4496
W. D. PHILLIPS, H. C. MILLERA N D E. L. ~ZUETTERTIES
4.6 mm.). It therefore appears that approximately 64% of the original non-condensable product was hydrogen. 0.825 rnmole of water was added to the residue. On warming t o room temperature the black solid began to turn white. The mixture was allowed to stand at room temperature for three wezks. The only vohtile product obtained by distilling at 18’ was NH: (0.21 rnmole; vapor pressure at -80’ found 37 mm., Ut. value*’ 39 mm.). On rapid heating above 200’ BHzCN often decomposed with explosive violence.
bitals would suggest that a transfer of hydrogen from the boron to the silicon could occur readily by the means of an intermediate complex of the tYPe RSSiCSBHp
-
Results and Discussion Diborane reacts with silyl cyanides a t low temperatures according to the general equation 2RsSiCN
+ BzHs
--f
2RzSiCN.BHa
(12)
where R = H or CH3. Available chemical evidence indicates that a borine adduct is formed. This is suggested by the reactions of (CH3)&3iCN.BH3 with HC1 and (CH3)3N as given in equations 7, 8 and 9. Of considerable interest is the irreversible thermal decomposition of the adducts to the corresponding silane and a solid whose composition corresponds to BHZCN, viz. RsSiCNeBHa -+- R3SiH
+ BHtCN
(13)
The fact that silicon has available vacant 3d or-
[CONTRIBUTION No. 549 FROM
THE
1.oL 81
*
I
1
:
H H H,B ;\‘csi~~
followed by the elimination of silane. This is consistent with the observation that hydrogen migration from the boron occurs more readily with the silicion derivatives than with the carbon analogs, as evidenced by the fact that some unchanged CHsCN and B2Ha can be obtained on heating CH3CN.BHs. Since the B-H hydrogen can presumably form a linkage more readily to the silicon in an intermediate complex than to the carbon of the CN group! it is understandable why the H migrates to the Si and not to the carbon as is the case in the forination of (CHaCH2NBH), from CH,CN.BH,. Reduction of the cyanide carbon only occurs on heating BHzCN to higher temperatures. PHILADELPHIA, PA. CHICAGO, ILL.
ASD
CENTRAL RESEARCH DEPARTMENT, EXPERIMENTAL STATION,E. I. DU PONTDE NEMOURS A S D COMPANY]
B1l Magnetic Resonance Study of Boron Compounds BY W. D. PHILLIPS, H. C. MILLERAND E. L. MUETTERTIES RECEIVEDAPRIL17, 1959
BI1 chemical shifts are presented for representative examples of most classes of boron-containing compounds. B1l-H1 coupling constants are given for the boron hydrides and b x a n e adducts. Some qualitative conclusions regarding bonding in these compounds are derived from observed chemical shifts and coupling constants. Temperature dependences of the R” and BloHlc are presented and discussed. spectra of B I H ~ . NCH8)z (
Introduction The B l1 magnetic resonance spectra of several boron-containing compounds already have been reportedl-10 and discussed, usually in connection with proofs of structure. However, with the exception of the work of Onak, et al.,1° no systematic study of the nuclear magnetic resonances of this important class of compounds has been made. I n the present study, boron chemical shifts and B”-H1 coupling constants for most types of boron-containing compounds are given, from which sotne qualitative conclusions regarding electronic st.rucR. A. Ogg, Jr., J . Chcm. P h y s . . 29, 1933 (1954). (2) J. N. Shoolery, Disc. Faraday Coc., 19, 215 (1955). (3) R. Schaeffer, J. N. Shoolery and R. Jones, THISJ O U R N A L , 7 9 , 4606 (1957). (4) R. Schaeffer, J. N. Shoolery and R. Jones, ibid., 80, 2870 (1)
(1958). (5) B. Siegel, J. I.. Mack, J. I J . Lowe, Jr., and J. Gallaghan. ibid., 80, 4523 (1958). ( 6 ) R. Schaeffer, paper presented before t h e Division of Inorganic Chemistry, National Meeting of the A.C.S.. San Francisco, California. April, 1958. (7) R. E. M‘illiams and I. Shapiro, J. Chem. P h y s . , 29, ti7i (1958). ( 8 ) A. H. Silver and P. J . Bray, ibid., 29, 984 (19.55). (9) R. E. Williams, S. G. Gibbins and I. Shapiio, i h i d . , SO, 320, 333 (1959). (10) T. P. Onak, H. Landesman, R. E:. Williams and I. Shapiro, paper presented before the Division of Inorganic Chemistry, Kntional ATlretiny of the A.C.S.. Boston. Mass., April. 1959.
tures are made. In addition, some temperature dependences of the B” spectra of BloH14and B2Hb. N(CH3)Z are reported and discussed. Results and Discussion example of a B1l magnetic resonance spectrum is that of B2Hb.N(CH& shown in Fig. 1. The three main triplet components centered around 237,367 and 497 c.P.s., arise from nuclear spin coupling between boron atoms and the two “terminal” hydrogen atoms bonded to each boron. The single “bridge” hydrogen atom splits each of the triplet components into doublets of spacing 29 C.P.S. Because of the low nuclear moment and probably weak coupling to the B1l nuclei, the “bridge” NI4 nucleus with a spin of I = 1 only contributes to the breadth of the R1’ resonance through spin-spin coupling. The RI1 nucleus has a nuclear spin of 3/2 and a nonzero electric quadrupole moment; and in general, because of quadrupole relaxation effects, it exhibits resonance line widths considerably greater than those observed for hydrogen and fluorine. For this reason, coupling constants between boron and hydrogen separated by more than one bond length have not been observed. Indeed line widths of boron resonances are such that only in a few in-
I. Bl1 Chemical Shifts.-An
Sept. 5, 1959
A BORON-11MAGNETIC RESONANCE STUDY OF BORON COMPOUNDS
M i \
TABLE I BI1 CHEMICALSHIFTS AND BILH1SPIN-SPIN SPLIITXNCS IN BORONCOMPOUNDS HC HB
-
HR 106
x
Fig.
+
-14.3
134
-12.3
136
-12.9 0 0.6 0.5 - 0.1
+ +
+ 0.5 A
l.--Bl1
resonance of BLHaS(CHz)2(-10’); sec.; ref. B(OCH&.
10
Mc
/
20 p.p.m. for H1 l1 and less than the 300 p . p m . for Fl9l2 and Pr1.13 Saika and Slichter14 treated Fl9 chemical shifts in terms of a paramagnetic shift arising from the difference in electron occupancy of the bonding pz and non-bonding px and p , orbitals. Attempts have been made to correlate P 3 I chemical shifts on a similar basis.lPJ6 Qualitatively, the major features of the B” chemical shifts can be accounted for on the basis of such a “paramagnetic shift.” The extreme low field resonance for the compounds listed in Table I is observed for B(CHS)3, and the high field resonance, for BHh-. Neglecting hyperconjugation, the bonding hybrid of boron in B(CHJ3 should be sp2with a vacant pz orbital; the bonding hybrid of boron in BH4- should be spa with full tetrahedral symmetry about boron. If such an effcct dominates B1l chemical shifts, all B” resonances would be expected to fall between those for B(CH3)s and BHh-. As is seen from Tables I, I1 and 111, TABLE I1
137 (terminal) 48 (bridge)
H +36.7
\
JB-H (C.P.S.) Coup. constant
-68.2 -66.6 -35.9 6.6 -29.2 -22.7 +23.3 -15.2 -14.3 -10.4
4497
130 (terminal) 29 (bridge)
€3” RESONANCES OF BORONFLUORIDES
+ 6.6 +19.0
CHI CHI
$20.2 +18.6 116
+18.8
+19.0
103
$20.3 +18.2 $19.1 +18.7
f31.4
90
+32.8 +24.9 +55.6 +16.1 f15.2 1-55.1 t61.0 $49.4
94 97 96
+14.5
86
s1
stances are splittings between Bl1 and directly “bridged” hydrogen atoms observed. Coupling constants between B l 1 and “bridge” hydrogen atoms appear to be only ‘/2 to I,’b as great as those between B l 1 and “terminal” hydrogen atoms. Chemical shifts are presented for several classes of boron compounds in Table I. The 130 p.p.m. range of chemical shifts for Bl1 is greater than the
such is the case for all Bl1 resonances of this study with the exception of the apex boron atom of BbHs. This trend also has been noted by Onak, et aZ.,1° in their survey of B1l resonances. The Bl1 resonances of the BHa adducts with ethers and amines are consistent with this interpretation in that the reduced paramagnetic shift observed for boron in these compounds reflects presumably partial electron occupation of the boron p. orbital as the result of donation of lone pair electrons from the oxygen and nitrogen atoms. Adduct formation is accompanied by a change of boron hybridization, (11) L. H. Meyer, A. Saika and H.3.Gutowsky, THIS JOURNAL, 76, 4567 (1953). (12) H. S. Gutowsky, D. W. McCall and C. P. Slichter, J. Chcm. P h r r . , 21, 279 (1953). (13) N. Muller, P. C. Lauterbur and J . Goldanson, THIS JOURNAL, 78, 3557 (1956). (14) A. Saika and C. P.Slichter, J . Chem. P h y r . , 28, 26 (1954). (15) J. R. Van Warer, C. F. Callis, J. N. Shoolery and R. C. Jones, THIS JOURNAL, 78, 5715 (1956).
M". I). PHILLIPS, H. C ~ I I L L E RAND E. L. MUETTERTIES
4498
Vol. 81
TABLE111 B" RESONANCES OF BORONHYDRIDES
BF3 with ethers and amines are presented in Table 11. The B" resonance of BF4- exhibits a B1'( Hc -H HR X r 106) paramagnetic shift of about 41 p.p.m. relative to Compound JB-E (C.U.S. . _ that of BH4-. Since both BF4- and BH4- possess tetrahedral symmetries, Bl1 resonances of the two 137, 48 species would be expected to be the same if orbital paramagnetism were the only term affecting these 130, 29 +36.7 chemical shifts. The chemical shifts between the two species is best ascribed to effects of the highly A electronegative fluorine atoms in reducing the diaCHI CHB magnetic shielding about boron in BFI-, and so $25.8 31 f 5 B&bO( C&)2 giving rise to a low field shift of the BF4- resonance NaBsHa (in H2O) f46.5 32 relative to that of BH4-. B4H13 BH2 BH BHz BH The boron resonances of the BF3 adducts with $25.0 f59.9 132 156 ethers and amines are essentially coincident with BSHS base apex base apex that of BF4-. This suggests that the electronic f30.8 +69.6 168 176 configuration around the boron atom in BF4- and $6.9, f l 9 . 5 , +53.9 124, 128, 159 BlOHl4 in the adducts are similar. It would appear from $4.0, f17.6, f60.S 136, 137 BioHd? these results that the boron is tetrahedrally bonded BsHii -16.2, f 0 . 7 , f49.8" 134, 162, 142' in these adductsI8 (sp3 hybrid) and that bonding a Data taken from ref. 3. between adduct components could be represented presumably from sp2 to sps, which also would be as F3B--N+R3. 111. B1l-H1 Coupling Constants.--Gutowsky, expected to reduce the paramagnetic shift, The B" resonances of BH4- and B ( C F C C ~ H ~ ) ~ -McCall and Slichter'? have suggested that magniare similar; however, the resonance for B(CsH5)4- tudes of coupling constants between hydrogen and exhibits a paramagnetic shift of 45 p.p.m. relative other directly bonded nuclei should depend sigto BH4-. That such a shift is not attributable to nificantly on the fractional s-character of the bondthe Pople ring currentI6 observed in aromatic ing hybrid of the directly bonded nucleus. Thus, systems is shown by comparison of the BI1 reso- coupling constants between hydrogen and sp? boron nances of C ~ H S B ( O Hand ) ~ CGHBB(OH)~ in which should be larger than those hetween hydrogen and substitution of phenyl for %-butylon boron produces sp3boron. The results of Table I suggest that such a trend a paramagnetic shift of only 0.9 p.p.m. Also, the magnitude of the BH4--B(C&)4shift is far exists in the boron hydrides. The bonding hybrid too large to be accounted for on the basis of a ring of boron in BH4- is sp3, and for this compound a current shift. The large shift might largely reflect B1'-H1 coupling constant of 81 C.P.S. is observed. the electronegativity differences between CcHs- The structurelgJO and electronic spectnrm2' of and H-. The chemical shifts of B(C6&)4- and borazole indicate that the electronic configuration of boron in this molecule may be described by BF4- are almost identical. The chemical shifts for boron in BC13, BBr3 and pzsp2,with the B-H u-bond consisting of a hydrogen BT3 parallel the electronegativities of the halogens. Is orbital and a boron sp2hybrid orbital. For boraHowever, the boron chemical shift for BF3 is be- zole the BII-H1 coupling constant is 136 c.p.s , and tween those of BBr3 and BI3. A possible explana- for N-trimethylborazole, 134 C.P.S.B1'-H1 coupling tion, first proposed by Onak, et al.,'O of this apparent constants found in BH3 adducts with ethers and anomaly is significant "hack coordination" to the amines (Table 1) lie between the 136 C.P.S.value boron p. orbital from the non-bonding electrons of for borazole and the 81 C.P.S. value for BH4-. the fluorine atoms. The n.rn.r. resuIts and interpre- Configurations and boron quadrupole coupling tations are consistent with arguments for back constants in BH3 adducts have been interpreted2? coordination based on the shortening of B-X bond in terms of boron hybridization intermediate bedistances in BX317aand the conclusion of Brown and tween sp2 and sp3. The magnitudes of B"-H' Holmes, 15b among others, based on the observation coupling constants of BH3 adducts as interpreted that BF3 is a weaker acceptor molecule than BBr3 in terms of the s-character of the boron bonding and BC13, a t least with respect to pyridine and to hybrid are seen to be qualitatively consistent with nitrobenzene. The borate esters have resonances conclusions regarding boron hybridization as clesimilarly shifted from the low field resonance posi- rived from other studies. IV. Resonances of Boron Hydrides.-B" chemtion expected solely on electronegativity considerations. In view of this shift and since the esters do ical shifts and B1I-H' coupling constants for display a rather low order of acceptor character, several boron hybrides are given in Table 111. and R3Hsit would appear that back coordination from the The spectra of all except .B?HB.N(CH3)2 non-bonding electrons of the oxygen atoms to the previously have been reported ant1 disboron p. orbital is present to a significant degree in (18) X-Ray diffraction studies have. in f a c t , shown boron to be nearly tetrahedral in several boron halide complexes, e x . , < F D F and the esters. < N B F are 107 and 1 1 Z 0 , respectively, in (CHa!sNBFt ( S Geller and 11. Resonances of BF3 and its Adducts.-B1' J. L. Hoard, Acta Cryst., 4, 359 (1951)). resonances of BF3 and BF,- and some adducts of (19) S Bailer. THISJOURNAL, 6 0 , 524 (1938). (1G) J. A . Pople, Pmc. R o y . SOC.( L o r d o t i ) . A239, 550 (1957). (17) (a) N.V. Sidgwick, "The Chemical Elements and Their Compounds." Vol. I. Oxford m i x . . P r e s Tmndon. l9SO. p 392: (b) H . C Brown and R . R . Holmes,
T H I S ]Ol.KNAL,
76. 2172 11956).
(20) A . Stock and R. Wierl, Z. G ~ O Y E .Chent., 203, 228 (1531). (21) C . C. J. Rnothaan and R. S Mulliken, J Chem. Phvs , 16, 11s (1948). 1221 T.P. I l a s , i h i d
,
27, 1 11557).
X BORON-11 MAGNETIC RESOXANCE STUDYOF BORONCO~IPOUNDS
Sept. 5 , 1959
cussed, 1-3,7,9,10 although in several cases calibrations were not given. I n most instances, assignment’J of resonances in these compounds to the proper boron atoms has followed directly from analysis of resonance intensity and multiplet structure. For and B5H113v9 some ambiguity exists in analysis of the spectra in terms of the structures deduced from X-ray studies, and assignment has been made on the basis of an assumed correlation between the resonance field and calculated electron densities a t boron atoms. However, as Schaeffer, Shoolery and Jones3 have pointed out and as was suggested by the previous discussion of chemical shifts in this study, no good theoretical or experimental reasons exist for believing boron chemical shifts should be dominated by diamagnetic shieldings. Coupling constants between boron and “conventional,” ;.e., non-bridged, hydrogen atoms in the boron hydrides vary between 129 and 176 C.P.S. (Table 111). If Bl1-H1 coupling constants do reflect the s-character of the boron bonding hybrid as was suggested in the previous section, boron hybrids in the boron hydrides would appear to vary between the equivalents of sp2 and sp hybrids. However, in view of the complexity of the electronic structures of the boron hydrides and in the absence of a quantitative treatment of B-H coupling constants and boron chemical shifts, any conclusions regarding the electronic structures of boron hydrides as derived from magnetic resonance data must be accepted with reservation. V. Spectra of B3Hs- and of B3H7.0(CzH5)2.The Bll magnetic resonance spectrum of an ether solution of NaB3H8 is presented in Fig. 2. The
Fig. 3.-Possible
4499
structure of BnH,-.
I1 hydrogen would be replaced by the oxygen atom of the ether molecule. R. W. Parryz3has pointed out that this structure type is inconsistent with most recent X-ray work on B3H7*N(CH3)3which indicates that this molecule does not possess a threefold symmetry axis in the solid state. W. N. Lipscombz4has suggested that the B” spectrum of B3HB- may actually consist of nine lines, with the outermost peaks being too weak for observation. Such an interpretation would be consistent with a more conventional structure for the anion, one containing non-equivalent boron atoms and conventional B-H hydrogen atoms as well as bridged hydrogen atoms, but with the equivalence of boron and hydrogen being achieved by some intramolecular tunnelling process. Such NaB3Hp / (C,H,),O an interpretation previously has been invoked by B” IO Mc./sec. Ogg and Rayz5to account for the apparent equivalence of the hydrogen atoms of Al(BH4)3. The H1 spectrum of NaB3Hs in D2O is rather diffuse but contains a discernible fine structure of spacing about 32 C.P.S. The Bll spectrum of the molecule exhibited no detectible temperature de-96 - 6 4 -32 0 32 64 96 lcps.) pendence to -60’, nor did the H1 spectrum to 90’. It would appear that spin saturation experiments Fig. 2.-B1] resonance of BsHa- in ether. and use of deuterium derivatives will be necessary spectrum appears to consist of seven components before further progress can be made in unravelling with a separation between adjacent peaks of 32 the magnetic resonance spectrum of B3Hs-. VI. BL1Temperature Dependence. a. B2Hb.NC.P.S. Quantitative intensity measurements are temperature dependence has been rendered difficult by the breadth and close spacing (CH&.-A found in the Bl1 spectrum of B*Hb.N(CH& and of the lines, but the intensities are approximately in the ratios to be expected for six hydrogen is shown in Fig. 4. It is seen that, as the temperaatoms coupled equally or nearly equally to each of ture is raised from - 10 to 40’, the BI1 resonance is three equivalent boron atoms. Similarly, the BI1 broadened and the doublet splittings almost elimispectrum of B ~ H ~ . O ( C ~ Hwhile S ) ~ , more diffuse nated. Such a temperature dependence is consistthan that of BaHs-, appears to be a symmetrical ent with some rate process possible t o the B2H5. N(CH& molecule that results in H1 exchange a t a sextuplet of spacing about 30 c.p.s. A structure of B3H8- consistent with this spec- rate of about lo2 sec.-l a t 40’. Cleavage of a trum is one of D 3 h symmetry, shown in Fig. 3 in bridge B-H bond could, through sequences parwhich each of three equivalent boron atoms is tially diagrammed below, produce the observed bonded to six “bridge” hydrogen atoms. Each of temperature dependence. Identical spectral rethe type I1 hydrogen atoms would be “bridged” sults would result through cleavage of the bridge (23) Private communication. to three boron atoms, and the molecule would con(24) Private communication. tain no “conventional” B-H bonds. The B3H75 ( 2 5 ) R. A O g g , Jr.. and J Ray. Dts, F n r n d n v S o c 19, 215 O(C2H& structure would be analogous; a type ( I 955)
-
l=&e?LJ
M:. D.
4500
PIiILLIps,
H. C. MILLERAND E. L. MUETTERTIES
Vol. s1
40" 23'
M
(0)
0 129 262 464 B" SPECTRUM OF BloH14 I N
620
-
BENZENE
h
I
I
131
IO
(bl Fig. 5.-B"
I
256
461
615
6" SPECTRUM OF B I o H I ~ I N ACETONE
resonance of BloHlr in ( a ) benzene, ( b ) acetone
ponents of relative intensities 1:2 :1 in the region between 0 and 262 c.P.s., consistent with the 367 4 97 above interpretation of the spectrum. However, Fig. 4.-Ternperature dependence of B'l resonance of the B" spectrum of BIcHla dissolved in acetone is B L H I . S [ C H ~ )10 ~ ;nic./sec.; ref. B(OCH& asymmetrical in this region, with the 256 C.P.S. R2N- group to give an intermediate terminal resonance being much weaker than the 10 C.P.S. R2N group. Since this latter sequence also neces. resonance. This asymmetry is increased a t resarily requires B-H bridge cleavages, the former duced temperature, with the 256 c.p.s. resonance sequence of only B-H bridge cleavage is the simpler nearly vanishing and the 10 C.P.S. resonance bebut not necessarily the more probable of the coming as intense as the 131 C.P.S. resonance a t the Bl1 two.?6 A quantum mechanical tunnelling similar --50°. Conversely, a t about +GOo, to that proposed by Ogg and Rayz6for A1(BH4)3 spectrum of BIoH,( in acetone becomes essentially and LipscombZ4for B3Ha- could also account for identical with that of the compound in benzene. The temperature dependence of the B" spectrum the observed spectra. Studies a t higher temperatures will be necessary to establish the kinetic of B1fiH14 in acetone suggests strong solvent-solute process responsible for the observed temperature interaction. On the Oasis of this interpretation, the spectrum of BIOHI, in benzene and in acetone a t dependence. GOo would represent the non-associated molecule. R? R? The -50' spectrum of BloHla in acetone would represent the BluHlc/acetone complex. The main effect of BIoH14-acetone complex formation then / mould be to cause the set of four boron atoms exhibiting a resonance a t 193 C.P.S. in the non-comH7 plesed form to become accidentally equivalent, 122 HI, /X\ in the n.m.r. sense, to the set of four atoms a t 70 c.p.s If this interpretation is correct, it is in13 B teresting to note that the equilibrium is slow (7 > &I' \H/ \H5 B I O H I ~ acetone f, BloH1c.acetone b. BlOH14.-A rather striking temperature dependence was noted in the B" spectra of acetone see.), since separate rather than averaged solutions of BILrHL4.Room temperature B1I spectra resonances for complesed and non-complexed of B1,Hlr dissolved in benzene and in acetone are B10H:4are observed shown in Fig. 5 While some controversy still Experimental exists regarding assignment of the B" spectrum of Materials and Method.-The compounds examined in B10H14,2~3~7.9 it appears to be probable that the this study were either purchased from commercial sources or doublet centered around 530 C.P.S. arises from a were prepared by literature procedures. I n most instances, pair of equivalent boron atoms, and those centered t h e materials were purified by standard methods. The B11 magnetic resonance spectra were obtained using a around 70 and 193 C.P.S. from two sets of equival'arian high resolution n.m.r. spectrometer and electromaglent or nearly equivalent boron atoms, each set nets7 at a frequency of 10 Mc. and a field of about 7.315 consisting of four atoms. The intensity of the gauss. Spectra were calibrated relative to the boron resopeak a t 131 C.P.S. is considered to result from nance of B(OCH& using the side band technique.m accidental overlap of components from the 70 Acknowledgments.-We are indebted t o Dr. and 193 C.P.S.doublets. V. D. Aftandilian and to Dr. W. H . Knoth who furThe room temperature BI1 spectrum of B10H14 nished us with some of the compounds used in this dissolved in benzene (Fig. 5) consists of three com- study. d
>-I,
j
I
\
237
N
+
(26) Bromodiborane and the a!kylated diboranes have the substituent groups in terminal positions T h e aminodiboranes are the only substituted diboranes in which the substituent group is known t o he in a bridge position, although i t is highly probable t h a t phosphino- and alkanethiodihoranes have the substituent group in the bridge position.
WILMIXGTON, DELAWARE (27) Varian Associates, Palo Alto, California. (28) J. T. Arnold and M. E. Packard, J . Chetn. P k y s . , 19, 6108 (1951).
