Bis­[μ-1-(2,4,6-tri­methyl­phenyl)­ethano­lato-O:O]­bis­(di­methyl­aluminium)

The title compound, [{Al(CH₃)₂}₂(C₁₂H₁₈O)₂], formed by the Me₃Al-mediated alkyl­ation of mesityl­aldehyde, is dimeric in the solid state, with a central Al₂O₂ planar ring core residing on an inversion centre.

Key indicators

• Single-crystal X-ray study
• T = 160 K
• Mean [sigma](C-C) = 0.005 Å
• R factor = 0.076
• wR factor = 0.216
• Data-to-parameter ratio = 15.4

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level C:



RADNW_01  Alert C The radiation wavelength lies outside the expected range
          for the supplied radiation type. Expected range 1.54175-1.54180
          Wavelength given =    1.54184

REFLT_03
         From the CIF: _diffrn_reflns_theta_max           67.50
         From the CIF: _reflns_number_total               2201
         TEST2: Reflns within _diffrn_reflns_theta_max
         Count of symmetry unique reflns         2380
         Completeness (_total/calc)             92.48%
          Alert C: < 95% complete

PLAT_743  Alert C Torsion Calc    0.00(11), Rep        0.00 ....    Missing s.u.
                     O   -AL  -O   -AL     3.665   1.555   1.555   3.665


 0 Alert Level A = Potentially serious problem

 0 Alert Level B = Potential problem

 3 Alert Level C = Please check

---

S1. Comment

For many years, aluminium alkoxides and aryloxides have been utilized as reagents in organic synthesis (Zietz et al., 1983). However, the development of heterobimetallic reagents for use in asymmetric transformations has led to an increased awareness of the importance of aluminium in synthesis (Yamamoto & Maruoka, 1988; Saito & Yamamoto, 1997; Saito et al., 1998; Saito et al., 1999; Cogan & Ellman, 1999). In this regard, we have recently reported the syntheses and solid-state structures of dimethylaluminium enolates and aryloxides, produced from the reactions of trimethylaluminium and aromatic methyl ketones (Allan et al., 2000). The unexpected formation of enolate species was found to be a consequence of steric crowding in the ketones, when both the 2- and 6-positions of the aromatic rings carried methyl groups. We report here the structure of the title compound, (I), which is produced from the addition reaction between trimethylaluminium and 2,4,6-trimethylbenzaldehyde (mesitylaldehyde).

As can be seen from Fig. 1, complex (I) adopts a dimeric structure with a planar central Al₂O₂ ring core. The molecule is crystallographically centrosymmetric. Each Al atom is tetracoordinate by bonding to two methyl groups and two bridging O atoms. Although the average of the angles around each metal is 108.4°, these vary from 80.20 (9)° for O—Al—Oⁱ to 119.49 (16)° for C1—Al—C2, giving a highly distorted pseudo-tetrahedral geometry at aluminium [symmetry code: (i) 1 - x, 1 - y, -z]. The internal Al₂O₂ ring angles at the metal atoms are smaller than at the oxygen atoms [80.20 (9) and 99.80 (9)°, respectively], which is consistent with sp² hybridization for the bridging O atoms (van der Steen et al., 1991). The two independent Al—O distances are essentially the same at 1.844 (2) and 1.848 (2) Å for Al—O and Al—Oⁱ respectively. Similarly, the remaining bond lengths and angles within (I) are in accord with those found in related systems (Schumann et al., 1996; Hitchcock et al., 1990; Sierra et al., 1989).

It is notable that the α-carbon atom C3 lies 0.434 Å out of the Al₂O₂ ring plane, and that the groups attached to this atom are staggered with respect to the ring, with torsion angles of -30.5 (4), 86.3 and -157.3 (2)° for Al—O—C3—C4, Al—O—C3—H3, Al—O—C3—C5, respectively. These differ markedly from those found in the closely related enolate analogue [Me₂AlOC(2,4,6-Me₃C₆H₂)=CH₂], (II), where the olefin bond lies close the Al₂O₂ ring plane (Allan et al., 2000). This difference is most likely a consequence of the sp³ versus sp² hybridization of the α-carbons in (I) and (II), respectively, which alters the minimum energy orientation of the attached groups.

S2. Experimental

Under an inert atmosphere of argon gas, mesitylaldehyde (0.35 g, 2.36 mmol) was added dropwise to a 195 K cooled solution of Me₃Al (2.4 mmol of a 2M solution in toluene) in 3 ml of toluene, producing a yellow solution. The reaction mixture was allowed to warm slowly to ambient temperature with constant stirring and stirred for a further 20 h. All toluene was removed in vacuo, the residue was dissolved in 5 ml of hexane and a crystalline product was obtained on cooling the mixture to 245 K for 24 h. Yield: 0.153 g, 29.5%; m.p. 417–419 K. ¹H NMR (C₆D₆, 400 MHz): δ 6.66 (broad s, 2H, m-H, Ph), 5.58 (q, J = 6.8 Hz, 1H, HCO), 2.54 (broad s, 3H, o-Me, Ph), 2.14 (broad s, 3H, o-Me, Ph), 2.01 (m, 3H, p-Me, Ph), 1.36 (m, 3H, Me), -0.44, -0.53, -0.62 (s, 6H, Me—Al). The presence of three signals for the methyl groups attached to the metal and the broad resonances obtained for some of the ligand H atoms indicates that a dynamic process is in operation in solution.

S3. Refinement

The high-angle data (θ = 60–67.5°) are incomplete because of restrictions imposed by the low-temperature device; data up to θ = 60° are 98% complete. H atoms were placed geometrically and refined with a riding model (including free rotation about C—C bonds), and with U_iso constrained to be 1.2 (1.5 for methyl groups) times U_eq of the carrier atom.

Computing details

Data collection: DIF4 (Stoe & Cie, 1988); cell refinement: DIF4; data reduction: local programs; program(s) used to solve structure: SHELXTL (Sheldrick, 1997); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and local programs.

Figures

Fig. 1. The molecular structure of (I) with unique atom labels and 50% probability ellipsoids for non-H atoms.

Bis[µ-1-(2,4,6-trimethylphenyl)ethanolato-O:O]bis(dimethylaluminium)

Crystal data

[Al(CH3)2(C12H18O)2]	F(000) = 480
Mr = 440.56	Dx = 1.110 Mg m^−3
Monoclinic, P21/c	Cu Kα radiation, λ = 1.54184 Å
a = 7.224 (3) Å	Cell parameters from 32 reflections
b = 15.380 (6) Å	θ = 19.0–28.0°
c = 12.204 (5) Å	µ = 1.12 mm^−1
β = 103.48 (4)°	T = 160 K
V = 1318.6 (9) Å^3	Block, colourless
Z = 2	0.7 × 0.7 × 0.6 mm

Data collection

Stoe-Siemens diffractometer	Rint = 0.000
Radiation source: fine-focus sealed tube	θmax = 67.5°, θmin = 4.7°
Graphite monochromator	h = −7→8
ω/θ scans	k = −14→18
2201 measured reflections	l = −10→14
2201 independent reflections	5 standard reflections every 60 min
1912 reflections with I > 2σ(I)	intensity decay: none

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.077	H-atom parameters constrained
wR(F^2) = 0.217	w = 1/[σ^2(Fo^2) + (0.1463P)^2 + 0.8656P] where P = (Fo^2 + 2Fc^2)/3
S = 1.05	(Δ/σ)max = 0.003
2201 reflections	Δρmax = 0.54 e Å^−3
143 parameters	Δρmin = −0.84 e Å^−3
0 restraints	Extinction correction: SHELXTL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.011 (2)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Al	0.44345 (12)	0.55944 (5)	0.07415 (7)	0.0324 (4)
O	0.4687 (3)	0.44128 (12)	0.05642 (16)	0.0322 (5)
C1	0.6373 (5)	0.6039 (2)	0.2005 (3)	0.0477 (9)
H1A	0.6204	0.6667	0.2075	0.072*
H1B	0.7639	0.5923	0.1875	0.072*
H1C	0.6252	0.5750	0.2701	0.072*
C2	0.1802 (5)	0.5975 (2)	0.0545 (3)	0.0480 (8)
H2A	0.1353	0.5831	0.1221	0.072*
H2B	0.1003	0.5680	−0.0109	0.072*
H2C	0.1727	0.6605	0.0423	0.072*
C3	0.3788 (4)	0.36716 (19)	0.0958 (3)	0.0352 (7)
H3	0.2566	0.3560	0.0393	0.042*
C4	0.3290 (5)	0.3876 (2)	0.2072 (3)	0.0497 (9)
H4A	0.4458	0.4000	0.2644	0.075*
H4B	0.2643	0.3375	0.2311	0.075*
H4C	0.2448	0.4383	0.1981	0.075*
C5	0.5027 (4)	0.28651 (18)	0.0991 (2)	0.0334 (7)
C6	0.6914 (4)	0.28329 (19)	0.1616 (2)	0.0364 (7)
C7	0.8002 (4)	0.2092 (2)	0.1550 (3)	0.0386 (7)
H7	0.9283	0.2072	0.1974	0.046*
C8	0.7267 (5)	0.13858 (19)	0.0884 (3)	0.0381 (7)
C9	0.5395 (5)	0.14193 (19)	0.0308 (3)	0.0384 (7)
H9	0.4863	0.0933	−0.0135	0.046*
C10	0.4243 (4)	0.21397 (19)	0.0350 (2)	0.0356 (7)
C11	0.7867 (5)	0.3558 (2)	0.2392 (3)	0.0491 (9)
H11A	0.7576	0.3490	0.3133	0.074*
H11B	0.7390	0.4121	0.2070	0.074*
H11C	0.9247	0.3530	0.2474	0.074*
C12	0.8498 (6)	0.0601 (2)	0.0815 (3)	0.0514 (9)
H12A	0.9802	0.0792	0.0840	0.077*
H12B	0.7989	0.0291	0.0107	0.077*
H12C	0.8500	0.0213	0.1452	0.077*
C13	0.2174 (5)	0.2098 (2)	−0.0277 (3)	0.0444 (8)
H13A	0.1361	0.2198	0.0251	0.067*
H13B	0.1899	0.1523	−0.0624	0.067*
H13C	0.1921	0.2545	−0.0865	0.067*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Al	0.0411 (7)	0.0270 (5)	0.0326 (6)	0.0017 (3)	0.0156 (4)	−0.0041 (3)
O	0.0418 (12)	0.0272 (10)	0.0307 (10)	0.0014 (8)	0.0145 (9)	−0.0001 (7)
C1	0.060 (2)	0.0455 (19)	0.0394 (17)	−0.0086 (15)	0.0158 (15)	−0.0112 (14)
C2	0.054 (2)	0.0391 (17)	0.057 (2)	0.0090 (15)	0.0259 (16)	0.0015 (15)
C3	0.0392 (17)	0.0300 (14)	0.0407 (16)	0.0009 (12)	0.0178 (13)	0.0018 (11)
C4	0.065 (2)	0.0439 (19)	0.0506 (19)	0.0099 (16)	0.0354 (17)	0.0080 (15)
C5	0.0413 (17)	0.0258 (14)	0.0359 (14)	0.0030 (12)	0.0144 (12)	0.0048 (11)
C6	0.0440 (18)	0.0336 (15)	0.0338 (14)	−0.0013 (13)	0.0137 (12)	0.0035 (11)
C7	0.0403 (17)	0.0358 (16)	0.0401 (15)	0.0049 (13)	0.0098 (13)	0.0058 (12)
C8	0.0494 (19)	0.0299 (15)	0.0392 (16)	0.0060 (13)	0.0183 (13)	0.0074 (12)
C9	0.0492 (19)	0.0291 (15)	0.0406 (16)	−0.0009 (12)	0.0182 (14)	0.0010 (12)
C10	0.0420 (18)	0.0322 (15)	0.0353 (15)	−0.0012 (12)	0.0142 (12)	0.0025 (12)
C11	0.051 (2)	0.0446 (19)	0.0481 (19)	0.0016 (15)	0.0040 (15)	−0.0079 (14)
C12	0.058 (2)	0.0350 (17)	0.065 (2)	0.0098 (14)	0.0218 (18)	0.0042 (15)
C13	0.0428 (19)	0.0386 (17)	0.0508 (19)	−0.0020 (14)	0.0087 (15)	−0.0006 (14)

Geometric parameters (Å, º)

Al—O	1.844 (2)	C5—C10	1.404 (4)
Al—O^i	1.848 (2)	C6—C7	1.397 (4)
Al—C1	1.950 (3)	C6—C11	1.520 (4)
Al—C2	1.950 (4)	C7—C8	1.386 (4)
O—C3	1.448 (3)	C7—H7	0.950
O—Al^i	1.848 (2)	C8—C9	1.372 (5)
C1—H1A	0.980	C8—C12	1.512 (4)
C1—H1B	0.980	C9—C10	1.394 (4)
C1—H1C	0.980	C9—H9	0.950
C2—H2A	0.980	C10—C13	1.514 (4)
C2—H2B	0.980	C11—H11A	0.980
C2—H2C	0.980	C11—H11B	0.980
C3—C4	1.518 (4)	C11—H11C	0.980
C3—C5	1.525 (4)	C12—H12A	0.980
C3—H3	1.000	C12—H12B	0.980
C4—H4A	0.980	C12—H12C	0.980
C4—H4B	0.980	C13—H13A	0.980
C4—H4C	0.980	C13—H13B	0.980
C5—C6	1.399 (4)	C13—H13C	0.980
O—Al—O^i	80.20 (9)	C10—C5—C3	118.3 (3)
O—Al—C1	111.46 (13)	C7—C6—C5	119.2 (3)
O^i—Al—C1	110.88 (13)	C7—C6—C11	117.1 (3)
O—Al—C2	113.67 (13)	C5—C6—C11	123.8 (3)
O^i—Al—C2	114.63 (13)	C8—C7—C6	122.0 (3)
C1—Al—C2	119.49 (16)	C8—C7—H7	119.0
C3—O—Al	132.26 (17)	C6—C7—H7	119.0
C3—O—Al^i	123.53 (16)	C9—C8—C7	117.9 (3)
Al—O—Al^i	99.80 (9)	C9—C8—C12	121.6 (3)
Al—C1—H1A	109.5	C7—C8—C12	120.6 (3)
Al—C1—H1B	109.5	C8—C9—C10	122.5 (3)
H1A—C1—H1B	109.5	C8—C9—H9	118.8
Al—C1—H1C	109.5	C10—C9—H9	118.8
H1A—C1—H1C	109.5	C9—C10—C5	119.0 (3)
H1B—C1—H1C	109.5	C9—C10—C13	118.4 (3)
Al—C2—H2A	109.5	C5—C10—C13	122.6 (3)
Al—C2—H2B	109.5	C6—C11—H11A	109.5
H2A—C2—H2B	109.5	C6—C11—H11B	109.5
Al—C2—H2C	109.5	H11A—C11—H11B	109.5
H2A—C2—H2C	109.5	C6—C11—H11C	109.5
H2B—C2—H2C	109.5	H11A—C11—H11C	109.5
O—C3—C4	110.7 (2)	H11B—C11—H11C	109.5
O—C3—C5	110.1 (2)	C8—C12—H12A	109.5
C4—C3—C5	113.9 (2)	C8—C12—H12B	109.5
O—C3—H3	107.3	H12A—C12—H12B	109.5
C4—C3—H3	107.3	C8—C12—H12C	109.5
C5—C3—H3	107.3	H12A—C12—H12C	109.5
C3—C4—H4A	109.5	H12B—C12—H12C	109.5
C3—C4—H4B	109.5	C10—C13—H13A	109.5
H4A—C4—H4B	109.5	C10—C13—H13B	109.5
C3—C4—H4C	109.5	H13A—C13—H13B	109.5
H4A—C4—H4C	109.5	C10—C13—H13C	109.5
H4B—C4—H4C	109.5	H13A—C13—H13C	109.5
C6—C5—C10	119.4 (3)	H13B—C13—H13C	109.5
C6—C5—C3	122.3 (3)
O^i—Al—O—C3	−156.1 (3)	C3—C5—C6—C7	−176.0 (3)
C1—Al—O—C3	95.2 (3)	C10—C5—C6—C11	−176.2 (3)
C2—Al—O—C3	−43.4 (3)	C3—C5—C6—C11	5.1 (4)
O^i—Al—O—Al^i	0.0	C5—C6—C7—C8	−0.1 (4)
C1—Al—O—Al^i	−108.71 (14)	C11—C6—C7—C8	178.9 (3)
C2—Al—O—Al^i	112.71 (15)	C6—C7—C8—C9	−2.1 (4)
Al—O—C3—C4	−30.5 (4)	C6—C7—C8—C12	178.8 (3)
Al^i—O—C3—C4	178.1 (2)	C7—C8—C9—C10	1.7 (4)
Al—O—C3—C5	−157.29 (19)	C12—C8—C9—C10	−179.2 (3)
Al^i—O—C3—C5	51.3 (3)	C8—C9—C10—C5	0.9 (4)
O—C3—C5—C6	57.3 (3)	C8—C9—C10—C13	−177.5 (3)
C4—C3—C5—C6	−67.7 (4)	C6—C5—C10—C9	−3.1 (4)
O—C3—C5—C10	−121.4 (3)	C3—C5—C10—C9	175.6 (2)
C4—C3—C5—C10	113.6 (3)	C6—C5—C10—C13	175.3 (3)
C10—C5—C6—C7	2.7 (4)	C3—C5—C10—C13	−6.0 (4)

Symmetry code: (i) −x+1, −y+1, −z.