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Matrix isolation study of the mechanism of the reaction of diborane with ammonia: pyrolysis of the H3B·NH3 adduct
Volume 197, number I,2
CHEMICAL PHYSICS LETTERS
4 September 1992
Matrix isolation study of the mechanism of the reaction of diborane with ammonia: pyrolysis of the H3B-NH3 adduct John D. Carpenter and Bruce S. Ault Department of Chemistry University of Cincinnati, Cincinnati, OH 45221, USA Received 15June 1992
Gas-phase pyrolysis of the H3B.NH9 adduct followed by trapping of the reaction product(s) in an argon matrix at 14 K was conducted to provide support for an earlier proposed mechanism for the reaction of B2H6with NH+ As the pyrolysis temperature increased from 65 to 3OO”C,a decrease in yield of adduct was noted, along with a concomitant growth in H2B=NHZ. The results obtained here support a mechanism for the reaction of B2H6with NH, in which the 1: 1 adduct H,B.NH3 is formed in an initial slow step, and is followed by rapid elimination of H2 to form H2B=NH2.
1. Introduction Diborane, BzHs, has long been of interest to chemists as a result of its electron deficient nature [ l-3 1, and subsequent reaction chemistry. BzH6 reacts with a range of Lewis bases (electron donors) both in solution and the gas phase. The reaction of B2H6 with NH3 is of particular interest in that it leads ultimately to thin films of boron nitride, BN( s) [ 4-6 1. These thin films have applications in the semi-conductor industry as well as protective coatings. Only recently have studies begun to shed light on the mechanism of the reaction of B2H6 with NHs, although several details remain unconfirmed. A recent matrix isolation study [7] in this laboratory copyrolyzed samples of Ar/B2H6/NH3 immediately before trapping into a 14 K argon matrix. The primary product was aminoborane, H2B=NH2. In this study, an initial reaction of B2H6 with NH3 to form the H3B.NH, adduct and BH3 was postulated, followed by rapid elimination of Hz at the elevated pyrolysis temperatures to form the observed H2B=NH2 product. However, the adduct was not isolated, and its intermediacy remains unconfirmed. The H,B.NH3 adduct, although a solid, is known to sublime at temperatures just above room temCorrespondence to: B.S. Ault, Department of Chemistry, University of Cincinnati, Cincinnati, OH 4522 1USA.
perature. Observation of H2B=NH2 after pyrolysis of the adduct would provide additional information concerning the overall reaction mechanism. As in the earlier study, argon matrix trapping [ 8-101 provides an excellent means for both isolation and characterization of pyrolysis products. Consequently, a study was undertaken to examine the products of sublimation of the H3B.NH3 adduct, and its pyrolysis.
2. Experimental The matrix isolation experiments in this study were all carried out on conventional equipment which has been described [ 7,111. H3B*NHJ (Johnson Matthey Co. ) was placed in a 3 inch long sample finger of 1/ 4 inch outer diameter stainless steel tubing sealed at one end. This was connected to the argon deposition line by an ultratorr tee approximately 150 cm from the cold window. A heating mantle was placed around the sample finger, so that the sublimation temperature could be adjusted from room temperature to as high as 150°C. Heating tape was wrapped around the entire length of the deposition line from the tee to the cold window to pyrolyze the flowing gas sample. Temperatures as high as 360’ C were used in the pyrolysis zone. Samples were deposited for 22-24 h at approximately 2 mmol/h of argon before final in-
0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
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frared spectra were recorded at 1 cm-’ resolution on a Nicolet IR42 Fourier transform infrared spectrometer.
3. Results and discussion In an initial experiment, argon was passed over a room temperature sample of H,B.NH,(s) and deposited through the room temperature deposition line onto the 14 K cold window for 24 hours. The only infrared absorptions noted at this time were weak bands due to residual HZ0 and CO*. In a subsequent experiment, the sample finger was gently heated to 65°C while the deposition line/pyrolysis zone remained at room temperature. Under these conditions, a number of weak but distinct infrared absorptions were noted at: 608, 971, 974, 994, 997, 1070, 1076, 1175, 1180, 1660, 1664, 2423, 2492, 2566, 3401 and 3472 cm-‘. The intensities of these bands ranged from 0.02 to 0.05 absorbance units, and the bands were relatively broad for argon matrix absorptions. In addition, very weak bands were noted with intensities less than 0.02 absorbance units at: 613, 618, 1295, 1298, 1301, 1305, 1311, 1334 and 2555 cm-‘. In the following experiment, the finger again held at 65°C and the deposition line/pyrolysis zone was heated just slightly, to 45°C. Under these conditions, all of the above bands were seen, with increased intensity. With the exception of the bands at 608, 971, 974 and 1334 cm-‘, all of the above bands are readily attributed to the argon matrix-isolated H3B.NH3 adduct by comparison to literature spectra of this species [ 12 1. The fact that gently heating the deposition line increased intensities suggests that some of the sublimed adduct was condensing on the walls of the deposition line. The weak band at 608 cm-’ and the very weak band at 1334 cm- ’ are due to the two most intense fundamentals of aminoborane, H2B=NH2. This indicates that even under very mild temperature conditions, some decomposition of the adduct to aminoborane occurs. However, the two observed bands were very intense in the spectrum of aminoborane, and their low intensity here indicates that only a very small amount of the adduct decomposes at 65 ’ C. The bands at 97 1 and 974 cm-’ may be assigned to the symmetric deformation mode of NH3, 172
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1992
a very intense absorber [ 13 1. This also indicates that slight decomposition occurs under these conditions, but most of the adduct survives passage through the pyrolysis zone to the matrix cold window. In further experiments, the sample finger was maintained at 65 ‘C, and the deposition line/pyrolysis zone heated stepwise to temperatures as high as 360°C. When the flowing sample was pyrolyzed at 100 ’ C,the above bands were all observed, along with a new set of absorptions of roughly comparable intensity. These were located at 1002, 1014, 1120, 1130,1216,1365,2499,2568,3437 and 3518 cm-‘. At yet higher temperatures, absorptions due to the H3B.NH3 adduct decreased in intensity, while this new set of bands continued to grow. When the pyrolysis zone was heated to 3OO”C,the bands of the adduct were no longer detectable, while the new set of bands was quite intense. In this experiment, additional new bands were observed at 97 1, 1170, 1592, 2608 and 2618 cm-‘. These last five bands are directly assigned to diborane, BzHs, by comparison to authentic spectra of this compound [ 7,141. Finally, the set of bands which grew in intensity as the pyrolysis zone was heated from 65 to 300 “C match exactly (along with the 608 and 1334 cm-’ bands described above) the known [ 71 absorptions of HIB=NH2. Fig. 1 shows representative spectra representing these results, in the B-H stretching region. The above sequence of experiments demonstrates that: (1) the H3B*NH3 adduct does survive the transit through the deposition line when the temperature is at or slightly above room temperature; (2) increased pyrolysis temperature destroys the adduct; (3 ) increased pyrolysis temperature produces aminoborane in a manner proportional to the destruction of the adduct; and (4) diborane and ammonia are minor byproducts, with the diborane only observed at quite high temperatures. These results are consistent with the earlier mass spectrometric observation [ 15 ] of H2B=NH2 over solid H3B*NH3, and the fact that Merer and co-workers [ 161 recorded the gas-phase spectrum of H2B-NH2 by heating H3B.NH3 to 400°C. It is a resonable conclusion that the formation of HIB=NHI is a direct consequence of the destruction of the H,B*NH3 adduct, i.e. that the adduct eliminates H2 to form H2B=NH2. The fact that no other intermediate products were observed under any temperature conditions sup-
4 September 1992
CHEMICAL PHYSICS LETTERS
Volume 197, number 1,2
-0.070 -I
I
I
I
1
,dPO
I 3OilO WAVEMHERS
I
I
I
8 25bO
I
1
2200.0
Fig. 1. Infrared spectra, in the B-H stretching region, of the products of sublimation, pyrolysisand matrix trapping of samples of HsB.NH, in argon. The bottom trace was taken from an experiment with the deposition line at room temperature, while in the middle trace the pyrolysis temperature was 100°C. In the top trace, the pyrolysis temperature was 300°C. In all cases, the sublimation temperature was slightly above room temperature.
ports this conclusion. Since NH3 and B2H6 were byproducts of the decomposition of the adduct, it is possible that they could react to form H2B=NH2. While this reaction has been shown to occur, the threshold temperature [ 7 ] for the reaction is at least 180°C. In the present experiments, the formation of H2B=NH2 was observed at temperatures below 100 ’ C. All of these points support the conclusion that the HsB.NH3 adduct is an immediate precursor to HIB=NH2 formation. The earlier matrix study of H2B=NH2 after pyrolysis of Ar/B2H6/NH3 mixtures postulated an initial step in which attack of NH3 on B2H6 led to adduct formation. It was then postulated that the adduct eliminated Hz to form H2B=NH2. The present observation that the adduct can serve as an immediate precursor to HIB=NHI support this mechanism. Further, the fact that the reaction of NH3 with B2H6 requires a higher temperature for initiation (than H2 elimination from the adduct) suggests that the formation of the adduct is the first step and the ratelimiting step in the production f HIB=NHI. Finally, experiments [ 17 ] with the methylamines and BzH6 support this mechanism, in that for the reactions of CHJNH2 and (CH3)2NH with B2Ha, both the ad-
ducts and the H2 elimination products (H2B=N(H)CH3 and H,B=N(CH,)2) were observed.
Acknowledgement
The National Science Foundation is gratefully acknowledged for support of this research through grant CHE-90-24474.
References [ 1] A. Stock and A. Massenez, Chem. Ber. 45 ( 1912) 3539. [ 2 ] E. Muetterties, Boron hydride chemistry (Academic Press, New
York,1975).
[3] C.F.
Lane, Chem. Rev. 76 (1976) 773. [4] M. Hirayama and K. Shohno, J. Electrochem. Sot. 122 (1975) 1671. [ 51 S.B. Hyder and T.O. Yep, J. Electrochem. Sot. 123 (1976) 1721. [6]A.C. Adams and C.D. Capio, J. Electrochem. Sot. 127 (1980) 399. [7] J.D. Carpenter and B.S. Ault, J. Phys. Chem. 95 (1991) 3502. [8] S. Craddock and A.J. Hinchliffe, Matrix isolation (Cambridge Univ. Press, Cambridge, I975 )
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CHEMICAL PHYSICS LETTERS
[9] H.E. Hallam, Vibrational spectroscopy of trapped species (Wiley, New York, 1973). [ lo] L. Andrews, Ann. Rev. Phys. Chem. 22 ( 197 1) 109. [ 111 B.S. Ault, J. Am. Chem. Sot. 100 ( 1978) 2426. [ 121 J. Smith, K.S. Seshadri and D. White, J. Mol. Spectry. 45 (1973) 327. [ 131 B.S. Ault, Inorg. Chem. 20 (1981) 2817.
174
4 September 1992
[ 141 B.S. Ault, Chem. Phys. Letters 157 (1989) 547. [ 15 ] P.M. Kuuznesof, D.F. Shriver and F.E. Stafford, J. Am. Chem. Sot. 90 (1968) 2557.
[ 16lM.C.L. Gerry, W. Lewis-Bevan, A.J. Merer and N.P.C. Westwood, J. Mol. Spectry. 110 (1985) 153.
[ 171 J.D. Carpenter and B.S. Ault, J. Phys. Chem. 95 (1991) 3507.
CHEMICAL PHYSICS LETTERS
4 September 1992
Matrix isolation study of the mechanism of the reaction of diborane with ammonia: pyrolysis of the H3B-NH3 adduct John D. Carpenter and Bruce S. Ault Department of Chemistry University of Cincinnati, Cincinnati, OH 45221, USA Received 15June 1992
Gas-phase pyrolysis of the H3B.NH9 adduct followed by trapping of the reaction product(s) in an argon matrix at 14 K was conducted to provide support for an earlier proposed mechanism for the reaction of B2H6with NH+ As the pyrolysis temperature increased from 65 to 3OO”C,a decrease in yield of adduct was noted, along with a concomitant growth in H2B=NHZ. The results obtained here support a mechanism for the reaction of B2H6with NH, in which the 1: 1 adduct H,B.NH3 is formed in an initial slow step, and is followed by rapid elimination of H2 to form H2B=NH2.
1. Introduction Diborane, BzHs, has long been of interest to chemists as a result of its electron deficient nature [ l-3 1, and subsequent reaction chemistry. BzH6 reacts with a range of Lewis bases (electron donors) both in solution and the gas phase. The reaction of B2H6 with NH3 is of particular interest in that it leads ultimately to thin films of boron nitride, BN( s) [ 4-6 1. These thin films have applications in the semi-conductor industry as well as protective coatings. Only recently have studies begun to shed light on the mechanism of the reaction of B2H6 with NHs, although several details remain unconfirmed. A recent matrix isolation study [7] in this laboratory copyrolyzed samples of Ar/B2H6/NH3 immediately before trapping into a 14 K argon matrix. The primary product was aminoborane, H2B=NH2. In this study, an initial reaction of B2H6 with NH3 to form the H3B.NH, adduct and BH3 was postulated, followed by rapid elimination of Hz at the elevated pyrolysis temperatures to form the observed H2B=NH2 product. However, the adduct was not isolated, and its intermediacy remains unconfirmed. The H,B.NH3 adduct, although a solid, is known to sublime at temperatures just above room temCorrespondence to: B.S. Ault, Department of Chemistry, University of Cincinnati, Cincinnati, OH 4522 1USA.
perature. Observation of H2B=NH2 after pyrolysis of the adduct would provide additional information concerning the overall reaction mechanism. As in the earlier study, argon matrix trapping [ 8-101 provides an excellent means for both isolation and characterization of pyrolysis products. Consequently, a study was undertaken to examine the products of sublimation of the H3B.NH3 adduct, and its pyrolysis.
2. Experimental The matrix isolation experiments in this study were all carried out on conventional equipment which has been described [ 7,111. H3B*NHJ (Johnson Matthey Co. ) was placed in a 3 inch long sample finger of 1/ 4 inch outer diameter stainless steel tubing sealed at one end. This was connected to the argon deposition line by an ultratorr tee approximately 150 cm from the cold window. A heating mantle was placed around the sample finger, so that the sublimation temperature could be adjusted from room temperature to as high as 150°C. Heating tape was wrapped around the entire length of the deposition line from the tee to the cold window to pyrolyze the flowing gas sample. Temperatures as high as 360’ C were used in the pyrolysis zone. Samples were deposited for 22-24 h at approximately 2 mmol/h of argon before final in-
0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
171
Volume 197, number
1,2
CHEMICAL
PHYSICS
frared spectra were recorded at 1 cm-’ resolution on a Nicolet IR42 Fourier transform infrared spectrometer.
3. Results and discussion In an initial experiment, argon was passed over a room temperature sample of H,B.NH,(s) and deposited through the room temperature deposition line onto the 14 K cold window for 24 hours. The only infrared absorptions noted at this time were weak bands due to residual HZ0 and CO*. In a subsequent experiment, the sample finger was gently heated to 65°C while the deposition line/pyrolysis zone remained at room temperature. Under these conditions, a number of weak but distinct infrared absorptions were noted at: 608, 971, 974, 994, 997, 1070, 1076, 1175, 1180, 1660, 1664, 2423, 2492, 2566, 3401 and 3472 cm-‘. The intensities of these bands ranged from 0.02 to 0.05 absorbance units, and the bands were relatively broad for argon matrix absorptions. In addition, very weak bands were noted with intensities less than 0.02 absorbance units at: 613, 618, 1295, 1298, 1301, 1305, 1311, 1334 and 2555 cm-‘. In the following experiment, the finger again held at 65°C and the deposition line/pyrolysis zone was heated just slightly, to 45°C. Under these conditions, all of the above bands were seen, with increased intensity. With the exception of the bands at 608, 971, 974 and 1334 cm-‘, all of the above bands are readily attributed to the argon matrix-isolated H3B.NH3 adduct by comparison to literature spectra of this species [ 12 1. The fact that gently heating the deposition line increased intensities suggests that some of the sublimed adduct was condensing on the walls of the deposition line. The weak band at 608 cm-’ and the very weak band at 1334 cm- ’ are due to the two most intense fundamentals of aminoborane, H2B=NH2. This indicates that even under very mild temperature conditions, some decomposition of the adduct to aminoborane occurs. However, the two observed bands were very intense in the spectrum of aminoborane, and their low intensity here indicates that only a very small amount of the adduct decomposes at 65 ’ C. The bands at 97 1 and 974 cm-’ may be assigned to the symmetric deformation mode of NH3, 172
LETTERS
4 September
1992
a very intense absorber [ 13 1. This also indicates that slight decomposition occurs under these conditions, but most of the adduct survives passage through the pyrolysis zone to the matrix cold window. In further experiments, the sample finger was maintained at 65 ‘C, and the deposition line/pyrolysis zone heated stepwise to temperatures as high as 360°C. When the flowing sample was pyrolyzed at 100 ’ C,the above bands were all observed, along with a new set of absorptions of roughly comparable intensity. These were located at 1002, 1014, 1120, 1130,1216,1365,2499,2568,3437 and 3518 cm-‘. At yet higher temperatures, absorptions due to the H3B.NH3 adduct decreased in intensity, while this new set of bands continued to grow. When the pyrolysis zone was heated to 3OO”C,the bands of the adduct were no longer detectable, while the new set of bands was quite intense. In this experiment, additional new bands were observed at 97 1, 1170, 1592, 2608 and 2618 cm-‘. These last five bands are directly assigned to diborane, BzHs, by comparison to authentic spectra of this compound [ 7,141. Finally, the set of bands which grew in intensity as the pyrolysis zone was heated from 65 to 300 “C match exactly (along with the 608 and 1334 cm-’ bands described above) the known [ 71 absorptions of HIB=NH2. Fig. 1 shows representative spectra representing these results, in the B-H stretching region. The above sequence of experiments demonstrates that: (1) the H3B*NH3 adduct does survive the transit through the deposition line when the temperature is at or slightly above room temperature; (2) increased pyrolysis temperature destroys the adduct; (3 ) increased pyrolysis temperature produces aminoborane in a manner proportional to the destruction of the adduct; and (4) diborane and ammonia are minor byproducts, with the diborane only observed at quite high temperatures. These results are consistent with the earlier mass spectrometric observation [ 15 ] of H2B=NH2 over solid H3B*NH3, and the fact that Merer and co-workers [ 161 recorded the gas-phase spectrum of H2B-NH2 by heating H3B.NH3 to 400°C. It is a resonable conclusion that the formation of HIB=NHI is a direct consequence of the destruction of the H,B*NH3 adduct, i.e. that the adduct eliminates H2 to form H2B=NH2. The fact that no other intermediate products were observed under any temperature conditions sup-
4 September 1992
CHEMICAL PHYSICS LETTERS
Volume 197, number 1,2
-0.070 -I
I
I
I
1
,dPO
I 3OilO WAVEMHERS
I
I
I
8 25bO
I
1
2200.0
Fig. 1. Infrared spectra, in the B-H stretching region, of the products of sublimation, pyrolysisand matrix trapping of samples of HsB.NH, in argon. The bottom trace was taken from an experiment with the deposition line at room temperature, while in the middle trace the pyrolysis temperature was 100°C. In the top trace, the pyrolysis temperature was 300°C. In all cases, the sublimation temperature was slightly above room temperature.
ports this conclusion. Since NH3 and B2H6 were byproducts of the decomposition of the adduct, it is possible that they could react to form H2B=NH2. While this reaction has been shown to occur, the threshold temperature [ 7 ] for the reaction is at least 180°C. In the present experiments, the formation of H2B=NH2 was observed at temperatures below 100 ’ C. All of these points support the conclusion that the HsB.NH3 adduct is an immediate precursor to HIB=NH2 formation. The earlier matrix study of H2B=NH2 after pyrolysis of Ar/B2H6/NH3 mixtures postulated an initial step in which attack of NH3 on B2H6 led to adduct formation. It was then postulated that the adduct eliminated Hz to form H2B=NH2. The present observation that the adduct can serve as an immediate precursor to HIB=NHI support this mechanism. Further, the fact that the reaction of NH3 with B2H6 requires a higher temperature for initiation (than H2 elimination from the adduct) suggests that the formation of the adduct is the first step and the ratelimiting step in the production f HIB=NHI. Finally, experiments [ 17 ] with the methylamines and BzH6 support this mechanism, in that for the reactions of CHJNH2 and (CH3)2NH with B2Ha, both the ad-
ducts and the H2 elimination products (H2B=N(H)CH3 and H,B=N(CH,)2) were observed.
Acknowledgement
The National Science Foundation is gratefully acknowledged for support of this research through grant CHE-90-24474.
References [ 1] A. Stock and A. Massenez, Chem. Ber. 45 ( 1912) 3539. [ 2 ] E. Muetterties, Boron hydride chemistry (Academic Press, New
York,1975).
[3] C.F.
Lane, Chem. Rev. 76 (1976) 773. [4] M. Hirayama and K. Shohno, J. Electrochem. Sot. 122 (1975) 1671. [ 51 S.B. Hyder and T.O. Yep, J. Electrochem. Sot. 123 (1976) 1721. [6]A.C. Adams and C.D. Capio, J. Electrochem. Sot. 127 (1980) 399. [7] J.D. Carpenter and B.S. Ault, J. Phys. Chem. 95 (1991) 3502. [8] S. Craddock and A.J. Hinchliffe, Matrix isolation (Cambridge Univ. Press, Cambridge, I975 )
173
Volume 197, number 1,2
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[9] H.E. Hallam, Vibrational spectroscopy of trapped species (Wiley, New York, 1973). [ lo] L. Andrews, Ann. Rev. Phys. Chem. 22 ( 197 1) 109. [ 111 B.S. Ault, J. Am. Chem. Sot. 100 ( 1978) 2426. [ 121 J. Smith, K.S. Seshadri and D. White, J. Mol. Spectry. 45 (1973) 327. [ 131 B.S. Ault, Inorg. Chem. 20 (1981) 2817.
174
4 September 1992
[ 141 B.S. Ault, Chem. Phys. Letters 157 (1989) 547. [ 15 ] P.M. Kuuznesof, D.F. Shriver and F.E. Stafford, J. Am. Chem. Sot. 90 (1968) 2557.
[ 16lM.C.L. Gerry, W. Lewis-Bevan, A.J. Merer and N.P.C. Westwood, J. Mol. Spectry. 110 (1985) 153.
[ 171 J.D. Carpenter and B.S. Ault, J. Phys. Chem. 95 (1991) 3507.