitudes below the pushing pressures normally used for the collection of gas-phase electron diffraction data.5,6 The intensity of the electron diffraction pattern is determined by the intensity of the primary beam, the concentration of the gas (mol L-l) in the scattering region, and the scattering power of the molecular species to be investigated. The scattering power of a molecule is proportional to the sum of the squares of the nuclear charges of the constituent atoms, XZi2. The concentration of the gas density in the scattering region is determined by the pushing pressure, which in turn is limited by the vapor pressure of the solid or liquid sample. Over the past years we have found that a primary beam intensity of about 50 nA combined with a pushing pressure between 0.5 and 5 Torr provides excellent conditions for the recording of electron diffraction data of organometallic compounds on our Balzers Eldigraph KDG 2 unit.'^^ Recently we have modified our instrument to allow us to increase the intensity of the primary beam: The magnetic lens, which in the original design was placed a t a distance of 55 cm from the filament, was replaced by a new and slightly modified lens at about 22 cm from the electron-emitting filament. This modification allows the utilization of a greater fraction of the electrons emerging from the electrostatic focusing device and increases the primary beam current by 1 order of (5) Ebsworth, E. A. V.; Rankin, D. W. H.; Cradock, S. Structural Methods in Inorganic Chemistry, Blackwell, Oxford, U.K., 1987; pp 304-315. (6) Hargittai, I. Gas-Phase Electron Diffraction In Accurate Molecular Structures; Domenicano, A., Hargittai. I., Eds.; Oxford University Press: Oxford, U.K., 1992; p 95-125. (7) Zeil, W.; Haase, J.; Wegmann, L. 2.Instrumentenkd. 1966,74, 84-88. (8) Bastiansen, 0.;Graber, R.; Wegmann, L. Bakers High Vacuum Report 1969, 25, 1-8.
0276-733319512314-3116$09.00/0 0 1995 American Chemical Society
Organometallics, Vol. 14,No. 6, 1995 3117
Notes
Table 1. Interatomic Distances (ra),Root Mean Square Vibrational Amplitudes ( I ) , and Valence Angles in (Cp*)Al, and Mole Fraction of the Pentamethylcyclopentadiene, Cp*H, Impuritp
Figure 1. Molecular model (Pluton13)of (r15-Cp*)Al.Molecular symmetry, C5u. magnitude to a maximum value of about 1500 nA. With a primary beam current of this magnitude we were able t o record the scattering pattern of Cp*Al with a reservoir and nozzle temperature of 139 f 4 "C. Crystalline Cp*Al was synthesised from solvated AlCl and Cp*zMg as described in ref 9. The NMR spectra in toluene contained no peaks indicating the presence of Cp*H or other impurities. Gas-phase electron diffraction data were recorded with a metal inlet system. Exposures were made with nozzle-to-photographicplate distances of about 50 and 25 cm; structure refinements were based on data from four plates from the 50 cm set and three plates from the 25 cm set. The plates were photometered and the data processed by a program written by T. G. Strand. Atomic scattering factors were taken from ref 10. Backgrounds were drawn as leastsquares-adjusted seventh (50 cm) or eighth (25 cm) degree polynomials to the difference between total experimental and calculated molecular intensity curves. Molecular structure refinements were carried out with the program KCED26, which was written by G. Gundersen, S. Samdal, H. M. Seip, and T. G. Strand. Structure refinements were based on a molecular model of CS, symmetry as shown in Figurel. Methyl groups were assumed to have CsUsymmetry with the symmetry axes coinciding with the exocyclic C(Cp)C(Me) bonds. The orientation of the methyl groups is such that one C-H bond points away from the metal as indicated in the figure. The molecular geometry is then determined by six independent parameters, e.g. the endocyclic C(Cp)-C(Cp) and the exocyclic C(Cp)-C(Me) bond distances, the Al-C and C-H bond distances, the valence angle LCCH and the angle between the exocyclic CC bonds and the ring plane which we denote by LC5,C-C and define as positive when the bonds are bent towards the metal atom. Several refinements showed that the magnitude obtained for LC5,C-C is independent of the orientation of the methyl groups. Since information on the vibrational spectrum of monomeric Cp*Al is missing, shrinkage corrections were neglected. (9) Dohmeier, C.; Loos, D.; Robl, C.; Schnockel, H.; Fenske, D. J. Organomet. Chem. 1993,448, 5-8. (10) Bonham, R. A.; Schafer, L. Complex Scattering Factors for the Diffraction of Electrons by Gases. In International Tables for X-Ray Crystallography; Ibers, J. A,, Hamilton, W. C., Eds.; Kynoch Press: Birmingham, U.K., 1974; Vol. 4.
A-C C(Cp)-C(Cp) C(Cp)-C(Me) C-H
238.8(7) 141.4(5) 152.9(6) 111.0(6)
12(1) 3.2(15Ib 3.7(15Ib 8.4(5)
nonbonded distances Al-- -C(Me) C(Cp)- - -C(CP) C(Cp)- - -C(Me) C(Cp)- - -C(Me) C(Me)- - -C(Me) C(Me)- - -C(Me)
327(2) 221(1) 262( 1) 377(1) 320(1) 518(1)
22(4) r5.81 5.9(8) 8.8(6) r13.11 14(1)
h
206.3(8)
LCCH LC5,C-C
112.3(6) 5(2)
x(CP*H)(5%)
7(3)
27(3)
R factorsC(%)
2.2 (50 cm)
8.6 (25 cm)
Distances and vibrational amplitudes in pm, angles in degrees. Estimated standard deviations in parentheses in units of the last digit. These vibrational amplitudes were refined with constant difference. R = [ C W ( Z & - z,,l,d)2~W(Z,b,)2]1'2; total, 2.9%. a
Exploratory refinements showed that the gas-phase electron diffraction data were incompatible with a gas consisting of (q-Cp*)Al only. Since the data had been collected at a temperature at which Cp*Al is known to suffer some thermal decomposition, and since the only volatile decomposition product was CP*H,~we then attempted to refine the mole fraction of such an impurity along with the structure parameters of Cp*Al. Cp*H has never been studied by gas-phase electron diffraction. A molecular model was therefore constructed from the known molecular structure of the bicyclic compound Cp*z by breaking the bond between the rings and adding a hydrogen atom.ll Separate refinements of the intensity data obtained with nozzle-to-plate distances of 50 and 25 cm indicated that, while the mole fraction of Cp*H in the former was about x(Cp*H) = 0.10, the mole fraction in the latter (which required longer exposure times) was about 0.30. The KCED26 program does not allow simultaneous refinement of different impurity mole fractions for different parts of the data. The 25 cm intensities data were therefore amended by subtraction of calculated intensity for Cp*H until the two data sets yielded equivalent values for x(Cp*H). The intensity data recorded with a nozzle-to-photographic plate distance of 25 cm are normally of poorer quality than the data recorded with a distance of 50 cm, and the present study is no exception. Structure refinements were therefore carried out with unit weight for the 50 cm data and a weight of W = 0.2 for the 25 cm data: this weighting scheme yielded square error sums, c w ( l o b s - Icalcd2, of about equal magnitude for the two data sets. The final refinements involved the six independent structure parameters of Cp*Al, 12 root mean square vibrational amplitudes, 1, and the mole fraction of the impurity, x(Cp*H). The best values are listed in Table 1. Since the refinements were carried out with diagonal weight matrices, the estimated standard deviations (11)Blom, R. Acta Chem. Scand., Ser. A 1988,42, 445-453.
3118 Organometallics, Vol. 14, No. 6, 1995
I
l
50 cm data
i
l
l
~
l
l
l
l I
1m> ,,,,, 100
,
i t I , ,
,,
200
I ,
1,
/
I
/
(
I
I
I
I
I , , #,,I
I
-
--
-
0
Notes
I , I,
1 , I , I I , I , ,
400
300
,
, , : ' I , , I
500
l
i
l
l
~
I I , I,
600
I ,
1,
I I , ,
I
r
' 1
700
r r pm
Figure 3. Experimental and calculated (-) radial distribution curves of (q5-Cp*)Al.The vertical scale is arbitrary. Difference curve is shown at the bottom of the figure. Artificial damping constant, k = 25 pm2. (e..)
listed in the table have been multiplied by a factor of 3.0 to include the added uncertainty due to data correlation12and our modeling of the structure of Cp*H. The parameter LC5,C-C may be particularly sensitive to errors in the modeling of Cp*H, the estimated standard deviation of this parameter was therefore, somewhat arbitrarily, further increased from 0.7" to 2". Experimental and calculated intensity curves and radial distribution curves are compared in Figures 2 and 3, respectively. Least-squares refinements were carried out on two more molecular models differing from the model in Figure 1 in the orientation of the methyl groups: another model of C5" symmetry in which all methyl (12) Seip, H. M.; Strand, T. G.; Stolevik, R. Cfiem.Phys. Lett. 1969, 3, 617-623. (13) Spek, A. L. The "Euclid" Package. In Computational Crystallography; Sayre, D., Ed.; Clarendon: Oxford, U.K., 1982.
l
groups had been rotated 180" from their oroginal positions in the Figure and a model of C5 symmetry in which they had been rotated 90". Refinement of these models yielded slightly, but not significantly, poorer agreement between observed and calculated intensities. We conclude that the data contain insufficient information to allow us to determine the equilibrium orientation of the methyl groups. The Al atom is the heaviest atom in the molecule, and the terms representing the five Al-C(Cp) bond distances at 239 pm and the five nonbonded Al- - -C(Me) distances at 327 pm are the largest terms in the molecular intensity curve along with the terms representing the nonbonded distances C(Cp)-- -C(Me)at 262 and 377 pm. The good agreement between experimental and calculated curves obtained for a model shows that the five Al-C bond distances must be equal or nearly equal: If the metal atom in Cp*Al was +bonded to the ring, the peak representing the five Al-C bond distances would split into one peak a t about 197 pm representing the AI-C o-bond distance, and two peaks, each representing two nonbonded Al- - -C(Cp)distances, a t about 280 and 360 pm. The nonbonded Al- - -C(Me) peak at 327 pm would be split in a similar manner. Such a model is clearly incompatible with the gas-phase electron diffraction data. This study shows that the break up of the tetramer in the gas phase leads to monomeric (q-Cp*)Aland lends indirect support to the conclusions reached in the 27Al NMR studies described above. Comparison of structure parameters are complicated by the fact that the monomer and tetramer structures have been determined in different phases and by different methods. In particular, bond distances obtained by X-ray crystallography at room temperature are expected to be systematically shortened unless corrected for thermal motion. It nevertheless appears likely that the perpendicular metal-to-ring distance h is about 3 pm shorter in the tetramer than in the monomer: The values obtained for h in the crystal range from 199.7 to 203.2 pm with a mean value of 201.5 pml, while the gas-phase value is 206.3(8) pm. The bonding radii of monovalent metals Al, Ga, or In are generally larger than the single-bond radii of the trivalent metals. Thus the bond distances in the gaseous monomeric monochlorides AlC1, GaC1, and InCl are 213,220, and 240 pm, respectively, while the bond distances in the gaseous monomeric trichlorides are 207, 211, and 229 pm, re~pective1y.l~The difference has been rationalized as a hybridization effect: The pair of nonbonding electrons in the monochlorides are assumed to occupy the valence shell s atomic orbital. This leaves a pure p A 0 for formation of the M-C1 bonding MO. The two-center bonding MOs in the trichlorides, on the other hand, are constructed from sp2 hybrid orbitals on the metal atom. The increased s-character is expected to lead to a better overlap with the ligand A 0 and to a shorter bond.I5 If real, the observed shortening of the metal t o ring distance in [AlCp*]d as compared to AlCp* (14)Haaland, A.; Hammel, A.; Martinsen, K.-G.; Tremmel, J.; Volden, H. V. J. Chem. SOC.,Dalton Trans. 1992,2209-2214. (15) The bond energies of the monochlorides are, however, lower than the mean bond energies of the trichlorides. See ref 14 for a discussion.
Organometallics, Vol. 14, No. 6, 1995 3119
Notes
experimental perpendicular metal-to-ring distance is 7 pm greater than ~ a l c u l a t e d . ~ In Figure 4 we compare the M-C bond distances in gaseous monomeric Cp*Al, Cp*Ga,16 Ga-C = 240.5(4) pm, Cp*In,17 In-C = 259.2(4) pm, and Cp*T1,18 T1-C = 266.3(5) pm, with the M-I bond distances in the gaseous monoiodides, AI-I = 253.7, Ga-I = 257.5, In-I = 275.4, and T1-I = 281.4 pm.19 The similarity of the M-C and M-I bond distance curves suggest that both M-C and M-I bond distances are primarily determined by the size of the metal atom.
290
280
-
270
-
min U
m C Y
260 -
in
E 'D C
250
-
240
-
m0
La"
1
AI
Ga
;1
TI
Figure 4. M-I bond distances in the gaseous monomeric monoiodides and M-C bond distances in gaseous (v5Cp*)M, M = Al, Ga, In, and T1. may, in a similar manner, be due to increased scharacter of the al hybrid A 0 pointing toward the ring center. The bond distances in the Cp* ligand are in reasonable agreement with those obtained by structure optimization of Cp*AI a t the dzp MP2 level, but the
Acknowledgment. We are grateful to the VISTA program of STATOIL and the Norwegian Academy for Science and Letters for financial support. OM9501586 (16) Haaland, A.; Martinsen, K.-G.; Volden, H. V.; Loos, D.; Schnockel, H. Acta. Chem. Scand. 1994,48, 172-174. (17)Beachley, 0. T., Jr.; Blom, R.; Churchill, M. R.; Faegri, K.; Fettinger, J. C.; Pazik, J. C.; Victoriano, L. Organometallics 1989, 8, 346-356. (18)Blom, R.; Werner, H.; Wolf, J. J.Organomet. Chem. 1988,354, 293-299. (19) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure; Van Nostrand: New York, 1979; Vol. 4.
