Stabilities and Structures of Halogenated Dialanes - American

American Research Institute, Suite I21 2, Upper Darby, Pennsylvania I9082 ... Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma...
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J. Phys. Chem. 1993,97, 2546-2554

Stabilities and Structures of Halogenated Dialanes Charles W. Bock' Chemistry Department, Philadelphia College of Textiles and Science, Philadelphia, Pennsylvania 191 44, and American Research Institute, Suite I21 2, Upper Darby, Pennsylvania I9082

Mendel Trachtman' Chemistry Department, Philadelphia College of Textiles and Science, Philadelphia, Pennsylvania I91 44

Gilbert J. Mains' Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received: August 5, I992

The a b initio structures and energies of all the isomers of A12H,,Xa-,, ( n = 0-6, X = F, C1) have been calculated a t the HF/6-31G* level. Correlation has been included a t the MP2/6-3lG*//HF/6-3lG* level in all cases and a t the MP4SDTQ/6-31G*//RHF/6-31G* and MP2/6-31G*//MP2/6-31G* levels in a few cases. Ab initio structures and energies are also presented for selected chlorofluorodialanes, A12H,C1,Fp (n m p = 6). Dissociation energies, including zero-point corrections, are presented for all molecules a t the MP2/63 lG*//RHF/6-3 l G * level. Acid-base interaction parameters deduced from thecomputed dissociation energies of 28 molecules are shown to be sufficient to predict the computed dissociation energies of 32 other chlorodialanes, fluorodialanes, and selected chlorofluorodialanes with an root mean square error of 1.2 kcal/mol. The applicability of these Lewis acidfbase interaction parameters in assigining Lewis acid strengths and Lewis base strengths is discussed, and the utility of describing the bonding in terms of three-center two-electron bonds is considered.

+ +

Introduction Aluminum hydrides and halides play important roles in a wide range of chemical processes. Aluminum chloride, for example, is used extensively as an alkylation and acylation catalyst in numerous Friedel-Crafts reactions and finds wide use as a Lewis acid polymerization catalyst.' Aluminum trifluoride is added to many glasses to improve durability and is an important component of some optical fibem2 These compounds also have favorable electrochemical properties and, combined with quaternary salts, form ionic liquids at room temperature. It is widely recognized that these electron-deficient molecules exist as monomers only at high temperatures or in unreactive matrices and that the dominant species in the gas phase are dimers and higher polymers. A12H6,3A12F6,4and Al$&,4.5have been studied theoretically and havebeenshown toexist asdiborane-typedimers in thegasphasea6 On the other hand partially halogenated alanes, Le., A12H,F6_, and A12HnC16-,,, ( n = 1-5), are apparently unknown. These compounds, whose preparation will undoubtedly present a formidable challenge to experimentalists,are extremely interesting in that they can give additional insight into the double dative bonds, Le., the two-electron three-center bonds, which bind such dimers. They also serve to highlight the differences between first- and second-row compounds of the group-13 elements, boron and aluminum. For example, although borane is certainly dimeric, boron trichloride and boron trifluoride are monomeric; in contrast, the analogous aluminum compounds are solid polymers at room temperature and are dominantly dimeric in the vapor phase. We report here the first theoretical studies of all of the isomers of A12HnF6-,,, A12HnC16-, ( n = 1-5), and the chlorofluorodialanes A12H4ClF, A12H3C12F, A12H3CIF2, and A12H2C12F2 in order to estimate the strengths of the Al(p-H)AI, Al(p-Cl)Al, and Al(p-F)Al bridge bonds.

Computational Methods Ab initio calculations were performed by using the GAUSSIAN 90 package of programs' on the CRAY Y-MP/832 computer at 0022-3654/93/2097-2546$04.00/0

the Pittsburgh Supercomputing Center, on IRIS 4D/35 and VAX 8250 computers located in Philadelphia, on the CRAY-2 and the RS/6000 cluster at the National Center For Supercomputing Applications (NCSA) in Champaign-Urbana, and the IBM 3090 in Stillwater. Gradient optimizations were employed throughout by using the 6-3 1G* basisset, included in thesepackages. Electron correlation was included by performing single-point calculations at the MP2/6-31G*//RHF/6-31G* level in all cases and up to theMP4(SDTQ)/6-31G*//RHF/6-3 1G*levels inisolatedcases. Vibrational frequencies were obtained from analytical second derivativescalculatedat theRHF/6-31GZ//RHF/6-3lG* level. Ina few instances, MP2=FULL/6-31G* optimizations werealso performed to assess the geometrical effects of including electron correlation or p-functions on the hydrogen atoms.

Results and Discussion

Halogenated Alanes. The structures of AlH,Clr,, and AIHnFh, = 0-3, have been discussed previously9 and are summarized here at the RHF/6-31G*//RHF/6-31G* level, augmented by MP2/6-31G*//RHF/6-31G*, MP3/6-31GS//RHF/6-31G*, MP2/6-3lG*//MP2/6-31G*, and MP2/6-31G**//MP2/63 1G** calculations in some instances as specifically noted. The symmetries are D3h for n = 3,O and C2r,for n = 1,2 as expected. Structures for AIF2CI and AICI?F,both exhibiting C2"symmetry, are also presented. The AI-H bond distance in AlH3 is 1.584 A at the RHF/6-31G*//RHF/6-31G* level and 1.577 A at the MP2/6-3 1G*//MP2/6-3 lG* level. For AlH2Cl the computed AI-H and A1-CI bond lengths are 1.57 1 and 2.1 1 1 A, respectively, and the Cl-AI-H bond angle is 117.5O. Inclusion of correlation at the MP2/6-31G* level for AlH2Cl increases the AI-H and decreases the AI-Cl bond lengths to 1.579 and 2.097 A, respectively, while the H-Al-Cl bond angle remains essentially unchanged, 117.6'. For A1HCl2, the AI-H and AI-Cl bond lengths are 1.559 and 2.092 A, respectively, and the H-A141 bond angle is 120.9O. For AlCL,, we find the AI-C1 bond distance to be 2.077 A, and correlation at the MP2/6-31G* level reduces this slightly to 2.069 A. For AIH2F, the AI-H and AI-F bond n

0 1993 American Chemical Society

The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 2541

Stabilities and Structures of Halogenated Dialanes

TABLE I: Monomers of Aluminum Hydrides and Halides ~~

~~

absolute energies, au, level molecule AlClH2 AIC12H AIFH2 AIHF2 AIC13 AlFi AlClFH AlCIlF AICIF2

RHF/6-3lG*//RHF/6-3lG* -702.607 -1161.594 -342.563 -441.510 -1620.576 -540.450 -801.552 -1260.535 -900.493

MP2/6-31G*//RHF/6-3IGZ

091

-702.785 318

119 328 192 087 452 568 351 476

-1 161.898 935

-342.789 849 -441.910 106 -1621,007 982 -541.022 398 -801.905 023 -1261.014 224 -901.018 974

lengths are 1.574 and 1.640 A, respectively, and the F-AI-H bond angle is 117.6'. Inclusion of correlation at the MP2/63 1G* level for AlH2F increases the AI-H and AI-F bond lengths to 1.580 and 1.663A, respectively, and reduces the H-AI-F bond angle to 117.4'. For AlF2H, the AI-H and A1-F bond lengths are 1.559 and 1.630 A, respectively, and the H-AI-F bond angle is 121.4'. For AlHFCl the AI-H, AI-F, and AI-Cl bond lengths are 1.559,1.630, and 2.092 A, respectively, and the H-AI-F and F-A1-Cl bond angles are 120.7' and 117.5', respectively. For AlF3, we find the AI-F bond length to be 1.620 at the RHF/ 6-31G* level. Correlation effects at the MP2/6-31G* level increase the AI-F bond length to 1.645 A, which is somewhat longer than the JANAF value, 1.63 f 0.01 A.lo An MP2/63 1lG(2d) optimization of AlF3 decreases the AI-F distance to 1.635 A and an MP2/6-31 lG(2df) optimization decreases it further to 1.634 A, bringing the computed AI-F bond lengths more into line with the expected value. Interestingly, the f functions play a negligible role in the length of the AI-F bond, whereas the second set of &functions appears to be rather significant. Thus, it seems likely that the RHF/6-31G* AI-F bond lengths are a lower bound and the MP2/6-3 lG* AI-F bond lengths are an upper bound to the correct values. For AlF2C1, the A1-CI and AI-F bond lengths are 2.072 and 1.621 A, respectively, and the Cl-AI-F bond angle is 120.6' at the HF/ 6-31G* levelcomparedwith2.06A, 1.63A,and 120°,respectively, reported in the JANAF table.l0 For AlC12F, the AI-F and AlC1 bond lengths are 1.623 and 2.074 A, respectively, and the F-AI-Cl bond angle is 119.4' at the HF/6-31GS level compared with 1.63 A, 2.06 A, and 120°, respectively, reported in the JANAF tableelo The relevant energies are provided in Table I as a convenience for the reader. MP4(SDTQ)/6-3 lG*//RHF/ 6-3 1G* energies are available el~ewhere.~ Halogenated Dialanes. Stable forms of A12HnX6-n(n = 0-6, X = C1, F) were found with the expected diborane-type structure, the symmetries of which depend upon the number and locations of the halogens in the isomers. Rather than present separate figures for all the chlorinated and fluorinated conformers, we present generic figures (see Figure 1-6) for the various halogenated structures where X is either chlorine or fluorine. Structural details for each compound are included in the captions. All dissociation energies reported include zero-point energy corrections. AlzXL( a h ) . The structures for A12X6are shown in Figure 1 and the energies are given in Tables I1 and 111. For A12C16at the RHF/6-3 lG*//RHF/6-3 lG* level, we find Al-Cl, (terminal) bond distances to be 2.083 A compared with AI-Clb (bridge) bond distances of 2.288 A. The JANAF tableslo use 2.065 f 0.002 A for AI-Cl, and 2.252 f 0.004 A for AI-Clb. The C1,AI-Cl, and Clb-Al-Clb bond angles are 119.2' and 89.2', respectively, compared with 123.4 f 1.6' and 91.0 f 0.5' in the JANAF tables. The agreement is satisfactory considering the limited basis set employed and somewhat better than those obtained by using the STO-3G basis set.4 The energy required to dissociate A12C16into two AlC13 molecules is 27.2 kcal/mol at the MP2/6-3 1G * / / H F/6- 3 1G * level, in reasonable agreement with the 30.4 kcal/mol calculated from the JANAF tables and

MP3/6-31G*//RHF/6-3IG1

MP4(SDQ)/6-31GS//RHF/6-31G*

-702.806 600 -342.795 301 -1621.044 278

-1621.045 919

Figure 1. A12X6. MP2/6-31G*//MP2/6-3IGS values are given in brackets, otherwise the structural data are at the HF/6-31GS level. Electron pair bonds are shown as heavy black lines. X = CI: bond lengths, A, AI-CI,= 2.083, 2.288; bond angles, deg, Clb-AI-Clb = 89.2, CI,-AI-CI, = 121.7; dihedral angles, deg, Clb-AI-AI-CI, = 90.0. X = F bondlengths,A,AI-F, = 1.621 [1.645],AI-Fb= 1.795 [1.186];bond angles, deg, Fb-AI-Fb = 80.0 [81.2], F,-AI-F, = 123.4 [123.3]; dihedral angles, deg, Fb-AI-AI-F, = 90.0 [90.0].

a significant improvement on the value computed by using the STO-3G basis set, 46.7 kcal/mol.4 At the MP4SDQ/6-31G*/ /RHF/6-31G* level, the dissociation energy is reduced to 25.6 kcal/mol. Note that the calculated dissociation energy of A12CI6 is greater than the energy released upon formation of an adduct9 between H F and AlC13, 18.9 kcal/mol, and, thus, H F would not be expected to depolymerize A12C16. For A12F6at the RHF/6-31G* level, the AI-F, (terminal) bond length is 1.621 A, compared with A1-S (bridge) bond distancesof 1.795 A, in relatively good agreement with the values used in the JANAF tables,I0 1.63 A for AI-F, and 1.80 A for AI-& At the MP2/6-31G*//MP2/6-31G* level, these bond lengths change to 1.645 and 1.8 16 A, respectively. The computed F,-AI-F, bond angle is 123.4' at the RHF/6-31GS//RHF/631G* and 123.3' at the MP2/6-31G*//MP2/6-3lG* level compared with 120.0' assumed in the JANAF tables. The FbAl-Fb bond angle is 80.0', compared with an assumed 90.0' in the JANAF tables and 80.0' reported by Curtiss at the STO-3G leveL4 The energy required to dissociate A12F6into two AlF3 molecules is 61.0 kcal/mol at the MP2/6-31G*//RHF/6-31G* level and62.3 kcal/molat theMP2/6-31G*//MP2/6-31G* level, in fair agreement with the 50.9 kcal/mol calculated in the JANAF tables from mass spectrometric data and a significantimprovement on the value computed by using the STO-3G basis set, 112.9 kcal/m01.~ It should be pointed out that, due to the high temperatures involved, structural parameters and dissociation energies are not as reliable for A12F6as for A12C16. Since the energy released when AlF3 forms an adduct with H F is only 23.6 kcal/m01,~ H F is not expected to depolymerize A12F6. The differences in the dissociationenergies for A12C16and A12F6reflect the stronger Lewis basicity of fluorine and the stronger Lewis acidity of A1F3,found earlier for H F and HCl adduct^.^ In both A12C16 and A12F6the two-electron three-center bond is longer than the two-electron two-center bond, as expected. Al2HX5. The structures of Al2HXs are shown in Figure 2 and the energies are given in Tables I1 and 111. Replacing one of the halogen atoms with a hydrogen atom produces two isomers, depending upon whether a terminal or bridge halogen atom is involved. For A12HClS,the double chlorine bridged isomer is 1.7 kcal/mol more stable than the chlorine and hydrogen bridged

2548 The Journal of Physical Chemistry, Vol. 97, No. I I, 1993

Bock et al.

TABLE 11: Dimers of Aluminum Chloride and Aluminum Chlorohydrides molecule

Figure

CI~AI(CI~)AICl2 CIHAI(C12)AlCI? C12AI(CIH)AIC12 CIHAI(C12)AIHCI HCIAI(C12)AIHCl H2AI(C12)AIC12 C12AI(HCI)AICIH CI?AI(H2)AIC12 H2AI(C12)AIHCI HCIAI(CIH)AICIH HCIAI(C1H)AIHCl C12AI(CIH)AIH2 HCIAI(H?)AIC12 H2Al(CIH)AIHCI H*AI(CI2)AIH2 CIHA1( H2)AIHCI HCIAl(H2)AIHCI H2AI(H2)AIC12 H2AI(CIH)AIH? H?AI(H2)AlCIH

1

2a 2b 3a 3b 3c 3d 3e 4a 4b 4c 4d 4e 5b 5a 5d 5e 5c 6a 6b

RH F/6-3 I G* / /RH F/6-3 1G* -3241.181 -2782.199 -2782,196 -2323.218 -2323.217 -2323.216 -2323.215 -2323.209 -1864.233 -1864.232 -1864.232 -1864.231 -1864.228 -1405.247 -1405.247 -1405.246 -1405.245 -1405.244 -946.261 -946.261

absolute energies, au, level MP2/6-3 1G* / /RH F/6-3 I G*

228 997 884 093 850 128 073 988 201 719 004 106 505 400 377 082 390 763 280 147

-3242.060 592 -2782.950 808 -2782.948 068 -2323.840 538 -2323.840 478 -2323.839 278 -2323.838 038 -2323.831 129 -1864.728 356 -1864.727 876 -1864,727 297 -1864.726 980 -I 864.722 406 -1405.616 024 -1405,615 447 -1405.613 019 -1405.612 558 -1405.613 130 -946.503 783 -946.502 766

MP2/6-3 I G*/ /MP2/6-3 1G*

-1405.655 580

-946.532 727

AE~~I?~ kcal/mol

0.0 1.7 0.0 0.0 0.8 1.6 5.9 0.0 0.3 0.7 0.9 3.7 0.0 0.4 1.9 2.2 1.9 0.0 0.6

' MP2/6-3 lG*//HF/6-3IG* TABLE 111: Dimers of Aluminum Fluoride and Aluminum Fluorohydrides molecule F?AI(F?)AIF2 F?AI(F2)AIFH F2AI( FH)AIF2 FHAI(F2)AIHF FHAI( F2)AIFH H?AI(F?)AIF2 FHAI(FH)AIF? F?AI(H?)AIF2 H2AI( F2)AIFH FHA]( FH)AIHF HFAI(FH)AIHF F?AI(FH)AIH2 FHAI( H?)AIF2 H?AI(F?)AIH? H?AI(FH)AIFH HzAI(H?)AIF? FHAI( H2)AlHF HFAI(H2)AIHF H2AI( FH)AIH2 FHAI(H2)AlHl

Figure 1

2a 2b 3a 3b 3c 3d 3e 4a 4b 4c 4d 4e 5a 5b 5c 5d 5e 6a 6b

RHF/6-31G*//RHF/6-31G* -1080,994 -982.050 -982.020 -883.105 -883.104 -883.102 -883.075 -883.046 -784.156 -784.130 -784.130 -784.129 -784.104 -685.206 -685.183 -685.163 -685.161 -685.161 -586.235 -586.218

absolute energies, au, level MP2/6-31G*//RHF/6-31G*

033 208 295 605 997 31 1 975 840 499 815 125 945 985 944 563 155 722 475 954 887

-1082.144 -983.028 -983.001 -883.910 -883.910 -883.906 -883.883 -883.855 -784.788 -784.765 -784.764 -784.763 -784.740 -685.665 -685.644 -685.625 -685.623 -685.623 -586.523 -586.508

337 014 102 902 468 735 472 165 565 175 569 8 13 185 748 526 777 736 700 633 297

MP2/6-31G*//MP2/6-31G*

AE~~I," kcal/mol

-1082.183 674 -983.035 628

-685.691 523 -685.649 961 -586.545 280 -586.529 310

0.0 16.9 0.0 0.3 2.6 17.2 35.0 0.0 14.7 15.1 15.1 30.4 0.0 13.3 25.1 26.4 26.4 0.0 9.6

' MP2/6-3 lG*//HF/6-3 1G*. isomer. There are small changes in the geometries of C12AI(CI2)AICl2 and C12AI(C12)AIHC1 (C,) when matching bond lengths and bond angles are compared as indicated in the captions for Figures 1 and 2a. The dissociation energy of C12AI(C12)AlHCl into AIC13 and AlHC12 is 26.5 kcal/mol, at the MP2/ 6-3 lG//HF/6-3 1G* level, which is not significantly different from that of A12C16. The ring structural parameters for C12A1(HCI)A1Cl2 (C2?) are substantially changed compared to those in CllAlC12AlHCl as a consequence of the smaller diameter of the hydrogen atom (compare captions to Figure 2), but the dissociation energy is only 2.6 kcal/mol lower, reflecting the difference in stability. Considering now A12HF5, the isomer in Figure 2a is 16.9 kcal/ mol more stable than the isomer in Figure 2b. There are small differences in the structures of F?AI(F>)AIF2and F?AI(F2)AIFH (see captions for Figures 1 and 2a). However, as in the case of the analogous chlorine compounds, there are significant changes in the ring structure when F2AI(F2)AIFH and F2A1(HF)A1F2 (CZ,.)are compared. The dissociation energy of F2AI(F*)AlFH into AIF, and AlHF2 is 58.1 kcal/mol, at the MP2/6-31G// HF/6-31G* level, which is nearly the same as the dissociation energy of F2A1(F2)AIF2,whereas that for F2AI(HF)AIF2 is reduced to40.6 kcal/mol, at thesamecomputational level. While

Figure 2. AIzHXs MP2/6-31G*//MP2/6-31G* values are given in brackets, otherwise the structural data are at the HF/6-31G* level. Electron pair bondsareshown as heavy black lines. (a) HXAI(X2)AIX2. X = CI: bond lengths, A, AI-HI = 1.555, AI-CIII = 2.094, AI-CI12 = 2.088, AI-CIt, = 2.086, AI-Clb = 2.279; bond angles, deg, Clb-AI-Clb = 89.9, CkAI-CIt = 121.0, Clb-AIrClb = 88.0; dihedral angles, deg, Clb-AI-AI-Cb = 90.0. X = F: bond lengths, A, AI-H, = 1.555, AI-F,, = 1.630, AI-F,? = 1.624, AI-F,, = 1.622, AI-Fb = 1.785, bond angles, deg, Fb-AI-Fb = 80.7, FI-AI-FI = 122.5, Fb-AI$-Fb = 78.8, Fl-AI-HI = 125.1; dihedral angles, deg, Fb-AI-AI-F, = 90.0. (b) X?AI(HX)AIX:. x = CI: bond lengths, A, AI-CI, = 2.082, AI-Hb = 1.714, AI-Clb = 2.285; bondangles.deg,Clb-AI-Hb = 85.3,Cll-AI-CIl = 123.0;dihedral angles, deg, Clb-AI-AI-CI, = 96.6. X = F: bond lengths, A, AI-F, = 1.623 [1.648], AI-& = 1.731 L1.724). AI-Fb = 1.790 (1.813]; bond angles,deg, Fb-AI-Hbrs8I.3 [82.5], F,-AI-F, = 122.7 [122.6];dihedral angles, deg, Hb-AI-AI-F, = 90.9 [91.3].

there is only a 2.5 kcal/mol difference in energy between a C1 and H bridge, the energy difference between a F and H bridge is some seven times as much, 17.5 kcal/mol.

Stabilities and Structures of Halogenated Dialanes

pg

Figure 3. A12H2X4. The structural data are at the HF/6-31G* level. Electron pair bonds are shown as heavy black lines. (a) XHAI(X2)AIHX. X = CI: bond lengths, A, AI-H, = 1.556, AI-CIt = 2.100, = 2.308; bondangles,deg,Clb-AI-clb = 88.7, H,-AI-CI, = 123.5;dihedral angles, deg, Clb-AI-AI-CI, = 90.0. X = F. Bond lengths, A, AI-HI = 1.556,AI-Fl =1.633,AI-Fb= 1.810; bondangles,deg, Fb-AI-Fb= 79.5, Fl-AI-H, = 124.2;dihedral angles,deg, F,-AI-AI-F, = 90.0. (b) HXAI(X2)AIHX. X = CI: bond lengths, A, AI-H, = 1.557, AI-CI, = 2.097, AI-Clb = 2.308; bondangles,deg, Clb-AI-Clb = 88.7, H,-AI-CI, = 123.5; dihedral angles, deg, Clb-A1-AI-CI, = 90.0. X = F: bond lengths, A, AI-H, = 1.558, A M , = 1.631, AI-&, = 1.810; bond angles, deg, FbAI-Fb = 79.4, F,-AI-F, = 124.0; dihedral angles, deg, Fb-AI-AI-F, = 90.0. (c) X>AI(X2)AIH2. X = CI: bond lengths, A, AI-HI = 1.564, = 2.269; bond angles, deg, Clb-Al-Clb = 87.0, AI-CI, = 2.091, Clb-Als-Clb = 90.9, CI,-AI-CI, = 120.2; dihedral angles, deg, Clb-AIAI-CI, = 90.0. X = F: bond lengths, A, AI-H, = 1.567, AI-F, = 1.625, AI-Fb = 1.845; bond angles, deg, Fb-AI-Fb = 77.9, Fb-AI5-Fb = 8 1.5, F,-AI-F, = 121.8, Ht-AI-Ht = 129.0; dihedral angles, deg, H,-AIAI-Fb = 90.0. (d) X2AI(HX)AIHX. X = CI: bond lengths, A, AI-H, = 1.557, AI-CI,] = 2.095, AI-CIi2 = 2.089, AI-CII3 = 2.085, A12-Hb = 1.738, A12-Clb = 2.317, Als-Hb = 1.701, A15-Clb = 2.274; bond angles, deg, CIb-AI2-Hb = 84.1, CI,-AI-CI, = 122.1, Clb-Als-Hb = 86.2, c l b AI-H, = 125.2; dihedral angles, deg, Hl-AI-A1-Clb = 95.1. X = F: bond lengths, A, AI-H, = 1.558, AI-F,, = 1.632, AI-F,> = 1.627, AI-FI3 = 1.626, Al2-Hb = 1.776, A12-Fb = 1.810, = 1.787; bond angles, deg, Fb-AI-Hb = 79.8, FI-AI-FI = 121.7, Fb-Als-Hb = 88.2, F,-AI-H, = 124.5; dihedral angles, deg, Hb-AI-AI-H, = 88.7. (e) X2AI(H2)AIX2. X = c1: bond lengths, A, AI-CI, = 2.083, AI-& = 1.733; bond angles, = 82.9, CI,-AI-CI, = 124.0; dihedral angles, deg, Hbdeg, &-AI-& AI-AI-CI, = 90.0. X = F: bond lengths, A, AI-F, = 1.626, AI-& = 1.73I ; bond angles, deg, Hb-AI-Hb = 84.1, F,-AI-F, = 122.7; dihedral angles, deg, Hb-AI-AI-F, = 90.0. A12HZX4. The structures of A12H2X4are shown in Figure 3 and the energies are given in Tables I1 and 111. Replacing another terminal halogen in Figure 2a gives rise to the three isomers shown in parts a-c of Figure 3. We shall refer to the geometry that places both substituents on the same A1 atom as the gem structure. Replacing a terminal halogen on Figure 2b gives rise to the structure in Figure 3d. Replacing the bridging halogen in Figure 2b gives Figure 3e. For A12H2C14, Figure 3a,b, the trans and cis isomers have nearly the same energy. Furthermore, placing both hydrogen atoms on the same aluminum atom, i.e., the gem structure, increases the energy a scant 0.8 kcal/mol at the MP2/6-31GS//RHF/6-3 lG* level, probably within the accuracy of the calculations. Similarly, the replacement of one of the CI bridges with an H bridge raises the energy only 1.6 kcal/mol, approximately the same as when this was done for A12HCl5, Le., Figure 2. However, replacing both bridging chlorine atoms with bridging hydrogen atoms raises the energy by 5.9 kcal/mol. As can be seen by comparing the geometrical parameters in the figure captions, isomers that preserve the bridge atoms are only slightly changed upon interchange of an H for CI in the terminal positions. The dissociation energies for the trans and cis isomers of HCIAI(C12)AIHCI into two AlHC12 are 25.6 and25.4kcal/mol,at the MP2/6-31G*//HF/6-31G*level. The dissociation of H2AI(C12)AlC12 into AICI, and AlH2Cl is 27.4 kcal/mol, at the same level. CI2AI(CIH)AIC1H can dissociate intoA1C13and AlHZCI, which requires 25.7 kcal/mol, or into two

The Journal of Physical Chemistry, Vol. 97, No. 1 1 , 1993 2549 AlHC12, which requires 23.2 kcal/mol, the latter being the energetically preferred channel. The dissociation of C12AI(H2)AlCl2 into two AlHCl2 requires only 18.2 kcal/mol, at the same level. Turning now to the fluorinated isomers we see that the trans isomer, Figure 3a, is only 0.3 kcal/mol more stable than the cis isomer, Figure 3b, hardly a significant difference. However, moving both hydrogen atoms to the same aluminum atom, giving the gem structure in Figure 3c, raises the energy by 2.6 kcal/mol at the MP2/6-31G*//HF/6-3 1G* level. The largest energy changes come about when the hydrogens are located in the bridge positions, parts d and e of Figure 3, in which the energy is raised 17.2 and 35.0 kcal/mol, respectively, at the MP2/6-31G*//HF/ 6-31G* level. The dissociation energies of trans and cis HFAl(F2)AIFH into two AlFH2 are 54.9 and 54.5 kcal/mol, respectively. For the gem structure, dissociating into AlF3 and A1FH2 requires 57.2 kcal/mol at the same level. The energetically preferred channel for the dissociation of F2AI(FH)AIFH is into two AlHF2 molecules, which requires 37.1 kcal/mol, whereas the dissociation into A1F3 and AlH2F requires 41.9 kcal/mol. The dissociation energy of FzAl(H*)AlF2 into two A1HF2 molecules is only 18.8 kcal/mol, at the comparable level. A12H3X3. The structures of A12H3X3 are shown in Figure 4 and the energies are given in Tables I1 and 111. Note that there is less than a 1 kcal/mol difference in the energies of H2A1(C12)AlHCI and the cis/trans/gem isomers with the single Cl bridge. This is a somewhat surprising result since replacement of a double chlorine bridge with a chlorine and hydrogen bridge changed the energies about 1.7 kcal/mol in both A12HC15 and A12H2C14. Indeed, replacing the double chlorine bridge with a double hydrogen bridge raised the energy only 3.7 kcal/mol as compared to 5.9 kcal/mol for AlzH2CI4. The reasons for these small differences are not obvious, however, there must be compensating factors that cannot be exclusively electrostatic. The dissociation energies of H2AI(C12)AlHCl and the trans/ &/gem isomers of A12H3C13 into AlHCl2 and AlH2C1 are 26.0, 24.9,24.5,and 24.5 kcal/mol,at the MP2/6-31G*//HF/6-31G* level, respectively, reflecting the small differences in stability. Of course the gem isomer can also dissociate into AIC13 and AlH3, but this requires a somewhat higher energy, 28.1 kcal/mol. Turning now to the A12H3F3 isomers, we find 15 kcal/mol difference between the double fluorine bridge isomer, Figure 4a, and the three fluorine and hydrogen bridged isomers, Figure 4bd, consistent with the -17 kcal/mol difference found for the AlzHF5 and AlzHzF4isomers. The arrangement of the remaining two hydrogen atoms and two fluorine atoms in the terminal position has less than a 0.4 kcal/mol effect on the relative energies. One might have expected a larger energy differences on the basis of simple electrostatics. The dissociation energy for HFAI(F2)AlH2 into AlHF2 and AlH2F is 53.2 kcal/mol at the MP2/631G*//HF/6-31G* level, which is similar to that found for breaking a double F bridge in the preceding isomers. The trans/ cis/gem isomer dissociation into AlHF2and AIH2Frequires 38.0, 37.6, and 37.4 kcal/mol at the same level, not too different from the 40-45 kcal/mol required to break the same bridge in the preceding isomers. Breaking the double hydrogen bridge in F2Al(H2)AlHFrequires 21.2 kcal/mol at the MP2/6-31G*//HF/ 6-31G* level, somewhat larger than the 18.8 kcal/mol for F>Al(H2)AlF2. A12H4X2. The structures of A12H4X2are shown in Figure 5 and the energies are given in Tables I1 and 111. The isomer distribution is the same as that for AlzHzX4, with the roles of H and X interchanged. For AlzH4C12, an interesting reversal takes place in that the chlorine and hydrogen bridge isomer is lower in energy than the double chlorine bridge isomer, but by only 0.4 kcal/molat theMP2/6-31G*//HF/6-31G* leve1,and thedouble hydrogen bridged isomers are only -2 kcal/mol higher in energy and separated from each other by only 0.3 kcal/mol. The latter

-

2550 The Journal of Physical Chemistry, Vol. 97, No. 11, 1993

Halogenated Dialanes Figure 4. AIzH3X3. The structural data are at the HF/6-31G* level. Electron pair bonds are shown as heavy black lines. (a) H2AI(X2)AIHX. X = CI: bond lengths, A, AI-Htl = 1.558, AI-H,2 = 1.566, AI-H, = 1.565, AI-CI, = 2.104, = 2.298; bond angles, deg, Clb-AI2-Clb = 89.7, CIb-AI5-Clb 87.7, Ht-AI-H, = 129.2, HII-AI-CII = 122.5; dihedral angles, deg, Clb-AI-AI-Ch = 90.0. X = F bond lengths, A, AI-H,, = 1.559, AI-Ht2 = 1.570, AI-H,, = 1.568, AI-FI = 1.634, AI-Fb = 1.801; bond angles, deg, = 80.2, Fb-AI5-Fb = 78.5, HtAI-HI = 127.8, H,-AI-F, = 123.3; dihedral angles, deg, Fb-AI-AI-F, = 90.0. (b) XHAI(HX)AIHX. X = CI: bond lengths, A, AI-H, = 1.558,AI-C1, = 2.102,AI-Hb= 1.723,Al-Clb =2.304;bondangles,deg, Clb-AI-Hb = 85.0, H,-AI-CIt = 124.2; dihedral angles, deg, CII-AIAI-Clb = 89.9. X = F: bond lengths, A, AI-HI = 1.560, AI-F, = 1.636, AI-& = 1.749, A1-h = 1.806; bond angles, deg, Fb-AI-Hb = 80.7, H,-AI-F, = 123S;dihedralangles,de F,-AI-AI-Fb = 89.3. (c) HXAI(HX)AIHX. X = CI: bond lengths, AI-H, = 1.560, AI-CI, = 2.099, AI-& = 1.724, Al-Clb = 2.304; bond angles, deg, Clb-AI-Hb = 84.9, H,-AI-CI, = 124.1; dihedral angles, deg, CII-A1-AI-Clb = 83.3. X = F bond lengths, A, AI-H, = 1.562, AI-F, = 1.634, AI-Hb = 1.749, AI-Fb = 1.806; bond angles, deg, Fb-AI-Ht. = 80.6, H,-A1-F1 = 123.4; dihedral angles, de , FI-AI-AI-Fb = 89.7. (d) X2AI(HX)AlH2. X = CI: bond lengths, AI-H, = 1.566, AI-C1, = 2.093, A12-Hb = 1.691, AIS-Hb = 1.758, A12-Clb = 2.262, A15-Clb = 2.352; bond angles, deg, CIb-AI-Hb = 87.4, Clb-A15 FzAI(HF)AIF* H 2A1( H F) AIH F H2AI(HF)AIH2 HFAI(HF)AIHF H*AI(HF)AIHCI HFAI(HF)AIHF F2AI(HF)AIH2 H2AI(H F)AlH F H FAI( H F)AI F2 HlAI( HF)AIHCI H2AI(HCI)AIHF H2AI(HCI)AIHCI H2AI(HCI)AIHCI C12AI(HCI)AIHCI C12AI(HCI)AIHz HCIAI(HCI)AIHCI C12AI( HCI)AIC12 CI*AI(HC1)AIHCI

interactions

eq 1

abinitio

56. I 58.0 53.1 26.6 25.8 26.4 26.5 26.0 23.7 23.8 23.5 21.5 23.7 20.7 41.9 37.7 37.7 35.0 36.1 35.0 39.8 35.0 36.9 36.4 26.6 24.9 24.6 25.3 24.4 22.1 25.1 21.9 1.23

51.2 58.1 53.2 21.4 26.0 26.5 26.8 26.6 23.7 25.2 24.5 21.2 23.7 20.8 40.6 37.7 36.9 38.0 36.1 37.6 37.4 37.4 37.1 36.2 27.1 26.8 25.7 25.7 24.5 24.5 23.9 23.2

base atom in B-H, and/or the Lewis acid, AlXYZ, can more readily accept the electrons in thedimer than in the X Y Z A b - E H adduct. Since the double Lewis acid-base interaction brings the two molecules into closer proximity, both explanations are reasonable. With use of BI, Le., F, as the Lewis base atom, the differences in interaction energies between A,, Az, A3, and As are probably ascribable to electrostatic effects and not to differences in the Lewis acid strength. Similarly, the interaction energies of Aq, As, Ab, and A7 with B, are not sufficiently different to permit assigning an order to their Lewis acidity. Perusal of Table VI with respect to Bl and BJ interacting with the various acids leads to the same conclusion; Le., the dimerization reaction cannot be used to predict Lewis acidity without careful consideration of the dipole4ipole interaction energy. A comparison of the interactions of various Lewis acids with halide ions, e.g., F-, C1-, etc., might be useful to assign relative Lewis acid strengths but care must be taken to account for polarization, i.e., induced dipole effects, and preliminary calculations suggest that these moieties react with the aluminum acids rather than form adducts. Only interactions of Lewis acids with atomic Lewis bases, say a inert gas atom, can avoid the complications arising from permanent dipole4ipole interactions and chemical reaction. Finally, one must question the utility of the three-center twoelectron bond in describing these interactions. Multicenter bonds are well-entrenched in the literature, especially boron literature, where the concept does have much utility. However, in the case of diborane and for the halogenated dialanes described here it is equally convenient to portray the dimeric interaction in terms of dative bonds. It was recognized 15 years ago1I that for symmetric molecules the overlap populations between the A1 atoms and the bridging atoms are identical and significantly smaller than the overlap populations between the A1 atoms and the terminal atoms. In the case of asymmetric molecules, the overlap populations between the bridging atom and the A1 atoms are not the same and the geometry is skewed. Thus, one is forced to take into

2554

The Journal of Physical Chemistry, Vol. 97, No. I I, I993

account asymmetry of the three-center two-electron bond about the bridge atom. It seems to us that this is an unnecessary complication that can more easily be explained in terms of the relative Lewis acid-base strengths of the interacting partners. The early concept of a donor-acceptor (dative) bond serves equally well for these electron-deficient dimers and, in view of its success in predicting dissociation energies of both symmetric and asymmetric molecules by decomposing them into double donoracceptor interactions, is probably more realistic.

Acknowledgment. M.T. acknowledges support from the Pittsburgh Supercomputer Center, Grant CHRSSJP, for computer time on the CRAY Y-MP/832. G.J.M. thanks the USAFOSR for an RIPGrant, which supported this research, and the National Center for Supercomputing Applications for Grant CHE890003N, for computer time on the CRAY-2 and RS/6000 cluster.

Bock et al. (3) (a) McKee, M. J. Phys. Chem. 1991,95,6519. (b) Lamertsma, K.; Leszcynski, J. J. Phys. Chem. 1990,94,2806. (c) Baird, N. C. Can. J . Chem. 1985,63,7 I . (d) Mains,G. J.; Bock,C.; Trachtman, M.; Finley, J.; McNamara, K.; Fisher, M.; Wociki, L.J. Phys. Chem. 1990, 94, 6996. (4) Curtiss, L. A. Int. J. Quantum Chem. 1978, 14, 709. ( 5 ) Beattie, I. R.; Blayden, H. E.;Hall, S. M.; Jenny, S.N.; Ogden, J. S. J. Chem. Soc., Dalton Trans. 1976, 666. (6) (a) Cotton, F. A.; Wilkinson, G. Aduancedfnorganic Chemistry. 5th ed.; Wiley-Interscience: New York, 1988. (b) See alsochapters by: Haaland, A.; Hargittai, M. In Sterochemical Applications of Gas-Phase Electron Dijfraction; Hargittai, I., Hargittai, M., Eds.; VCH Publishers, Inc.: New York, 1988. (7) Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foresman, J. B.; Schlegel, H. B.; Raghavachari, K.;Robb, M. A.; Binkley, J. S.;Gonzalez, C.; Defrees, Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S.; Pople, J. A. Guussiun 90; Gaussian, Inc.: Pittsburgh, PA, 1990. (8) Moller, C.; Plesset, M. S.Phys. Reu. 1934, 46, 678. (9) Wilson, M.; Coolidge, M. B.; Mains, G.J. J. Phys. Chem. 1992,96,

References and Notes

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( I ) Olah, G. A. Friedel-Crafts and Related Reactions; Interscience Publishing: New York, 1963-1965; Vols I-IV. (2) (a) Pureza, P. C.; Brower, D. T.: Aggarawal, I. D. J. Am. Ceram. SOC.1989, 72, 1980. (b) Yasui, 1.; Hagihara, H.; Inoue, H. J . Non-Cryst. Solids 1992, 140, 130.

(IO) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N.J. Phys. Chem. Ref. Data 1985,14; Suppl I , JANAF tables. (11) Lappert, M. F.; Pedley, J. B.; Sharp, G.J. J. Chem. SOC.,Trans. Faraday Sor. 2 1916, 539.