Infrared Spectra of Trimethylamine-Carbon Monoxide- Borane and Its Deuterated Derivatives

James C. Carter,* George W. Luther, I I I , t and Alfred L. Moy6

Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received 28 January 1974; revision received 16 April 1974)

Low temperature infrared spectral data are given for the adducts H3BC(O)N(CH3)~, I).~BC (O)N (CH3).~, H3BC (O)N (C 1).~)3, I)3BC (O)N (C 1).~) 3 over the range 3000 to 300 cm~L Boron- 11 NMl~ data are given for H~BCO and H3BC(O)N(CH3)~. The spectra allow the assign- ment of the structure of the adducts as (CH~)~N--~C---~BH~.

II o

INDEX HEADINGS: Infrared spectroscopy; Nuclear magnetic resonance; Isotope studies.

I N T R O D U C T I O N

During our investigation of the chemistry of BH3CO and in particular its adduct formation with tr imethyl- a m i n e / w e found it advantageous to prepare the series of adducts H3BC(O)N(CH3)3, D3BC(O)N(CH3)3, H3BC(O)N(CD3)3, and D~BC(O)N(CD3)3. The infra- red spectra of these adducts clearly demonstrate the nature of the carbon-oxygen bond and the position of a t tachment of the nitrogen atom. We also found it of interest to compare these adducts with H~BCO and N(CH~)3 and the amine-boranes which result from the base displacement of carbon monoxide at high tempera- tures.

Because of the low symmetry, C,, of the carbon monoxide-borane-trimethylamine adduct and the num- ber of a toms in the adduct (19), a complete vibrational analysis was not performed. Instead, the infrared bands were assigned by analogy to group frequencies and by consideration of the various selectively deuter- ated species. The pitfalls related to the assignment of particular infrared band frequencies to various valence bond modes of motion in complex molecules are recog- nized. The cases where one a tom is very light (hydrogen modes) and where formal multiple bonding is present (CO modes) are particularly well suited to this type of approximat ion/

I. E X P E R I M E N T A L

BH3CO was prepared by the method of Carter and P a r r y / I t was stored at - 1 9 6 ° C and measured and transferred as rapidly as possible to avoid decomposi- tion. After transfer, any CO was removed and measured in a Toepler system, and any B2H8 was removed at --165°C. The amount of HaBCO was then corrected if decomposition occurred during transfer. This also allowed preparation of samples without amine-borane being formed simultaneously from diborane and tri- methylamine.

* To whom correspondence should be addressed. Current ad- dress : Department of Chemistry, Memphis State University, Memphis, Tenn. 38152.

t Current address: Department of Chemistry and Physics, Kean College of New Jersey, Union, N. J. 07083.

BD3CO was prepared from B~D6 and carbon mon- oxide by reaction in a stainless steel cylinder in a fashion similar to the preparat ion of BH3CO.

BeD8 was prepared from boron trifluoride-ethyl etherate and lithium aluminum deuteride obtained from Alfa Inorganics. I t was purified b y low tempera- ture distillation.

Trimethylamine was obtained from Matheson Com- pany. I t was distilled from --83.6°C and the first volatile fraction discarded. I t was then dried over P4010 for several days prior to use.

(CD3)3N was prepared by the method of Tanaka, Dunning, and Carter. 4 I t was dried over P4010 prior to use.

The various adducts were prepared by direct reaction as described by Carter, Moy6, and Luther3

The infrared spectra were taken on a Beckman IR-10 at approximately --180°C in a low temperature cell with CsI optics. The spectra were measured between 4000 and 250 cm -1 using in each case polystyrene film for calibration. The values given for absorption bands are accurate to d:=8 cm -1 over the region 4000 to 2000 c m % and ::t= 4 cm -1 over the region 2000 to 250 cm-L

The ~lB N M R spectra were taken at 32.1 M H z using a Varian HA-100. Samples were sealed in 5-mm tubes, and the spectra are referenced to (CH3)3B as an exter- nal standard. All spectra reported were taken at - 65° C .

II. R E S U L T S A N D D I S C U S S I O N

In order to interpret the infrared spectrum, it was necessary to assign the hydrogen modes and, particu- larly, the carbon-oxygen stretching mode. To do this, a var ie ty of deuterated species were prepared. I t was felt tha t the structure of the adduct could be assigned if the carbon-oxygen band were known. The rationale for this is as follows.

The frequency of the CO stretching vibrat ion of several related compounds has been determined. For example, the CO stretch in BH3CO occurs at 2165 cm-1, 5 in B4HsCO at 2170 cm-1. 6 These values may be compared to the v(CO) band at 2143 cm -1 7 for the

Volume 28, Number 5, 1974 APPLIED SPECTROSCOPY 427

gaseous carbon monoxide molecule itself. The v(CO) mode in the boranocarbamate derivatives occurs in the range of 1630 to 1640 cm -1) It is apparent, then, that adduct formation of carbon monoxide with the Lewis acid BH3 or the more complex Lewis acid B4H8 does not greatly shift the CO stretching frequency; the shift that does occur is to higher energy. In contrast, the addition of a Lewis base such as the NR2- group to the carbon atom of H3BCO to form the boranocarbamate ion lowers the CO stretching frequency considerably. The mode at 1630 to 1640 cm -t is then very closely related to the CO stretching mode in amides; for example, N-methylacetamide, which is isoelectronic with and thus formally related to the N-methylborano- carbamate ion, has the v(CO) band occurring at 1635 cm-1. 8 Therefore, it can be generalized that removal of

TABLE I. Comparison of infrared frequencies of H3BCO. N(CH3)3 and deutera ted molecules from 3000 to 300 cm -z.

HaBCO. DaBCO. HaBCO. D3BCO- Tentative N(CHa)a N(CHa)8 N(CD~)* N(CD~)3 assignment

2955,w 2950,w 2069,w 2065, w v(C--H) 2020,w 2018,w v(C--H)

2400,s 1750,s 2395sh,s 1745sh,s v(B--H) 2335sh,m 1725sh,m 2355,s 1715sh,m v(B--H) 2295,s 1690,m 2305,s 1690sh,m v(B--H) 2258sh, w 2258sh, w 1670sh, w g (B--H) 1798,s 1800,s 1790,s 1790,s v(C--O) 1478,s 1480,s 1205,m 1200sh,w a(CH~) 1463,m 1465,s l lS0sh,s 1175,s $(CH3) 1397,w 1400,m 1145sh,m 1145,m ~(CHa) 1255, w 1260, w 1055, m 1055, m r(CH3) 1241 ,w 1235,w I025,w 1027,w r(CH3) 1162, m 932, m 1160, s 940, m ~ (B Ha) l143sh,w l l l 0 , w l l l 0 , m a(BH3) 1094, m 1100, m 930sh, w 955sh, w t (CH~) 1000sh, w 993, s 992, s w (C Ha) 975, m 978, m 812, s 810, m v(N--CH~) 890,s 605, w 890, s 600, w r(BH~) 880sh, In 870, m 870sh,s 870, m

840, m 830sh, w 824, s 820, m 700,s 695, s v (N--C H~) 802sh, m 805sh, m 770, s 770sh, m 785, s 775, s v (C--N)

745,s 732, m

534, s 535, s 495, s 480, s 425,s 425, s 480sh, m 455sh, m 485, m 418,s 403,s 365,s 360,m 358, m 323, m 325sh, m 318sh, m 310sh, m

310, m 306, m 298, m

t(BH3) ~(CHa) or NC3

~(B--C)

electron density from the carbon increases the fre- quency of the ~(CO) band, and, conversely, increasing electron density of the carbon decreases the v(CO) frequency.

Because there are three positions of possible attach- ment of the trimethylamine molecule, we must consider the effect of each position upon the CO stretching mode. The first position is attachment at the boron giving a five-coordinate boron. Although a coordination number of five is not generally common in chemistry, it is well established in boron compounds such as BsH9 and B10H16. It is assumed, although no exactly analo- gous system with the higher coordination number of the boron is known, that addition of the trimethylamine nitrogen to the boron would result in only a small change of the v(CO) mode from 2165 em -1 which is found in BH~CO and it should appear near 2150 cm -1 in the adduct; attachment of the nitrogen to the carbon would give a compound more analogous to the amide- type derivative of the boranocarbamate, although in this case without the negative charge. The NR3 group is a less effective electron donor than the NR2- ion. It would appear reasonable that this type of adduct, although no direct analogy is known, would have a CO stretching vibration at a much lower frequency than H3BCO but not as low as [H~BC(O)N(CH3)2-], presumably in the range of 1700 to 1850 cm -1. If the nitrogen attaches to the oxygen, no direct analogy is again available, but this would appear most closely related to the ethers, and one would expect the CO stretching mode to occur below 1400 cm -1. Intuitively, this appears the least likely point of attachment. Thus, assignment of the v(CO) band in H3BCO.N(CH3)a would provide structural information on the adduct.

Further support of the structure could be derived from the chemical shift of the nB NMR. Attachment of trimethylamine to the boron would be expected to shift the resonance considerably. Little change in the chemical shift would be expected if the amine attaches to the carbon or the oxygen. Combination of 11B NMR data and infrared spectral data should thus provide sufficient information to determine the structure.

The infrared spectral data presented in Table I and Fig. 1 allow assignment of the CO stretching mode. The various deuterated species were prepared in which the BH bonds were replaced by BD bends and also the CH bonds were replaced by CD bonds. This allows

t 0 0

6O

40

2 C i I I [ I

4 0 0 0 3 0 0 0 2 0 0 0 I I I I I I I I

1600 1200 8 0 0 4 0 0 WAVENUMBER CM - t

FIG. 1. Infrared spectrum of solid H3BC(O)N(CH3)3 at -180°C.

428 V o l u m e 28 , N u m b e r 5 , 1974

C

2 A 1_ ? \

Frequency Fro. 2. 32.1 MHz nB NMI/~ spectra of H~BC0, neat liquid at -65°C (A) and HaBCO.N(CHa)a in ethyl ether at -65°C (B). Chemical shifts in ppm upfield from (CH~)~B = 64 (A); 62 (B). J for both A and B = 105 Hz.

assignment of the bands in the region above 1200 cm -1 using well established group frequencies. To eliminate the possibility that a simple mixture was obtained, comparison spectra were obtained under identical con- ditions to establish the absence of BH3CO, BDaCO, (CH3)3N, (CD3)~N, (CH~)3NBH3, (CD3)aNBH3, (CH3)3NBD3, and (CD3)3NBD3 front the spectra of the various 1:1 adducts of carbon monoxide-borane and trimethylamine. For example, the spectrum of H3BCO.N(CH3)a shows an absence of bands at 2165 cm -~, 1042 cm -~, and 691 cm -~ which have been as- signed to v(CO)in BH3CO, 5 v (C- -N) in N(CHa)a ~, and v(B--C) a in BHaCO, respectively. Moreover, the BHa stretching modes are intermediate between those for BH~CO ~ and (CHa)aNBHa ~, indicating no decomposi- tion of the adduct ~o (CHa)aNBHa. The only band which occurs in all spectra of the 1 : 1 adducts is a band centered at 1790 to 1800 cm-L This intense band is assigned to the ~(CO) mode.

Assignments below 1200 cm -~ must be considered tentat ive and are supported as follows.

The absorptions at 1162 and 1110 cm -~ are assigned to the BH~ bending modes by comparison with those found for (CHa)aNBHa at 1169 cm -1 and 1117 cm-L ~ In the alkyl amine boranocarbamates these bands occur at 1190 and 1130 cm-1. a The relative intensities for these three types of compounds also agree.

The band at 970 cm -1 in the N(CH3), adducts is assigned to ~ (H,C- -N) of the amine on coordination with BH3CO. This band has been assigned at 1003 cm -1 in (CH3)~NBH3. ~ This band appears to be shifted to 810 cm -1 on deuterating the amine.

The strong band at 890 cm -I appears only in the BH3CO adducts and has the structure of the BH~ rock band in BHaCO which occurs at 809 cm-L 5 This assign- ment also agrees with the BH3 rock in (CH3)~NBHa which has been established at 915 cm -~ by Taylor 2 and Rice et al. ~° The absence of a strong band in this region for the BDsCO adducts further supports this assign- ment.

The strong absorption near 770 cm -~ for all the ad- ducts is possibly ~(C--N). The only other possibility should be v(B--C) but is discounted because deutera- tion of the borane group does not result in the expected shift to lower frequency.

The 534 cm -~ strong band is seen only in the BH3CO adducts and not in the BD~CO adducts; this is at tr ib- uted to a BH3 motion--possibly a torsion.

The 495 cm -1 absorption appears only in the N(CH~)3 series and is shifted to near 420 cm -~ in the N(CD3)3 series. This strong band may be either a CH~ or NCa deformation.

The intense absorption occurring at 418 cm -~ appears to be a ~(B--C) mode. This is in contrast to the v(B--C) mode in BH~CO which occurs at 690 cm-*. 5 However, a shift to lower frequency is expected for this band on coordination with N(CH~)3 and also on deuteration of the boron and deuteration of the amine as seen for the tentat ive assignments of the deuterated compounds.

The '~B N M R spectra are shown in Fig. 2 for H~BCO as the neat liquid and H3BCO.N(CH3)3 in ethyl ether solution. The multiplicity establishes the continued presence of a BH3 group in the adduct; the identical coupling constants and nearly identical chemical shift suggest highly similar boron environments for boron in both compounds. The substantial quadrupolar broad- ening in the adduct results from incorporation of ~4N.

The spectral evidence presented here and chemical evidence presented elsewhere strongly support the fol- lowing structure

0 / /

(CH3)~N ~ C

BH3

If considered as a derivative of carbon monoxide, the carbon functions as both a Lewis acid and a Lewis base accepting electrons from nitrogen and donating elec- trons to boron.

ACKNOWLEDGMENTS

We wish to acknowledge financial support of this research by the American Cancer Society (Grant T-325) and the National Inst i tutes of Heal th (Grant CA- 08222). George W. Luther I I I also acknowledges

APPLIED SPECTROSCOPY 429

support by a National Defense Education Act Fcllow- ship.

1. J. C. C'arter, A. L. Moy5, and G. W. Luther, III , J. Am. Chem. Soc. in press.

2. R. C. Taytor; Adv. in Chem. Series 42, 59 (1964). 3. J. C. Carter and R. W. Parry, J. Am. Chem. Soc. 87, 2354

(1965). 4. J. Tanaka, J. E. Dunning, and J. C. Carter, J. Org. Chem.

31, 3431 (1966).

5. G. W. Methke and M. K. Wilson, J. Chem. Phys. 26, 1118 (1957); 27,978 (1957).

6. G. TerHaar, Doctoral dissertation, University of Michigan, Ann Arbor (1963).

7. E. Amaldi, Z. Physik 79,492 (1932). 8. S. Mizushimu, T. Simanouti, S. Magakara, D. Kuratani,

M.~Tsuboi, It. Bab, and O. Fujioku, J. Am. Chem. Soc. 72, 3490 (1950).

9. J. T. Edsall, J. Chem. Phys. 5,225 (1937), 10. B. Rice, R. J. Galiano, and W, J. Lchmen, J. Phys. Chem.

61, 1222 (1957).

A Quantitative Method for Infrared Spectroscopy of Surface Species

A. Ahmed and E. Gallei*

Institut liar Chemische Technologie, TH Darmstadt, 61 Darmstadt, Hochschulstr., West Germany

(Received 4 February 1974; revision received 19 March 1974)

A quantitative method for studying surface species by infrared spectroscopy has been de- scribed. The solid particles are not subjected to high pressures or contacted with any extrane- ous substance during sample preparation. The method is quantified by using the solid substrate as an internal standard. Reproducibility measurements on surface concentration of trimethyl- chlorosilane adsorbed on silica gel and Aerosil gave 3.29 and 3.49% standard errors, respec- tively, Quantitative measurement of surface hydroxyl groups in open systems is also described. IND J.:X H f,:ADINGS : Infrared analysis; Adsorbed species; Infrared, general.

I N T R O D U C T I O N

Infrared spectroscopy is a well established tool for studying the surfaces of powdered solids. Earlier in- frared studies 1 were carried out by sprinkling powered samples on an infrared t ransmit t ing plate mounted horizontally in the sample compar tment of the spectro- photometer . This method has fallen into disuse because of serious light scattering problems and the necessity of redesigning the optical pa th of the spectrophotometer. At present the most popular procedure for preparing samples for surface investigations is to press the finely divided solids into self-supporting discs. The major disadvantage of the pressed disc technique is tha t the surface of particles may be altered during the pressing process, which can produce localized high temperatures. McDonald ~ found tha t compressing Cab-o-sil silica particles a t pressures of approximately 100,000 lb/in. ~ decreased the intensity of isolated surface hydroxyl groups. Hamble ton and co-workers 3 showed tha t the O H - - O D exchange reaction on the surface is altered by the pressing process.

Adsorbed species have also been studied by using the mull technique 4 or by depositing solid particles on an infrared t ransmit t ing plate from a slurry in an organic liquid. 5 The disadvantages of these methods are possible

* To whom correspondence should be addressed.

contamination of the surface during sample preparat ion and the difficulty of obtaining quant i ta t ive results.

This paper prese.nts a quant i ta t ive method for study- ing powered solid surfaces. The solid particles are not subjected to high pressures, and contact with any extraneous liquid or solid during sample preparat ion is avoided. The particles are dusted between two window plates and held vertically in the radiation path. Since it is difficult to measure the weight of sample interacting with the spectrophotometer beam, the need for this is eliminated by using the solid substrate as an internal standard. This is readily justified by considering the Beer-Lamber t law. The absorbance of a substance at wave-length Xk is given by

Ak = Ekcd (1)

where c is its concentration and d is the pa th length. Usually c and d are given in moles per liter and centime- ters, respectively. Ek is the molar extinction coefficient with dimensions of liters per centimeter per mole. Eq. (1) may be rearranged as

Ak = E~Xk (2)

where Xk is the weight of adsorbed species per square centimeter of the infrared beam. The proport ionali ty factor E J has different dimensions than Ek.

Similarly, the absorbance of substrate at wavelength

430 Volume 28, Number 5, 1974 APPLIED SPECTROSCOPY