Organometallics 1996, 14, 4732-4735
4732
Gas-Phase Characterization by Photoelectron Spectroscopy of Unstabilized a-Unsaturated Arsines: Ethylidene- and Ethylidynearsines' V. MBtail,? A. Senio,? L. Lassalle,t J.-C. Guillemiqt and G. Pfister-Guillouzo*vt Laboratoire de Physicochimie Moltculaire, URA CNRS 474, 64000 Pau, France, and Laboratoire de Synthkses et Activations de Biomolbcules, URA CNRS 1467, 35700 Rennes, France Received May 18, 1995@ Primary unsaturated arsines, vinylarsine (l),prop-l-enylarsine (2), and ethynylarsine (31, prepared by a chemoselective reduction of the corresponding dichloroarsines, have been characterized in the gas phase by their photoelectron (PE) spectra. Their base-induced rearrangements on solid %COS, in vacuum gas-solid reactions (VGSR) conditions, led, respectively, to ethylidenearsine (4), propylidenearsine (5), and ethylidynearsine (6). For the first time, electronic structures of primary a-unsaturated arsines and low-coordinate arsenic compounds are evidenced and the coherence with phosphorus analogues is confirmed. In particular, six narrow and well-resolved bands at 9.6, 10.6, 11.9, 12.7, 14.0, and 15.4 eV are observed i n the PE spectra of 1,and the spectrum of 3 exhibits three ionizations at 9.9, 10.6, and 11.6 eV. The spectra attributed to 4 and 5 display a broad band with a shoulder at 9.6 and 10.3 eV and at 9.5 and 10.2 eV, respectively. Two well-resolved bands at 9.6 and 12.1 eV are observed for 6.
Introduction Primary heterocompoundswith double or triple bonds such as CH2=CHOH, HCWOH, CH24HNH2, and HCWNH2 are well-known for their ability t o isomerize into the corresponding heteroalkenes, -allenes, and -alkynes. This property depends mainly on the acidity of the hydrogen(s) bonded to heteroatom. The acid properties are related with the stability of the anionic species and the neutral molecule, the last one depending on the interactions between the nc-c system and the heteroatom lone pair. Experimental characterization of these molecules has been performed either in a matrix2 or in the gas phase by mass spectrometry or photoelectron spectroscopy (PES).3 This last technique has proved to be particularly suitable for direct observation of compounds in which this type of interaction occurs. In this area, primary a-unsaturated phosphorus derivatives and the corresponding carbon-phosphorus multiple-bonded derivative^^-^ have been investigated +
Laboratoire de Physicochimie.
t. Laboratoire de Syntheses et Activations de Biomolecules. Abstract published in Advance ACS Abstracts, September 15,1995. (1) Application of Photoelectron Spectroscopy to Molecular Properties. Part 49. Part 48: Chuburu, F.; Lacombe, Pfister-Guillouzo, G.; Wentrup, C. New J . Chem. 1994,18, 879-888. (2) (a)Ethenol: Holmes, J. L.; Lossing, F. P. J.Am. Chem. Soc. 1982, 104,2648-2649. Rodler, M.; Bauder, A. J.Am. Chem. SOC.1984,106, 4025-4028. Hawkins, M.; Andrews, L. J . Am. Chem. SOC.1983,105, 2523-2530. Ripoll, J . L. New J . Chem. 1979,3,195-198. Capon, B.; Rycroft, D. S.; Watson, T. W.; Zucco, C. J . Am. Chem. SOC.1981,103, 1761-1765. (b) Ethynamine: Lasne, M. C . ; Ripoll, J. L. Bull. SOC. Chim. Fr. 1986, 766-770. (3) (a) Ethenol: Matti, G. Y.; Osman, 0. I.; Upham, J . F.; Suffolk, R. J.; Kroto, H. W. J . Electron. Spectrosc. Relat. Phenom. 1989, 49, 195-201. (b) Ethenamine: Lafon, C.; Gonbeau, D.; Pfister-Guillouzo, G.; Lasne, M. C.; Ripoll, J . L.; Denis, J . M. N o w . J . Chim. 1986, 10, 69-72. (c) Ethynol: Von Baar, B.; Weiske, T.; Terlouw, J. R;Schwarz, H. Angew. Chem., Int. Ed. Engl. 1986,25,282-284. (d) Ethynamine:
both from the experimental and theoretical points of view. Recent publications devoted to the preparation of the first primary a-unsaturated arsinesg and unstabilized arsaalkynesgbprompted us to study alkenyl- and alkynylarsines and their rearrangement reactions with the aim to observe low-coordinate arsenic derivatives by photoelectron spectroscopy.
Experimental Section Photoelectron spectra were recorded with a Helectros 0078 photoelectron spectrometer equipped with an 127" cylindrical analyzer using 21.21 eV He I and 40.81 eV He I1 radiation as the photon source and monitored by a microcomputer supplemented with a digital analog converter. Helium ionization at 4.98 eV and nitrogen ionization at 15.59 eV were used as references. Arsines 1-3. The vinyl- and ethynyldichloroarsines were synthesized as previously r e p ~ r t e d .The ~ corresponding primary arsines, vinylarsine (l),prop-l-enylarsine (2), and ethynylarsine (3)were prepared by chemoselective reduction of the dichloroarsines with tributylstannane or dichloroalane in tetraglymes (Scheme 1). To record the photoelectron spectra, mbar) two cold traps were fitted on a vacuum line (ca.
@
s.;
Wentrup, C . ; Briehl, H.; Lorencak, P.; Vogelbacher, U. J.; Winter, H. W.; Maquestiau, A.; Flammang, R. J.Am. Chem. Soc. 1988,110,13371343. Von Baar, B.; Koch, W.; Lebrilla, C.; Terlouw, J.; Weiske, T.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1986,25, 827-828.
0276-7333f95/2314-4732$09.00fO
(4) (a) Gonbeau, D.; Lacombe, S.; Lasnes, M. C.; Ripoll, J. L.; PfisterGuillouzo, G. J . Am. Chem. SOC.1988,110,2730-2735. (b) Lacombe, S.; Dong, W.; Pfister-Guillouzo, G.; Guillemin, C.; Denis, J . M. Inorg. Chem. 1992,31, 4425-4427. ( 5 ) (a)Lacombe, S.; Gonbeau, D.; Cabioch, J . L.; Pellerin, B.; Denis, J . M.; Pfister-Guillouzo, G. J . Am. Chem. SOC.1988,110, 6964-6967. (b) Dong, W.; Lacombe, S.; Gonbeau, D.; Pfister-Guillouzo, G. New J . Chem. 1994, 18, 629-641. (6) Guillemin, J. C.; Janati, T.; Denis, J. M. J . Chem. Soc., Chem. Commun. 1992, 415-416. (7) Bock, H. Phosphorus, Sulfur Silicon Relat. Elem. 1990,49150, 3-53. (8) (a) Bock, H.; Bankman, M. Angew. Chem., Int. Ed. Engl. 1986, 25,265-266. (b) Bock, H.; Bankman, M. Angew. Chem., Int. Ed. Engl. 1989.28. 911-912. (Si (a)hkenylarsines: Guillemin, J. C.; Lassalle, L. Organometallics 1994,13, 1525-1527. (b) Alkynylarsines: Guillemin, J . C.; Lassalle, L.; Drean, P.; Wlodarczak, G.; Demaison, J. J . Am. Chem. SOC.1994, 116, 8930-8936 and references therein for other gas-solid reaction
experiments.
0 1995 American Chemical Society
Gas-Phase Characterization of a-Unsaturated Arsines
Organometallics, Vol. 14, No. 10, 1995 4733
Scheme 1
2
I
c’s
m
a
t
Scheme 2 RCH=CH-AsH2
RCH,CH=AsH
1,4:R=H 2 ,S : R = CH3
4,s
1,Z HCIC-ASH~ 3
VGSR
CH3-CEAs 6
and the last one was connected to the PES inlet. The dichloroarsine (5 mmol) was then added slowly (10 min) by syringe through a septum to the reducing agent solution cooled at 273 K. Due to their instability, volatile arsines 1-3 were continuously distilled in uucuo from the reaction mixture during and after the addition of dichloroarsine. The first cold trap (213 K) removed selectively the less volatile products, and compounds 1-3 were condensed in the second cold trap (liquid nitrogen bath, 77 K). At the end of the reaction, this cold trap was warmed t o 183 K to remove traces of AsH3. After subsequent heating of the trap t o the suitable temperature (1,163K 2,173 K 3,193 K), the analysis of the arsines 1-3 was completed by recording the photoelectron spectra of the gaseous flow. Base-Induced €&arrangement of Arsines 1-3. The base-induced rearrangement of 1-3 was performed by contading the gaseous a-unsaturated arsines with solid potassium carbonate at 373 K in vacuum gas-solid reactions (VGSR) conditionssb (Scheme 2). Powdered and dried potassium carbonate (15 g) was introduced into a VGSR reactor (1 = 30 cm, i.d. = 3.5 cm, Pyrex tube) and then horizontally distributed between two pads of glass wool 20 cm distant from each other. This reactor was fitted in an oven onto a vacuum line connected to the PES inlet. Arsines 1-3 (3.0 mmol) were prepared as reported above, and each was vaporized slowly in umuo through the reactor. In addition,the purity of arsines 1-3 before their reaction on solid K&O3 heated to 373 K was directly checked by PES (due to the parallel connection). The products were analyzed in the gas phase by photoelectron spectroscopy without further purification.
Results and Discussion Only three primary and one secondary vinylarsinesga and the alk-l-ynylarsine parent compoundgbhave been described in the literature. Such compounds exhibit a half-life of about 30 min at room temperature in a solvent. Thus, their stability ranges between these of the corresponding phosphines516 and amine^.^^,^ The decomposition of the arsenic derivatives leads to an insoluble brown oligomeric material and not to the corresponding carbon-arsenic multiple-bonded compounds. Several arsaalkenes1°-13 and three arsaalkynesgbJ4 have been described. Due to the high reactivity of the C-As double bond, its stabilization can be achieved only be aromatic conjugation (benzazarsolelO and 1,3-aza(lo)Richter, R.; Sieler, J.; Richter, A,; Heinicke, J.; Tzschach, A.; Lindqvist, 0. 2.Anorg. Allg. Chem. 1983,501,146-152. (11)Miirkl, G.; Dietl, S.; Ziegler, M. L.; Nuber, B. Angew. Chem., Znt. Ed. Engl. 1988,27,709-710. (12) Becker, G.; Gutekunst, G. 2.Anorg. Allg. Chem. 1980,470, 157-166. (13)Werner, H.; Paul,W.; Zolk, R. Angew. Chem., Znt. Ed. Engl. 1984,23,626-627. (14)(a) Markl, G.;Sejpka, H. Angew. Chem., Znt. Ed. Engl. 1986, 25, 264. (b) Seyferth, D.; Merola, J. S.; Henderson, R. S. Organometallics 1982,1, 859-866.
IP(.V) 20
15
10
1
s b
Figure 1. Photoelectron spectra of (a) vinylarsine (1)and (b) l-propenylarsine (2). arsinine’l), by steric hindrance (MeAs=C(OSiMe& But),12or by coordination to transition metal fragments (Ph.4s=ScH~)RhCp*).~~ The kinetically stabilized 2-(2,4,6tri-tert-b~tylphenyl)-l-arsaethynel~~ has been isolated in form of pale yellow cristals. In situ generation then complexing of R-CsAs (R = H, CH3, C6H5, or the chemical trapping of tert-butylarsaalkyne (tBuC=As)15 have also been described, but all attempts to characterize these derivatives spectroscopically failed. Moreover, even the isolated compounds cannot be vaporized. The base-induced rearrangement of ethynylarsine 3 in the gaseous phase is the first approach to a volatile and unstabilized arsaalkyneegbWe used this facile rearrangement to prepare and to characterize by PES the ethylidynearsine 6 and then to study the rearrangement products of the vinylarsines 1 and 2. Vinyl-(l,2) and Ethynylarsine (3). Vinylarsines (1, 2) and ethynylarsines (3) are cleanly produced by a chemoselective reduction of the corresponding dichloroarsinesag The reaction is performed in a vacuum line directly connected, via a cryogenictrap, to the PES inlet, and the gas flow is directly analyzed. The spectra of vinylarsine (l),prop-l-enylarsine (2), and ethynylarsine (3)are displayed in Figures la,b and 2, respectively. The He I1 spectrum of compound 1 is shown in Figure l a . In the ethenylarsine 1 spectrum, six narrow and wellresolved bands are observed at 9.6,10.6,11.9,12.7,14.0, and 15.4 eV. With He I1 radiation, the intensities of the first, third, and fourth bands decrease. For the prop-l-enylarsine (21,three well-resolved ionizations at 9.3,10.2, and 11.6 eV and a broad band a t 13.0 eV with marked shoulders at 13.8 and 14.4 eV have been (15)Hitchcock, P. B.; Johnson, J. A,; Nixon, J. F. Angew. Chem., Znt. Ed. Engl. 1993,32,103-104.
4734 Organometallics, Vol. 14,No. 10,1995
Mitail et al.
x
.t.
AE (kJ/mol)
c
c/s
!V IP(eV) . . , 2b
I
. ' " . . ' l ' ' . , ' . '
1s
H
A
1'0
. . . , . . . , . s .
Figure 2. Photoelectron spectrum of ethynylarsine (3).
observed. The ethynylarsine 3 spectrum exhibits three ionizations at 9.9, 10.6, and 11.6 eV. The PE spectrum of vinylphosphine has been analyzed on the basis of free rotation of the phosphino moiety, in contrast to ethenol and ethenamine for which the rotamer with the lone pair eclipsing the n system was characterized in the gas phase.5 The stabilizing interaction nx-n*c=c is weaker for ethenylphosphine due to the less pronounced directional character of the lone pair (s-character increasing). In fact, the He I photoelectron spectra of the series X H 3 and XMe3 (X = N, P, As, Sb)16show that the first ionization potentials assigned to the nx lone pair are constant for all heteroatoms. Since this result is not in agreement with other atomic and molecular properties, such as basicity, atomic ionization potentials, and electronegativity, the anomalous constancy of the first IP values of XH3 and XMe3 was interpreted in terms of increasing in scharacter of the lone pair on going from N to Sb. Experimentally, with He I1 radiation, we observed for 1 a greater decrease in the first band intensity than for the second one, associated to greater participation of lone pair in the ionization. Compound 2, which is methylated on the double bond, shows a similar shift to lower potentials for the first two bands (attributed to the ejection of one electron from the molecular orbitals resulting from interactions between the C=C system and the arsenic lone pair). Thus, not only can the existence of the unique rotamer with the lone pair eclipsing the ac-c bond be ruled out, but the presence of a large population of rotamers with considerable arsenic lone pair-nc=c interactions can be supposed. In addition, compounds 1 and 2 exhibit between the two first bands an energy gap of 0.95-1.0 eV (compared with 1.25 eV for vinylphosphine) resulting from weak nh-nc=c interactions which are directly related to a greater C-As bond length and a more pronounced pyramidal configuration of the arsenic atom. These observations suggest a free rotation of k i n o group as previously observed for phosphino-analogue. Counterbalance between stabilizing (n-n*, n - 8 ) and destabilizing (n-n, n-a) interactions induces rotation tendency of heteroelement group around the ac-x bond. For vinylarsine (l),the proposed free rotation of a n arsino moiety around the C-As bond was checked by a theoretical study (3-21G basis set including d polariza(16)Elbel, S.; Bergman, H.; Enblin, W. J. Chem. SOC.,Faraday Trans. 2 1974, 70,555-559.
0
a
Figure 3. Rotation of the AsH2 group about the As-C bond. Relative energies as a function of the dihedral angle between the arsenic lone pair and the C-C-As plane.
tion orbitals on the arsenic atom, geometrical parameters of different stationary points optimized after a second-order Moller-Plesset perturbation method). The curve of the potential energy hypersurface shows the existence of two minima at 0 and 127" (dihedral angle between arsenic lone pair and C-C-As plane) and two saddle points at 90 and 180". The heights of rotation barriers are very low (5.68 and 4.68 kJ mol-l, respectively). The same order of magnitude was observed for vinylphosphine (8-10 k J mol-'). The more energetically stable rotamer is assigned the geometrical structure with the arsenic lone pair eclipsing the ac-c bond (Figure 3). This first characterization of vinylarsine by photoelectron spectroscopy emphasizes electronic structure similar to that of vinylphosphine (analogous profile of potential hypersurfaces curves with same energetically favored conformation, ionic state energies closed). An electronic similarity occurs as well between ethynylphosphineand ethynylarsine 3.6 For these systems, owing to the local cylindrical symmetry of acetylenic triple bond, all rotamers have the same energy and orbital energies are identical. It is worth noticing that for two rotamers, the heteroatom lone pair of which interacts strongly with only one of the acetylenic n orbitals, the first and third observed ionization potentials are described as a linear combination of the acetylenic n orbital and the arsenic lone pair. We observed a lower gap for 3 (1.7 eV), compared with its phosphorus analogues (2.06 eV), revealing weak interactions. The ionization potential of the other unpertubed n orbital is 10.6 eV and can be compared with 11.40 eV for acetylene. Thus, the destabilizing inductive effect of the arsino group is large. Base-InducedRearrangement of Vinyl- and Ethynylarsines. Vinylarsines (1,2)and ethynylarsine (3) have been vaporized over solid K2CO3 heated to 373 K. The gaseous flow was directly analyzed by photoelectron spectroscopy without further purification. As observed for the corresponding phosphorus derivative^,^ the expected arsaalkenes, ethylidenearsine (4), and propylidenearsine (5), are very unstable compounds. The ethylidynearsine (6), formed in the base-induced re-
Gas-Phase Characterization of a-Unsaturated Arsines
Organometallics, Vol. 14, No. 10, 1995 4735
l
IP(eV) 20
5
10
1s
h
b
vi
m
iP!ev) 20
,
,
,
,
15
,
,
, r
to the ionization of the nc=p orbital and the second band at 10.35 eV resulted from the ejection of one electron from the phosphorus lone pair.4 These assignments are supported only by experimental results because really sophisticated calculations, even with a n extended basis set, cannot provide a satisfactory interpretation of the ionic states of these molecules (see ref 17 for phosphorus analogues). Taking our experimental observations into account, it is reasonable to associate these two photoelectron spectra to ethylidenearsine (4) and propylidenearsine (5), respectively. The ionization at 9.6 eV for 4 (9.5 eV for 5) is associated with the ejection of an electron of the nc-h orbital. Weak intensity ionizations a t 10.3 eV for 4 (10.2 eV for 5) are due to the ejection of a n electron of the lone pair of arsenic atom ( n h ) . We observed a similar value in the spectrum of the corresponding phosphorus derivative. Thus, for the first time, a carbon-arsenic double bond has been characterized in the gas phase by its photoelectron spectrum. Quantum mechanical calculations confirm the thermodynamic stability of this entity; CH&H=AsH (ET= -2302.08858 au) is 12.1 kJ molv1 more stable than CH2-CH-AsH2 (ET= -2302.08397 au) (3-21G**MP2). Ethylidynearsine. In the PE spectrum displayed in Figure 5, two well-resolved bands observed a t 9.6 eV (a strong one) and 12.1 eV (a narrow band) characterize, without ambiguity, ethylidynearsine (6). The first band is assigned to ionization of the nc=h orbital, and the second-one is assigned to ionization of the orbital localized on the arsenic sp lone pair. The shift of the first band to lower energy relative to ethylidynephosphine (9.77 eV) corresponds to a slightly more diffuse effect of the carbon-arsenic bond. Ionization of the arsenic lone pair appears at the same energy in the phosphorus case. In addition, according to quantum calculations C H 3 C d s (ET= -2300.92354 au) is 93.3 kJ mol-l more stable than the starting comDound HCWAsH, (Et = -2300.88797 au). This energy difference probably explains the quantitative yield of triple-bond isomerization product and the partial doublebond rearrangement. Thus, this photoelectron spectroscopy study led to the characterization in the gas phase of the carbon-arsenic triple bond and supports the results obtained by microwave s p e c t r o s ~ o p y . ~ ~
L1 10
5
Figure 4. Photoelectron spectra of the gaseous products of reaction (a) of 1 and (b) of 2 over KzCO3. '9 m
1
CIS
,
Figure 5. Photoelectron spectrum of the gaseous product of reaction of 3 over KzCO3. arrangement of the arsine 3,is sufficiently stable to be condensed, revaporized, and analyzed.gb A good purity is obtained as shown by the low-temperature lH and 13C NMR spectra. Thus, we can conclude that we obtained a PE spectrum of a quite pure product. The PE spectra of the vinylarsine (1)and propenylarsine (2) rearrangements and of the ethylidynearsine (6) are shown Figures 4a,b and 5, respectively. Ethylidenearsine(4) and Propylidenearsine(5). The spectrum shown in Figure 4a presents a broad band at 9.6 eV with a shoulder at 10.3 eV followed by a second ionization , a t 11.3 eV. For 1-propenylarsine (21, the reaction is always partial. The spectrum obtained by digital subtraction of the spectrum of the starting compound is slightly shifted toward lower energy: it displays a broad signal a t 9.5 eV with a shoulder and a second ionization at 10.7 eV. This spectrum shows numerous similarities with that of ethylidenephos~ h i n e .Thus, ~ the first band at 9.75 eV was assigned
Conclusion
Primary alkenyl- and alkynylarsines have been characterized in the gas phase by their photoelectron spectra. The base-catalyzed isomerization of these compounds proves unambiguously the existence of lowcoordinate arsenic derivatives, such as ethylidenearsine, propylidenearsine, and ethylidynearsine in the gas phase. Quantum mechanical calculations of the electronic structure of these systems are presently being undertaken with the aim of understanding the influence of hydrogen's strength of acid on isomerization reactions. OM950359+ (17)Watts, J. D.; Rittby, M.; Bartlett, R. J. J.Am. Chem. SOC.1989, 111, 4155-4160.
