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Arsenic-boron derivatives—III. A spectroscopic investigation of methylarsine adducts of boron trihalides
Spedrochimica Acre. Vol. 4lA. No. 10. pp I197-1203, 1985. Printed m Great Bntain.
0
0584-8539/8513.00 + 0.00 1985Pergamon Press Ltd.
Arsenic-boron derivatives-III. A spectroscopic investigation of methylarsine adducts of boron trihalides* JAAFAR M. CHEHAYBERand JOHN E. DRAKE
Department of Chemistry, University of Windsor, Windsor, Ontario, Canada N9B 3P4 (Received 31 January 1985; infinalform
17 April 1985)
Abstract-Methylarsine adducts of the type CHsAsHsBXs where X = Cl, Br, I were prepared and characterized by i.r. and Raman spectroscopy. The fundamental vibrations were assigned on the basis of C, molecular symmetry. A modified Urey-Bradley force field was utilized to calculate frequencies, potential energy distributions and force constants
INTRODUCTION Numerous investigations of amine adducts of boron trihalides and borane have been reported [l-7]. The phosphorus analogs have drawn even more attention. The vibrational spectra and normal coordinate analysis have been reported for Me2PHBH3 [8], Me2PHBX3 [9] and Me3PBXJ [lo]. In addition, the ‘H and “B NMR spectra of several alkyl and aryl phosphine adducts [ 11, 121 and the X-ray structures of Me,PBX, [13] have been reported. Similar species with an As-B bond have received relatively less attention. This has been attributed to the relative instability of the As-B bond compared to P-B [ 14,151. Among studies reported are the microwave study of MeJAsBH, [ 161, the preparation of Me3AsBCI, [ 17, 181, the ‘Hand ilB NMR spectraofMe,AsBX, where X = Br and I, the vibrational spectra and normal. coordinate analysis of Me,AsBX, [19, 201 and most recently the X-ray structures of Me,AsBX, [21] where X = Cl, Brand I. No study has yet been centered on the spectroscopic investigation of the methylarsine adducts of boron trihahdes.
nates. The Cartesian coordinates were calculated using a Euclidean construction devised by HILDERBRANDT[~~]. All three adducts are white moisture-sensitive solids with melting points of 46, 68 and 78°C for the chloride, bromide and iodide compounds, respectively. RESULTS AND DISCUSSION The eleven-atom molecules MeAsH2BX3 were assumed to have a staggered configuration with C, point group symmetry. The vibrational representation is TV = 16a’ + 1 la” with a11modes active in both the i.r. and Raman effects. The in-plane (a’) and out-of-plane (a”) modes should give rise to polarized and depolarized Raman bands, respectively. The description of the normal modes and the numbering of the frequencies are shown in Table 1 and selected regions of the i.r. and Raman spectra in Figs l-4. Tables 24 contain the full spectroscopic information on each adduct along with calculated frequencies and potential energy distributions.
EXPERIMENTAL The preparation of methylarsine, CHsAsHs, was carried out by the reduction of sodium methylarsenate with zinc dust and HCI 1221. Its purity was checked by its i.r. and Raman spectra[23]. The boron trihalides were purified and checked for purity as previously reported[24]. The CH,AsHrBX, adducts were prepared under vacuum line conditions by direct combination [lo]. Raman spectra were recorded on solid samples on a Spectra-Physics Model 700 spectrometer in conjunction with a Model 164 argon-ion laser and Model 265 exciter unit using the 488 nm argon line. Polarization data were obtained using CHJ and CH,CI, solutions of the adducts. Infrared succtra were recordedon-a Perkin-Elmer 180 spectrometer as CsI pellets for the mid i.r. and as Nujol mulls between polyethylene plates for the far i.r. region. The normal coordinate analysis was performed using a computer program (Larmol) [25] in which the normal coordinates are expressed as mass-weighted Cartesian coordi*For parts I and II see Refs [ 191 and [21], respectively. 1197
Table 1. The approximate description and symmetry species of the fundamental vibrations of MeAsH,BX, (X = Cl, Br, I) a’
d’
“1 v2
v17 “1s
“3 “4
“19
v5 V6 v20
V-i “21
“s “9
“10 VI1 “12
“13 “14 “15
“16
“2.2 “23
Description CHs stretch (a) CHs stretch (s) ASH, stretch (a) AsH2 stretch (s) CHs def. (a) CHs def. (s) ASH* scissors CHs rock (a) CH, rock (s) ASH, twist ASH, wag As-B stretch C-As stretch ASH, rock BX, stretch (a) BXJ stretch (s) C-As-B def. BX, def. (a) BX, def. Is) BX; rock‘ CHs torsion BX, torsion
JMFAR M. CHEHAYBER and JOHN E. DRAKE
1198
(cm-’ ) 250 I
200 I
150 I
100 I
I
Fig. 1. Far i.r. spectra of MeAsH2BC13 (A), MeAsH,BBr, (B), and MeAsH*BIs (C).
1 2300
I I 2130 750
I 550
1 350
1 80
Fig. 2. !&h&d regions of the solid Raman spectrum of MeAsH2BCl,.
Vibrational assignments The methyl group stretching modes occur in the same region as in the free donor[23], with two asymmetric CHS stretches (vi, vi,) appearing in the Raman spectra in the range 3020-3000 cm-‘. The symmetric CHs stretch (vs) is assigned to an intense band in the Raman spectra at 2936, 2921 and 293Ocm-i in MeAsHsBCIJ, MeAsHsBBrs and MeAsHsBIs, respectively. The CHs symmetric (vs)
and asymmetric (v+ vis) deformation modes are assigned to relatively weak bands in the ranges 1230-1260 and 1400-1430 cm-‘, respectively. These vibrations appear to be shifted slightly to lower frequencies relative to the free base and have been assigned in the same region as in related molecules [ 19, 20,23,27,28]. The two CHs rocking modes (v,, vsO) are considered degenerate although they are split into a’ and a” modes. The region 930-950 cm- ’ is assigned
1199
Methylarsine adducts of boron trihalides
I 2300
1
I
1
2130 750
550
I
I
350
80
Fig. 3. Selected regions of the solid Raman spectrum of MeAsH2BBr,.
I 2300
I
I
2130 750
,
550
350
1 80
Fig. 4. Selected regions of the solid Raman spectrum of MeAsH2B13.
to these vibrations and there is no particular trend along the adduct series although these modes seem to be shifted slightly to higher frequency upon formation of the adducts. The various ASH, fundamental vibrations in the adducts were assigned by comparison with the corresponding modes in free MeAsHz. The symmetric (vg) and asymmetric (v,s) AsHz stretching modes are readily distinguished in the Raman spectra. They occur at significantly higher wavenumber values than in the free base, and this behavior was also observed for the PH3 stretching modes in the PHJBXJ [24] adducts. This effect is consistent with an increase in the ASH bond strength attributable to an increase in the scharacter contribution as the environment around arsenic approaches a tetrahedron upon adduct formaSA(A)41:1O-F
tion [15]. The definite trend toward lower AsHz stretching frequencies as the acceptor ability of the Lewis acid increases may suggest an increase in the drift of charge away from the ASH bonds without a significant change in the hybridization on arsenic [l 1, 241. The AsHz scissoring mode (v6) appears as a medium weak band in the i.r. spectrum at ca 1050-l 100 cm-’ which is higher than the corresponding motion in the free base. The AsHz rocking frequency (vz2) appears as a weak vibration in the 400 cm- ’ region in both effects, lower than the corresponding mode in the free base by ca 250 cm- ‘. The very weak and difficult to assign bands in the 850-900cm-1 region have been associated with the twisting (vzl) and wagging (vs) motions of AsHa. The CAs stretching frequency (vr,,) appears as a strong
JAAFARM. CHEHAYBER and JOHN E. DRAKE
1200
Table 2. Observed i.r. and Raman frequencies (cm - I, f 3 cm-i) for MeAsH BCl, with potential energy distributions Assianment
i.r. CsI*
Raman (solid)
Raman (soln.)?
CalC.
P.E.D.S
VI,
v5
2982 m 2920 s 2257 w 2233 w 1415 m 124Ow
3018 w 2936 m 2254 m 2235 vs 1410 w 1242 w
2935 VW 2250 m, dp 2228 w, p Solvent -
3018 2936 2254 2235 1409 1246
v6
1080 s
1086 VW
lOOf(CHa)- lJ(CH/CH) 98f(CH2) + 2f(CH/CH) 99f(AsH2) + If ((AsH/AsH) lOOf(AsH’)- lf(AsH/AsH) 93f(HCH2) + 6f(HCAs2) 54f(HCH2) + 45f(HCAs2) - 19f(HCAs/HCAs) 84f(HAsH2) + 23f(HAsC2) + lOf(HCAs/HCAs) 59f(HCAs2) + 23f(HAsC2) + lOf((HCAs/HCAs) 22f(HCAs’) + 50f(HAsC2) + 2Of (HAsB2) 20f(HCAs’) + 55f(HAsC2) + 17f(HAsB2) 75 K(BX) + 9H(XBX) 47K(AsB)+25K(BX)+ lOH(XBX) + 8F(XX) 89f(CAs2) 74f(HAsB2) + 16f(HAsC2) + 7 K(BX) 27K(BX)+48F(XX)+lZK(AsB) + 9F(AsX) 6OF(XX)+23H(XBX)+6F(AsX) 20f((CAsB2)+19F(XX)+28F(AsX) + 12H(AsBX) 12F(XX) + 36F(AsX) -t 20f(CAsB2) -f lSH(AsBX) 66F(AsX) + 28H(AsBX)
v17
v2 V18 V3 v4,
VI9
-
1082
942 VW
950 s
957 VW,dp
952
v21
896 VW
907w
893 VW, dp
882
VS
872 w
885 VW
887 VW,p
871
v11, v23
705 mw 653 s
690~ 646s
639 br, p
690 647
6ooms 420 m 376 w
604s 418 w 373 vs
590 VW,p 418 w, dp 372 s, p
604 418 368
V13
255 mw 200 br
255 m 200s
255 m, dp 195 sh, p
253 203
v15
172~
174m
170m, p
170
13ow
142 w
138 VW,p?
134
v7,
v20
v9
VlO v22 v12
h4r
v24
vl6r
v25
-
*Low frequencies taken from far i.r. spectra in Nujol. t Spectrum in CH2Cl, and MeI. *Contribution greeter than 10% except for significant interactions. Table 3. Observed i.r. and Raman frequencies (cm-‘, -&3 cm-‘) for MeAsH BBr, with potential energy distributions Assignment
i.r. CsI*
Raman (solid)
Raman (soln.)t
VI> v17 VZ VIS V3 v5
3025 w 2930 w 2240 ms 2218 ms 1400s 1245 m
3020 w 2930 m 2240 mw 2217 s 1407 w 1258 VW
Solvent 2935 w, p 2240 m, dp 2217 mw, p Solvent -
v6
1050 mw
1055 VW
v21
940 m 880 ms
945 m 890 VW
947 w, dp 892 VW,dp
VS
850 w
847 VW
844 VW,p
v9
649s
644W
64Om, p
VI Ir v23
620 s 595 s 410w
618 m 600mw 408~
620 w, dp 6OOw, P 410 VW,dp
238 s 19Om
235 vs 190w
238 vs, p 187 VW,p
v24
VI5
155m 137w
150ms 132 wsh
150 m, dp? 130sh, p
v16~ v25
102m
102 w
100 sh, dp
v4,
“19
b
“20
VI0 v22
v12 v13
VI&
Calc.
P.E.D.$
3020 100f(CH2) - lf(CH/CH) 2930 98f(CH2) + lf(CH/CH) 2240 99f(AsH2) + lf(AsH/AsH) 2217 100f(AsH2) - Ij(AsH/AsH) 1400 92f(HCH2) + 6f(HCAs2) 1246 53f(HCHz) + 57f(HCAs2) - 18f(HCAs/HCAs) 1049 84f(HAsH’) + 7f(HAsC2) + 9f(HAsB2) 947 66f(HCAs’) + 13f(HAsC’) 866 13f(HCAs’) + 57f(HAsC2) + 24f(HAsB2) 853 12f(HCAs’) + 61f(HAsC2) + 22f(HAsB2) 638 58K(AsB)+ 16K(BX)+8H(AsBX) + 6H(XBX) 617 63K(BX) + 8H (AsBX) + 7H (XBX) 594 82f(CAs’) 410 67f(HAsBZ) + 17f(HAsC’) + 13K(BX) 244 33K(BX)+46F(XX)+ lOF(AsX) 188 44f(CAsB2)+ 16F(XX)+ lZK(BX) + 8F (ASX) 158 69F(XX) + 16H(XBX) + 7K(BX) 134 25F(XX)+23F(AsX)+ 14/(CAsB2) + lOH(AsBX)+ llH(XBX) 100 44F(AsX) + 42H (AsBX)
*Low frequencies taken from far i.r. spectra in Nujol. tSpectrum in CH2C12 and MeI. $Contribution greater than 10% except for significant interactions.
Methylarsine adducts of boron trihalides Table 4. Observed Assienment
“16, “25
’ , + 3 cm-‘) for MeAsH
i.r. and Raman frequencies
(cm-
ir. CsI*
Raman
Raman
3018 m 2923 s 2197 m 2182s 1400s 1260~
3010 2921 2207 2183
IlOOm
1201
BI, with potential
energy distributions
Calc.
P.E.D.$
1265 w
3018 2923 2207 2183 1398 1263
1090 mw
1098
930 sh
935 VW
933
890 s
892 VW
877 br,
850 wsh
860 w
870 br, p
866
640 mw 600m 545 m 405 s
640w 595 w 537 w 405 mw
637 w, p 598 w, dp 545 w, dp 400 VW, dp
645 597 539 401
192~
189ms
187 mw, p
193
174w 124m 1lOwsh
175s 120 sh 109s
170ms, p 125 mw, dp 110 wsh, p
173 124 115
85 ms
89 wsh, dp
86
lOOf(CH’) - 1fU-I/CW 99_i(CH2) + l/(CH/CH) 99f(AsH’) + 1 f (ASH/ASH) lO$(AsH’) -if(AsH/AsH) 931(HCH*) + 6f(HCAs2) 52j”(HCH’) + 54f(HCAs’) - 13J(HCAs/HCAs) 85,f(HAsH’) + 6f(HAsC2) + Sf(HAsB’) 49f(HCAs2) + 35f(HAsC’) + 7f(HAsB’) 4Of(HCAs’) + 35f(HAsC’) + 15f (HAsB*) 34f(HCAs2) + 14f(HAsC*) + l?f(HAsB’j 67K (AsB) + 9K (BX) + 7H (AsBX) 86j-(CAs2) 57K (BX) + 13H (AsBX) + 7H (XBX) 52f(HAsB’)+ 13f(HAsC’) ’ + 25;u‘(BX) 32f(CAsB2) + 25F(XX) + 19K (BX) + 9F (AsX) 27K (BX) + 37F(XX) +25f(CAsB’) 74F (XX) + 9K (BX) + 11 H (XBX) 43F(XXj+ 14F‘(AsX)+ llfi(AsBX) + 12H(XBX) 33F(AsX) + 49H(AsBX)
83 w
(solid) w m m s
1405W
(soln.)t
Solvent 2920 w, p 2213 w, dp 2187 VW, p Solvent
dp
*Low frequencies taken from far i.r. spectra in Nujol. t Spectrum in CH&l, and MeI. $Contribution greater than lo?<, except for significant
polarized band in the Raman spectra at ca 600 cm- ‘, compared with 563cm-’ in MeAsH [23]. This vibration has been assigned at 610 cm- ’ in MeJAsBH, [29], and 575618 cm-’ in MeJAsBXJ [19,20]. Upon formation of the adducts, the strengthening of the CAs bond is consistent with the re-hybridization argument presented above for the ASH bonds. The CAsB bending mode (vrJ) is associated with medium to strong polarized bands at 200,190 and 189 cm- ’ for MeAsH,BCl,, MeAsHzBBr, and MeAsH2BIB, respectively. Extensive mixing of these modes with those of BX, is indicated in the potential energy distribution. The AsB stretching frequency (vg) varies slightly from 653 cm- ’ for MeAsH,BCl, to 640 cm-’ in MeAsH,BI,. It appears as a strong and polarized band in the Raman spectra. In Me,AsBX,, this vibration was initially assigned in the 650670 cm- 1region [ 191 and then subsequently in the 670-720cm-i region [20]. The assignment of the BX3 fundamentals was best achieved by comparison with studies undertaken for other BX3 compounds[9, 10, 19, 20, 24, 301. The polarized, intense bands at 373,238 and 175 cm-’ are assigned to the symmetric BXs stretch (viz) and are shifted to lower wavenumbers in comparison with those assigned at 471, 278 and 190cm-’ for planar BC13, BBr, and BIJ, respectively [31, 321. These shifts can be related to the rearrangement of the Lewis acid from the planar to the approximate tetrahedral con-
877
”
interactions.
figuration on coordination with the arsine [33]. The asymmetric BXs stretching frequencies (v,,, v& display descending wavenumbers at 690, 618 and 537 cm-’ compared to those of Me,AsBX, [19] at 700,615 and 548 cm- ‘. In free BXJ, these asymmetric modes were associated with bands at 956, 819 and 704cm-’ for BCl,, BBr, and B13, respectively [31, 321. The deformation modes (vr4, vz4, and vrs) naturally decrease in wavenumber along the series BCIJ > BBr, > B13as seen in the far i.r. spectra (Fig. 1) with the asymmetric and symmetric deformation modes of BCIB, BBr, and B13 adducts assigned at 225, 174; 155, 132; and 124, 110 cm- r, respectively. The remaining low frequency bands at 130, 102 and 83 cm- ’ are assigned to the BX3 rocking modes (vr6, v&. Normal coordinate analysis
The force constants for the adducts and for the free arsine are listed in Table 5. The calculated frequencies and the potential energy distributions are presented in Tables 2-4. The geometric parameters (listed in Table 6) were based initially on the parameters of MeAsH [23]. By the time this work on the vibrational analysis neared completion, the structures of the Me3AsBX, compounds had been solved[21]. Small adjustments were then made to the geometric parameters but these resulted in minimal changes in the calculations. Use was made of both the i.r. and Raman spectra to fit the calculated frequencies.
1202
JAAFARM. CHEHAYBER and JOHNE. DRAKE
Table 5. Modified Urey-Bradley force fields of MeAsH,BXs*t$ Force constant
x = Cl
X = Br
x=1
MeAsH, 5
f(CH*) f (HCH’)
4.850 0.495
4.848 0.488
4.836 0.488
4.870 0.529
$;;g$z’ ;;;;;;:;
2.930 0.515 0.866 0.674
0.519 2.880 0.797 0.628
0.504 2.800 0.850 0.693
0.430 2.611 0.646
0.403 0.597 2.810 1.437(1.79) 1.179(1.97) 0.338(0.64) 0.280(1.13) 0.535 0.190 0.030 0.033 0.082 -
0.426 0.705 2.812 1.680(1.92) 0.800(1.57) 0.405(0.62) 0.246(1.33) 0.550 0.120 0.020 0.034 0.063
$!;$; /K$<)) K (BX) H (AsBX) H (XBX) F(XX) F (AsX) /(CH/CH) f(AsH/AsH) f(HCAs/HCAs)
0.386 0.549 2.878 1.269(1.78) 1.460(2.32) 0.189(0.62) 0.368(1.07) 0.529 0.290 0.038 0.028 -0.085 -
2.466
*General valence force constants 6; Urey-Bradley force constants K (stretch), H (bend), F(repulsions). t Stretching constants in mdyn A - ’ (10’ Nm- i) and bending in mdyn A rade2 (1O’Nm rad-‘). *The corresponding F-matrix values are listed in brackets. $Values from Ref. [24].
Table 6. Geometric parameters of MeAsH,BXz Parameter
x = Cl
X = Br
x=1
CH CAs ASH AsB BX HCH HCAs HASH HAsC CAsB AsBX XBX
1.09 1.90 1.52 2.065 1.83 109.5 109.5 100.0 105.0 111.0 109.5 109.5
1.09 1.90 1.52 2.04 2.00 109.5 109.5 100.0 105.0 111.0 109.5 109.5
1.09 1.93 1.52 2.03 2.22 109.5 109.5 100.0 105.0 111.0 109.5 109.5
*Bond lengths in A and bond angles in degrees. Initially, a modified valence force field was used. This force field consists of the diagonal force constants and a small number of important interaction force constants. A fairly reasonable fit was obtained for frequencies belonging to the arsine donor. However, the results for the BXB end of the molecule were less satisfactory. For example, using acceptable values for the force constants, the best result using the MVFF fits all observed frequencies except the BCl, symmetric stretch. This calculates to ca 270cm-’ whereas the observed value is 373cm-’ and the band is so characteristic that there can be no doubt in the assignment. It is well established that when nonbonding interactions are significant, as in Ccl* and Ti&, the Urey-Bradley model is a better choice of force field [34, 353. Also, for the sake of comparison with other related molecules such as PH,BXB [24],
MezPHBXS [9], MesPBXs [lo] and MesAsBXs [19], a modified Urey-Bradley force field was adopted. The MeAsH end of the molecule was treated entirely as a valence force field which involved the diagonal force constants along with a few important interaction force constants such asf(CH/CH)andf(HCAs/HCAs). For the BXJ part of the molecule and the AsB bond, a Urey-Bradley force field was used by introducing two non-bonding repulsion force constants, the F(XX) between halogens and the F(AsX) between arsenic and halogens. The methyl group modes are well represented by f(CH’), j(HCH’), f(HCAs2), f(CH/CH) and f(HCAs/HCAs) force constants. The inclusion of f(HCAs/HCAs) has rendered the value off(HCAs’) artificially higher than that in free MeAsH [23] where the interaction force constant f(HCAs/HCAs) was not considered. The increase in f(AsH’), j(HAsH2) and f(CAs2) upon adduct formation is consistent with the increased s-orbital participation on rehybridization of the arsenic atom resulting in strengthening of the As-H and C-As bonds. The progressive increase inf(CAsB2) along the series Cl c Br < I has also been shown to occur for MeJAsBXS adducts. The Urey-Bradley K (AsB) exhibits a gradual increase along the sequence BCIB < BBr, < BIJ which agrees with the increase in acidity of the BXJ group along the same sequence. For this series MeAsH2BXJ, the average value of K (AsB) is 1.46, whereas for Me,AsBX, it is 1.67 [19]. These results are consistent with established orders of base strengths based on methyl group substitutions. The bending force constants H (AsBX) and H (XBX) and the repulsion force
Methylarsine adducts of boron trihalides
constants F(XX) and F(AsX) are close to those reported for related compounds indicating only a slight sensitivity of the BX3 group with respect to the donor molecule. The value of K (BX) decreases in the order BClj > BBr, > BIS as expected from the relative strengths of these bonds. The F-matrix elements are shown in brackets in Table 5 for the Urey-Bradley force constants for the sake of comparison with other studies. Acknowledgements-We wish to thank the Natural Sciences and Engineering Research Council for financial support.
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0584-8539/8513.00 + 0.00 1985Pergamon Press Ltd.
Arsenic-boron derivatives-III. A spectroscopic investigation of methylarsine adducts of boron trihalides* JAAFAR M. CHEHAYBERand JOHN E. DRAKE
Department of Chemistry, University of Windsor, Windsor, Ontario, Canada N9B 3P4 (Received 31 January 1985; infinalform
17 April 1985)
Abstract-Methylarsine adducts of the type CHsAsHsBXs where X = Cl, Br, I were prepared and characterized by i.r. and Raman spectroscopy. The fundamental vibrations were assigned on the basis of C, molecular symmetry. A modified Urey-Bradley force field was utilized to calculate frequencies, potential energy distributions and force constants
INTRODUCTION Numerous investigations of amine adducts of boron trihalides and borane have been reported [l-7]. The phosphorus analogs have drawn even more attention. The vibrational spectra and normal coordinate analysis have been reported for Me2PHBH3 [8], Me2PHBX3 [9] and Me3PBXJ [lo]. In addition, the ‘H and “B NMR spectra of several alkyl and aryl phosphine adducts [ 11, 121 and the X-ray structures of Me,PBX, [13] have been reported. Similar species with an As-B bond have received relatively less attention. This has been attributed to the relative instability of the As-B bond compared to P-B [ 14,151. Among studies reported are the microwave study of MeJAsBH, [ 161, the preparation of Me3AsBCI, [ 17, 181, the ‘Hand ilB NMR spectraofMe,AsBX, where X = Br and I, the vibrational spectra and normal. coordinate analysis of Me,AsBX, [19, 201 and most recently the X-ray structures of Me,AsBX, [21] where X = Cl, Brand I. No study has yet been centered on the spectroscopic investigation of the methylarsine adducts of boron trihahdes.
nates. The Cartesian coordinates were calculated using a Euclidean construction devised by HILDERBRANDT[~~]. All three adducts are white moisture-sensitive solids with melting points of 46, 68 and 78°C for the chloride, bromide and iodide compounds, respectively. RESULTS AND DISCUSSION The eleven-atom molecules MeAsH2BX3 were assumed to have a staggered configuration with C, point group symmetry. The vibrational representation is TV = 16a’ + 1 la” with a11modes active in both the i.r. and Raman effects. The in-plane (a’) and out-of-plane (a”) modes should give rise to polarized and depolarized Raman bands, respectively. The description of the normal modes and the numbering of the frequencies are shown in Table 1 and selected regions of the i.r. and Raman spectra in Figs l-4. Tables 24 contain the full spectroscopic information on each adduct along with calculated frequencies and potential energy distributions.
EXPERIMENTAL The preparation of methylarsine, CHsAsHs, was carried out by the reduction of sodium methylarsenate with zinc dust and HCI 1221. Its purity was checked by its i.r. and Raman spectra[23]. The boron trihalides were purified and checked for purity as previously reported[24]. The CH,AsHrBX, adducts were prepared under vacuum line conditions by direct combination [lo]. Raman spectra were recorded on solid samples on a Spectra-Physics Model 700 spectrometer in conjunction with a Model 164 argon-ion laser and Model 265 exciter unit using the 488 nm argon line. Polarization data were obtained using CHJ and CH,CI, solutions of the adducts. Infrared succtra were recordedon-a Perkin-Elmer 180 spectrometer as CsI pellets for the mid i.r. and as Nujol mulls between polyethylene plates for the far i.r. region. The normal coordinate analysis was performed using a computer program (Larmol) [25] in which the normal coordinates are expressed as mass-weighted Cartesian coordi*For parts I and II see Refs [ 191 and [21], respectively. 1197
Table 1. The approximate description and symmetry species of the fundamental vibrations of MeAsH,BX, (X = Cl, Br, I) a’
d’
“1 v2
v17 “1s
“3 “4
“19
v5 V6 v20
V-i “21
“s “9
“10 VI1 “12
“13 “14 “15
“16
“2.2 “23
Description CHs stretch (a) CHs stretch (s) ASH, stretch (a) AsH2 stretch (s) CHs def. (a) CHs def. (s) ASH* scissors CHs rock (a) CH, rock (s) ASH, twist ASH, wag As-B stretch C-As stretch ASH, rock BX, stretch (a) BXJ stretch (s) C-As-B def. BX, def. (a) BX, def. Is) BX; rock‘ CHs torsion BX, torsion
JMFAR M. CHEHAYBER and JOHN E. DRAKE
1198
(cm-’ ) 250 I
200 I
150 I
100 I
I
Fig. 1. Far i.r. spectra of MeAsH2BC13 (A), MeAsH,BBr, (B), and MeAsH*BIs (C).
1 2300
I I 2130 750
I 550
1 350
1 80
Fig. 2. !&h&d regions of the solid Raman spectrum of MeAsH2BCl,.
Vibrational assignments The methyl group stretching modes occur in the same region as in the free donor[23], with two asymmetric CHS stretches (vi, vi,) appearing in the Raman spectra in the range 3020-3000 cm-‘. The symmetric CHs stretch (vs) is assigned to an intense band in the Raman spectra at 2936, 2921 and 293Ocm-i in MeAsHsBCIJ, MeAsHsBBrs and MeAsHsBIs, respectively. The CHs symmetric (vs)
and asymmetric (v+ vis) deformation modes are assigned to relatively weak bands in the ranges 1230-1260 and 1400-1430 cm-‘, respectively. These vibrations appear to be shifted slightly to lower frequencies relative to the free base and have been assigned in the same region as in related molecules [ 19, 20,23,27,28]. The two CHs rocking modes (v,, vsO) are considered degenerate although they are split into a’ and a” modes. The region 930-950 cm- ’ is assigned
1199
Methylarsine adducts of boron trihalides
I 2300
1
I
1
2130 750
550
I
I
350
80
Fig. 3. Selected regions of the solid Raman spectrum of MeAsH2BBr,.
I 2300
I
I
2130 750
,
550
350
1 80
Fig. 4. Selected regions of the solid Raman spectrum of MeAsH2B13.
to these vibrations and there is no particular trend along the adduct series although these modes seem to be shifted slightly to higher frequency upon formation of the adducts. The various ASH, fundamental vibrations in the adducts were assigned by comparison with the corresponding modes in free MeAsHz. The symmetric (vg) and asymmetric (v,s) AsHz stretching modes are readily distinguished in the Raman spectra. They occur at significantly higher wavenumber values than in the free base, and this behavior was also observed for the PH3 stretching modes in the PHJBXJ [24] adducts. This effect is consistent with an increase in the ASH bond strength attributable to an increase in the scharacter contribution as the environment around arsenic approaches a tetrahedron upon adduct formaSA(A)41:1O-F
tion [15]. The definite trend toward lower AsHz stretching frequencies as the acceptor ability of the Lewis acid increases may suggest an increase in the drift of charge away from the ASH bonds without a significant change in the hybridization on arsenic [l 1, 241. The AsHz scissoring mode (v6) appears as a medium weak band in the i.r. spectrum at ca 1050-l 100 cm-’ which is higher than the corresponding motion in the free base. The AsHz rocking frequency (vz2) appears as a weak vibration in the 400 cm- ’ region in both effects, lower than the corresponding mode in the free base by ca 250 cm- ‘. The very weak and difficult to assign bands in the 850-900cm-1 region have been associated with the twisting (vzl) and wagging (vs) motions of AsHa. The CAs stretching frequency (vr,,) appears as a strong
JAAFARM. CHEHAYBER and JOHN E. DRAKE
1200
Table 2. Observed i.r. and Raman frequencies (cm - I, f 3 cm-i) for MeAsH BCl, with potential energy distributions Assianment
i.r. CsI*
Raman (solid)
Raman (soln.)?
CalC.
P.E.D.S
VI,
v5
2982 m 2920 s 2257 w 2233 w 1415 m 124Ow
3018 w 2936 m 2254 m 2235 vs 1410 w 1242 w
2935 VW 2250 m, dp 2228 w, p Solvent -
3018 2936 2254 2235 1409 1246
v6
1080 s
1086 VW
lOOf(CHa)- lJ(CH/CH) 98f(CH2) + 2f(CH/CH) 99f(AsH2) + If ((AsH/AsH) lOOf(AsH’)- lf(AsH/AsH) 93f(HCH2) + 6f(HCAs2) 54f(HCH2) + 45f(HCAs2) - 19f(HCAs/HCAs) 84f(HAsH2) + 23f(HAsC2) + lOf(HCAs/HCAs) 59f(HCAs2) + 23f(HAsC2) + lOf((HCAs/HCAs) 22f(HCAs’) + 50f(HAsC2) + 2Of (HAsB2) 20f(HCAs’) + 55f(HAsC2) + 17f(HAsB2) 75 K(BX) + 9H(XBX) 47K(AsB)+25K(BX)+ lOH(XBX) + 8F(XX) 89f(CAs2) 74f(HAsB2) + 16f(HAsC2) + 7 K(BX) 27K(BX)+48F(XX)+lZK(AsB) + 9F(AsX) 6OF(XX)+23H(XBX)+6F(AsX) 20f((CAsB2)+19F(XX)+28F(AsX) + 12H(AsBX) 12F(XX) + 36F(AsX) -t 20f(CAsB2) -f lSH(AsBX) 66F(AsX) + 28H(AsBX)
v17
v2 V18 V3 v4,
VI9
-
1082
942 VW
950 s
957 VW,dp
952
v21
896 VW
907w
893 VW, dp
882
VS
872 w
885 VW
887 VW,p
871
v11, v23
705 mw 653 s
690~ 646s
639 br, p
690 647
6ooms 420 m 376 w
604s 418 w 373 vs
590 VW,p 418 w, dp 372 s, p
604 418 368
V13
255 mw 200 br
255 m 200s
255 m, dp 195 sh, p
253 203
v15
172~
174m
170m, p
170
13ow
142 w
138 VW,p?
134
v7,
v20
v9
VlO v22 v12
h4r
v24
vl6r
v25
-
*Low frequencies taken from far i.r. spectra in Nujol. t Spectrum in CH2Cl, and MeI. *Contribution greeter than 10% except for significant interactions. Table 3. Observed i.r. and Raman frequencies (cm-‘, -&3 cm-‘) for MeAsH BBr, with potential energy distributions Assignment
i.r. CsI*
Raman (solid)
Raman (soln.)t
VI> v17 VZ VIS V3 v5
3025 w 2930 w 2240 ms 2218 ms 1400s 1245 m
3020 w 2930 m 2240 mw 2217 s 1407 w 1258 VW
Solvent 2935 w, p 2240 m, dp 2217 mw, p Solvent -
v6
1050 mw
1055 VW
v21
940 m 880 ms
945 m 890 VW
947 w, dp 892 VW,dp
VS
850 w
847 VW
844 VW,p
v9
649s
644W
64Om, p
VI Ir v23
620 s 595 s 410w
618 m 600mw 408~
620 w, dp 6OOw, P 410 VW,dp
238 s 19Om
235 vs 190w
238 vs, p 187 VW,p
v24
VI5
155m 137w
150ms 132 wsh
150 m, dp? 130sh, p
v16~ v25
102m
102 w
100 sh, dp
v4,
“19
b
“20
VI0 v22
v12 v13
VI&
Calc.
P.E.D.$
3020 100f(CH2) - lf(CH/CH) 2930 98f(CH2) + lf(CH/CH) 2240 99f(AsH2) + lf(AsH/AsH) 2217 100f(AsH2) - Ij(AsH/AsH) 1400 92f(HCH2) + 6f(HCAs2) 1246 53f(HCHz) + 57f(HCAs2) - 18f(HCAs/HCAs) 1049 84f(HAsH’) + 7f(HAsC2) + 9f(HAsB2) 947 66f(HCAs’) + 13f(HAsC’) 866 13f(HCAs’) + 57f(HAsC2) + 24f(HAsB2) 853 12f(HCAs’) + 61f(HAsC2) + 22f(HAsB2) 638 58K(AsB)+ 16K(BX)+8H(AsBX) + 6H(XBX) 617 63K(BX) + 8H (AsBX) + 7H (XBX) 594 82f(CAs’) 410 67f(HAsBZ) + 17f(HAsC’) + 13K(BX) 244 33K(BX)+46F(XX)+ lOF(AsX) 188 44f(CAsB2)+ 16F(XX)+ lZK(BX) + 8F (ASX) 158 69F(XX) + 16H(XBX) + 7K(BX) 134 25F(XX)+23F(AsX)+ 14/(CAsB2) + lOH(AsBX)+ llH(XBX) 100 44F(AsX) + 42H (AsBX)
*Low frequencies taken from far i.r. spectra in Nujol. tSpectrum in CH2C12 and MeI. $Contribution greater than 10% except for significant interactions.
Methylarsine adducts of boron trihalides Table 4. Observed Assienment
“16, “25
’ , + 3 cm-‘) for MeAsH
i.r. and Raman frequencies
(cm-
ir. CsI*
Raman
Raman
3018 m 2923 s 2197 m 2182s 1400s 1260~
3010 2921 2207 2183
IlOOm
1201
BI, with potential
energy distributions
Calc.
P.E.D.$
1265 w
3018 2923 2207 2183 1398 1263
1090 mw
1098
930 sh
935 VW
933
890 s
892 VW
877 br,
850 wsh
860 w
870 br, p
866
640 mw 600m 545 m 405 s
640w 595 w 537 w 405 mw
637 w, p 598 w, dp 545 w, dp 400 VW, dp
645 597 539 401
192~
189ms
187 mw, p
193
174w 124m 1lOwsh
175s 120 sh 109s
170ms, p 125 mw, dp 110 wsh, p
173 124 115
85 ms
89 wsh, dp
86
lOOf(CH’) - 1fU-I/CW 99_i(CH2) + l/(CH/CH) 99f(AsH’) + 1 f (ASH/ASH) lO$(AsH’) -if(AsH/AsH) 931(HCH*) + 6f(HCAs2) 52j”(HCH’) + 54f(HCAs’) - 13J(HCAs/HCAs) 85,f(HAsH’) + 6f(HAsC2) + Sf(HAsB’) 49f(HCAs2) + 35f(HAsC’) + 7f(HAsB’) 4Of(HCAs’) + 35f(HAsC’) + 15f (HAsB*) 34f(HCAs2) + 14f(HAsC*) + l?f(HAsB’j 67K (AsB) + 9K (BX) + 7H (AsBX) 86j-(CAs2) 57K (BX) + 13H (AsBX) + 7H (XBX) 52f(HAsB’)+ 13f(HAsC’) ’ + 25;u‘(BX) 32f(CAsB2) + 25F(XX) + 19K (BX) + 9F (AsX) 27K (BX) + 37F(XX) +25f(CAsB’) 74F (XX) + 9K (BX) + 11 H (XBX) 43F(XXj+ 14F‘(AsX)+ llfi(AsBX) + 12H(XBX) 33F(AsX) + 49H(AsBX)
83 w
(solid) w m m s
1405W
(soln.)t
Solvent 2920 w, p 2213 w, dp 2187 VW, p Solvent
dp
*Low frequencies taken from far i.r. spectra in Nujol. t Spectrum in CH&l, and MeI. $Contribution greater than lo?<, except for significant
polarized band in the Raman spectra at ca 600 cm- ‘, compared with 563cm-’ in MeAsH [23]. This vibration has been assigned at 610 cm- ’ in MeJAsBH, [29], and 575618 cm-’ in MeJAsBXJ [19,20]. Upon formation of the adducts, the strengthening of the CAs bond is consistent with the re-hybridization argument presented above for the ASH bonds. The CAsB bending mode (vrJ) is associated with medium to strong polarized bands at 200,190 and 189 cm- ’ for MeAsH,BCl,, MeAsHzBBr, and MeAsH2BIB, respectively. Extensive mixing of these modes with those of BX, is indicated in the potential energy distribution. The AsB stretching frequency (vg) varies slightly from 653 cm- ’ for MeAsH,BCl, to 640 cm-’ in MeAsH,BI,. It appears as a strong and polarized band in the Raman spectra. In Me,AsBX,, this vibration was initially assigned in the 650670 cm- 1region [ 191 and then subsequently in the 670-720cm-i region [20]. The assignment of the BX3 fundamentals was best achieved by comparison with studies undertaken for other BX3 compounds[9, 10, 19, 20, 24, 301. The polarized, intense bands at 373,238 and 175 cm-’ are assigned to the symmetric BXs stretch (viz) and are shifted to lower wavenumbers in comparison with those assigned at 471, 278 and 190cm-’ for planar BC13, BBr, and BIJ, respectively [31, 321. These shifts can be related to the rearrangement of the Lewis acid from the planar to the approximate tetrahedral con-
877
”
interactions.
figuration on coordination with the arsine [33]. The asymmetric BXs stretching frequencies (v,,, v& display descending wavenumbers at 690, 618 and 537 cm-’ compared to those of Me,AsBX, [19] at 700,615 and 548 cm- ‘. In free BXJ, these asymmetric modes were associated with bands at 956, 819 and 704cm-’ for BCl,, BBr, and B13, respectively [31, 321. The deformation modes (vr4, vz4, and vrs) naturally decrease in wavenumber along the series BCIJ > BBr, > B13as seen in the far i.r. spectra (Fig. 1) with the asymmetric and symmetric deformation modes of BCIB, BBr, and B13 adducts assigned at 225, 174; 155, 132; and 124, 110 cm- r, respectively. The remaining low frequency bands at 130, 102 and 83 cm- ’ are assigned to the BX3 rocking modes (vr6, v&. Normal coordinate analysis
The force constants for the adducts and for the free arsine are listed in Table 5. The calculated frequencies and the potential energy distributions are presented in Tables 2-4. The geometric parameters (listed in Table 6) were based initially on the parameters of MeAsH [23]. By the time this work on the vibrational analysis neared completion, the structures of the Me3AsBX, compounds had been solved[21]. Small adjustments were then made to the geometric parameters but these resulted in minimal changes in the calculations. Use was made of both the i.r. and Raman spectra to fit the calculated frequencies.
1202
JAAFARM. CHEHAYBER and JOHNE. DRAKE
Table 5. Modified Urey-Bradley force fields of MeAsH,BXs*t$ Force constant
x = Cl
X = Br
x=1
MeAsH, 5
f(CH*) f (HCH’)
4.850 0.495
4.848 0.488
4.836 0.488
4.870 0.529
$;;g$z’ ;;;;;;:;
2.930 0.515 0.866 0.674
0.519 2.880 0.797 0.628
0.504 2.800 0.850 0.693
0.430 2.611 0.646
0.403 0.597 2.810 1.437(1.79) 1.179(1.97) 0.338(0.64) 0.280(1.13) 0.535 0.190 0.030 0.033 0.082 -
0.426 0.705 2.812 1.680(1.92) 0.800(1.57) 0.405(0.62) 0.246(1.33) 0.550 0.120 0.020 0.034 0.063
$!;$; /K$<)) K (BX) H (AsBX) H (XBX) F(XX) F (AsX) /(CH/CH) f(AsH/AsH) f(HCAs/HCAs)
0.386 0.549 2.878 1.269(1.78) 1.460(2.32) 0.189(0.62) 0.368(1.07) 0.529 0.290 0.038 0.028 -0.085 -
2.466
*General valence force constants 6; Urey-Bradley force constants K (stretch), H (bend), F(repulsions). t Stretching constants in mdyn A - ’ (10’ Nm- i) and bending in mdyn A rade2 (1O’Nm rad-‘). *The corresponding F-matrix values are listed in brackets. $Values from Ref. [24].
Table 6. Geometric parameters of MeAsH,BXz Parameter
x = Cl
X = Br
x=1
CH CAs ASH AsB BX HCH HCAs HASH HAsC CAsB AsBX XBX
1.09 1.90 1.52 2.065 1.83 109.5 109.5 100.0 105.0 111.0 109.5 109.5
1.09 1.90 1.52 2.04 2.00 109.5 109.5 100.0 105.0 111.0 109.5 109.5
1.09 1.93 1.52 2.03 2.22 109.5 109.5 100.0 105.0 111.0 109.5 109.5
*Bond lengths in A and bond angles in degrees. Initially, a modified valence force field was used. This force field consists of the diagonal force constants and a small number of important interaction force constants. A fairly reasonable fit was obtained for frequencies belonging to the arsine donor. However, the results for the BXB end of the molecule were less satisfactory. For example, using acceptable values for the force constants, the best result using the MVFF fits all observed frequencies except the BCl, symmetric stretch. This calculates to ca 270cm-’ whereas the observed value is 373cm-’ and the band is so characteristic that there can be no doubt in the assignment. It is well established that when nonbonding interactions are significant, as in Ccl* and Ti&, the Urey-Bradley model is a better choice of force field [34, 353. Also, for the sake of comparison with other related molecules such as PH,BXB [24],
MezPHBXS [9], MesPBXs [lo] and MesAsBXs [19], a modified Urey-Bradley force field was adopted. The MeAsH end of the molecule was treated entirely as a valence force field which involved the diagonal force constants along with a few important interaction force constants such asf(CH/CH)andf(HCAs/HCAs). For the BXJ part of the molecule and the AsB bond, a Urey-Bradley force field was used by introducing two non-bonding repulsion force constants, the F(XX) between halogens and the F(AsX) between arsenic and halogens. The methyl group modes are well represented by f(CH’), j(HCH’), f(HCAs2), f(CH/CH) and f(HCAs/HCAs) force constants. The inclusion of f(HCAs/HCAs) has rendered the value off(HCAs’) artificially higher than that in free MeAsH [23] where the interaction force constant f(HCAs/HCAs) was not considered. The increase in f(AsH’), j(HAsH2) and f(CAs2) upon adduct formation is consistent with the increased s-orbital participation on rehybridization of the arsenic atom resulting in strengthening of the As-H and C-As bonds. The progressive increase inf(CAsB2) along the series Cl c Br < I has also been shown to occur for MeJAsBXS adducts. The Urey-Bradley K (AsB) exhibits a gradual increase along the sequence BCIB < BBr, < BIJ which agrees with the increase in acidity of the BXJ group along the same sequence. For this series MeAsH2BXJ, the average value of K (AsB) is 1.46, whereas for Me,AsBX, it is 1.67 [19]. These results are consistent with established orders of base strengths based on methyl group substitutions. The bending force constants H (AsBX) and H (XBX) and the repulsion force
Methylarsine adducts of boron trihalides
constants F(XX) and F(AsX) are close to those reported for related compounds indicating only a slight sensitivity of the BX3 group with respect to the donor molecule. The value of K (BX) decreases in the order BClj > BBr, > BIS as expected from the relative strengths of these bonds. The F-matrix elements are shown in brackets in Table 5 for the Urey-Bradley force constants for the sake of comparison with other studies. Acknowledgements-We wish to thank the Natural Sciences and Engineering Research Council for financial support.
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