A laser desorption ionization mass spectrometry investigation of triarylboranes and tri-9-anthrylborane photolysis products

Article abstract of DOI:10.1016/j.jorganchem.2004.11.002

Laser desorption ionization (LDI) mass spectra have been obtained for the photolysis products of tri-9-anthrylborane (TAB) in the solvents toluene, cyclohexene, and tetrahydrofuran; and for the solid triarylboranes: TAB, trimesitylborane (TMB), and tri-(2,6-dimethylphenyl)borane (TXyB). The major single solution photolysis product of TAB is 9,9′-dianthryl. Other products are rationalized in terms of an anthrylborylene intermediate (AnthB:). LDI mass spectrometry of the triarylboranes yields molecular ions in good abundance, providing a simple and selective method of characterization. Fragmentation patterns in some cases are interpreted in terms of photochemical reactions prior to ionization.

Full text of DOI:10.1016/j.jorganchem.2004.11.002

Journal of Organometallic Chemistry 690 (2005) 962–971
www.elsevier.com/locate/jorganchem
A laser desorption ionization mass spectrometry investigation of
triarylboranes and tri-9-anthrylborane photolysis products
Brian G. Ramsey, Mark E. Bier *
Center for Molecular Analysis, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA
Received 3 June 2004; accepted 3 November 2004
Abstract
Laser desorption ionization (LDI) mass spectra have been obtained for the photolysis products of tri-9-anthrylborane (TAB) in
the solvents toluene, cyclohexene, and tetrahydrofuran; and for the solid triarylboranes: TAB, trimesitylborane (TMB), and tri-(2,6-
0
dimethylphenyl)borane (TXyB). The major single solution photolysis product of TAB is 9,9 -dianthryl. Other products are ratio-
nalized in terms of an anthrylborylene intermediate (AnthB:). LDI mass spectrometry of the triarylboranes yields molecular ions
in good abundance, providing a simple and selective method of characterization. Fragmentation patterns in some cases are inter-
preted in terms of photochemical reactions prior to ionization.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Borylene; Arylboranes; Laser desorption ionization; Mass spectrometry; Photochemistry
1
. Introduction
This result has precedence. The photolysis of tri-1-
naphthylborane (TNB) in hydrocarbon solvents does
form 1,1 -dinaphthyl in 5–10% yields [1,2]. There is dis-
0
In the process of closing down his laboratory, one of
us (B.G.R.) made the serendipitous discovery, that in a
cyclohexane solution of tri-9-anthrylborane or TAB, 1,
exposed for over a year to the ambient fluorescent light
of the laboratory, the ultraviolet spectrum of TAB had
agreement [2] whether the analysis of the products of
TNB photolysis in cyclohexene supports the intermedi-
acy of naphthylborylene as suggested by Ramsey and
Anjo [1]. It was agreed that a major pathway in the solu-
tion photochemistry of TNB is simple fragmentation of
TNB into neutral naphthyl and dinaphthylboryl radicals
[1,2].
Studies of arylborane photochemistry over the years
have been sparse. An early study of the photolysis of
triphenylborane in cyclohexane by Williams et al. [3]
found no hydrocarbon products; photolysis in methanol
did yield small amounts of biphenyl, attributed to pho-
tochemistry of the triphenylborane methanol complex.
This photochemistry is analogous to the extensively
studied [4] photolysis of the tetraphenylborate anion to
form biphenyl and products from another carbene ana-
0
been replaced with that of 9-(9 -anthryl)anthracene or
dianthryl in almost quantitative yield. This reaction
had been previously overlooked, because, as we have
subsequently found, dianthryl itself is photochemically
destroyed by the usual light irradiation sources even
when the light is filtered through Pyrex. The discovery
that the photolysis of solutions of TAB yields dianthryl
as a major product raises the exciting possibility that the
remaining fragment, 9-anthrylborylene (AnthB:), was
also produced.
*
log, Ph B:. Along similar lines, heterolytic boron carbon
2
Corresponding author. Tel.: +1 412 269 3540; fax: +1 412 268
bond cleavage has been reported in the photolysis of
tribenzylborane solvent complexes in ether/methanol
6897.
E-mail address: mbier@andrew.cmu.edu (M.E. Bier).
0
022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.11.002

B.G. Ramsey, M.E. Bier / Journal of Organometallic Chemistry 690 (2005) 962–971
963
mixtures where high yields of toluene were attributed
to formation of an intermediate benzyl anion, which is
then protonated by methanol [5]. The observation by
Hancock and Dickson [6] of alkyl boron carbon bond
cleavage in the photochemistry of aminoboranes (alkyl-
of ‘‘organo-borylene’’ chemistry is provided. Although
‘‘borylene’’ products have been reported in a number
of reactions, there is some question if free borylenes,
RB:, are always actual intermediates. Attempts to dem-
onstrate the formation, of MeB: in a mixture of
MeBBr , potassium-graphite laminate (C K) and ben-
(
N-methylanilino)-phenylboranes) seems to us an inter-
2
8
esting extension of the idea that photochemical boron
carbon bond cleavage is facilitated by negative charge
density at boron.
The first UV absorption maximum in the spectra of
triarylboranes such as triphenyl and trinaphthyl borane
is an intense broad band in the region between 280 and
zene, by trapping the borylene with alkynes has met with
ambiguous results. For example, efforts to reproduce the
reported [9] trapping of MeB: with alkynes, followed by
dimerization of the borirene to 1,4-dibora-2,5-cyclohex-
adienes, 4, have not been successful [10]. Eisch and
Becker [11] have reported the formation of the aromatic
borirene, 5, utilizing di-t-butylacetylene as the trapping
agent, although the structure was assigned solely on
mass spectral fragmentation data.
3
55 nm. These transitions [7] have been characterized as
intramolecular charge transfer transitions. Neglecting
configuration interaction, the transition promotes an
electron from a highest occupied p molecular orbital
(
HOMO), localized on the aromatic rings, to a predom-
inantly boron 2p lowest unoccupied orbital (LUMO).
We have utilized LDI mass spectrometry (MS) [8] to
examine the products of the photochemical reaction of
TAB in several different solvents: cyclohexene, toluene,
and tetrahydrofuran. The LDI method illustrates the
advantage of selectively ionizing only those reaction
products, from a complex mixture, absorbing at the la-
ser wavelength of 337 nm – in our case anthracene or
other aryl chromophores.
To our surprise, there appear to be no reports in the
literature of a systematic study of the mass spectra of
arylboranes or even reports of the mass spectra of such
common individual compounds such as triphenylbo-
rane. This may be at least in part due to the fact that,
in our own experience, electron ionization MS of triaryl-
boranes do not exhibit parent ion peaks. We have there-
fore continued our LDI studies on unreacted solids of
TAB, trimesitylborane (TMB) 2; and tri-(2,6-dim-
ethylpheny)borane (TXyB) 3, both to extend our under-
standing of the behavior of arylboranes under the
conditions of LDI MS and to look for evidence of laser
induced photochemistry in the melt or gas phase.
The reaction of MeBBr and C K with cyclohexene in
2
8
refluxing benzene yields as the major product the bora-
tricylotridecane, 6 [12]. Its formation was rationalized as
an allowed p2s + r2a concerted addition product of 7 to
a second molecule of cyclohexene. Boracyclopropanes
are boron analogs of the cyclopropylcation, which is
well known not to be a minimum on the potential energy
þ
surface of a C H ion. And although both semi-
3
5
empirical and ab initio calculations [13] do indicate that
a boracyclopropane is an energy minimum, it may be
overly optimistic to assume that they would survive in
refluxing benzene.
The product might be formed through a reaction ini-
tiated by a radical boron anion. Haloboration mecha-
nisms have also been suggested [9,10] as alternatives.
In any case, the ‘‘methylborylene’’ generated by MeBBr2
reaction with C K or NaK does not undergo addition to
8
C@C aromatic bonds, or C–H bonds except in benzene
chromium tricarbonyl and ferrocene [14].
There have been reported attempts with inconclusive
Since the results of LDI of TAB photolysis products,
and the triarylboranes: TAB, TXyB, and TMB are dis-
cussed in terms of photochemical reactions producing
arylborylenes (ArB), a brief review of the current state
results, to generate C
petroleum ether [15] or Li in THF [16] and to then trap
H B: from C H BBr with K in
5 6 5 2
6

964
B.G. Ramsey, M.E. Bier / Journal of Organometallic Chemistry 690 (2005) 962–971
the borylene with diphenylacetylene. The early reported
success of Joy et al. [15] could not be reproduced by
Eisch and Gonsior [16].
until the absorbance of the 474 nm CT maximum de-
creased to less than one tenth its original value. Pho-
tolysis of five mg of TAB in THF was carried out for
three days under the GE Grow Light, after which
time the 474 nm CT band of TAB was reduced to a
shoulder on the side of a new maximum at 436 nm.
The photolysis reactions of 5–8 mg of TAB in cyclo-
hexene were carried out with the Hanovia lamp for 5,
16, and 23 h. After five hours photolysis, the appear-
ance is observed of the new maxima at 416 and 438
nm. Photolysis was continued until both these new
maxima and the TAB CT maximum had largely dis-
appeared: absorbance values less than 0.1. We note
that the new absorbance maxima in sample aliquots
will disappear over the course of several hours even
without irradiation of the solution. Solvent volumes
of photolyzed solutions were reduced to 5–10 ml by
distillation under nitrogen, and spotted directly onto
a LDI plate. Spotted samples were not protected from
exposure to the atmosphere.
More recently, Grigsby and Power [17] reported that
the reaction of 2,5-dimesityl-phenyl-boron dibromide
with lithium or C K in diethylether or benzene gave
8
products of intramolecular borylene insertion into the
carbon–carbon bond of the nearby methyl group. The
resulting product was a methylborafluorene derivative.
This is the only example reported thus far of what might
be an organo-borylene carbon–carbon bond insertion
reaction.
Strikingly, different results are obtained [18] from
reactions of the diisopropylaminoboron difluoride,
(
(
Me CH) NBF , Na/K system in 1,2-dimethoxyethane
2 2 2
DME) with toluene, m-xylene, benzene, and naphtha-
lene. The products, whose structures in the case of toluene
and xylene were determined by X-ray crystallography, are
the result of multiple insertions of the (Me CH) NB: frag-
2
2
ment between the aromatic ring carbons, forming prod-
ucts in which the borons are tricoordinate.
Trimesitylborane and tri-(2,6-dimethylphenyl)borane
in cyclohexane solutions in Pyrex flasks were purged
with nitrogen and then photolyzed under nitrogen for
23 h in front of the Hanovia lamp.
2
. Experimental
2
.1. Photolysis reactions
2.2. Materials
After a 250 ml cyclohexane solution of tri-9-ant-
hrylborane was left exposed to laboratory fluorescent
lighting for over one year, the charge transfer spectrum
of TAB was largely replaced with that of 9,9 -dianthryl:
kmax nm, (absorbance), 391 (0.6), 370 (0.5), 350 (0.3),
The triarylboranes, tri-(9-anthryl)borane, trimes-
itylborane, and tri-(2,6-dimethylphenyl)borane were
synthesized by standard methods previously reported
[20]. Carbon 13 NMR spectra (67.93 MHz) of tri-9-
anthrylborane in deuterochloroform were run in the
laboratory of Florida State University by Tom Gedris.
Both broadband and off resonance decoupled spectra
were obtained. The spectrum is reported as chemical
shift ppm down field, number of H on C, ring number
assignment: 147.88, C (very weak), 9; 134.58, C, 8a;
131.40, C, 4a; 131.17, CH, 10; 129.28, CH, 1;
128.49, CH, 2; 126.07, CH, 4; 124.86, CH, 3. The
assignment at carbon 10 is unambiguous as the inten-
sity is roughly half that of other CH resonances.
0
3
34 (0.32). Solvent reduction to 20 ml, followed by chro-
matography on neutral alumina, gave a white crystalline
hexane eluent fraction, whose ultraviolet spectrum [19]
0
was that of 9,9 -bianthryl and whose LDI mass spec-
trum consisted of a mass at 354.13 u with no other mass
greater than 0.5% abundance between 100 and 2500 u.
Subsequent photochemical reactions of TAB in tolu-
ene, cyclohexene, and tetrahydrofuran (THF) were run
under nitrogen in 50–60 ml of solvent, which had been
purged with nitrogen or argon for 30 min. The reaction
vessel was a standard 100 ml one necked Pyrex round
bottom flask with a side arm Teflon stopcock. The light
source was either a General Electric (GE) 75 W Grow
Light, for plants, or a water cooled Hanovia 450 W med-
ium pressure quartz mercury vapor lamp. The emission
from the Hanovia lamp was filtered through both a Pyr-
ex sleeve and the body of a 500 ml Ace Glass Pyrex pho-
tochemical reaction vessel, limiting sample irradiation to
wavelengths longer than 280 nm. A typical final reaction
time in front of the Hanovia lamp would be 12–16 h. In
some runs, the reactions were followed by withdrawing 5
ml aliquot samples for ultraviolet spectra analysis.
In toluene, 2–5 mg of TAB was photolyzed under
nitrogen, for 2–3 days in front of the GE Grow Light
2.3. Laser desorption ionization mass spectrometry
The LDI mass spectrometer was an Applied Biosys-
tems Voyager DE sSTR (Framingham, MA). We
operated the instrument in reflectron mode for all
experiments and used anthracene, m/z 178.0777 and
0
9,9 -dianthryl product, m/z 354.1403, as internal stan-
dards for calibration. Experimental masses are all
within 0.01 u of the assigned empirical formulas. This
instrument uses a nitrogen laser at 337 nm and the la-
ser power was changed in the typical way by rotating
a neutral density wheel with computer control. The
instrument control and acquisition software version
was Voyager version 5.1 with Data Explore, Rev. A.

B.G. Ramsey, M.E. Bier / Journal of Organometallic Chemistry 690 (2005) 962–971
965
Sample preparation of arylboranes was done by dis-
solving the analyte of interest in toluene followed by
spotting 1 lL directly onto the LDI stainless steel
plate. No additional chromophore containing molecule
or acid was added to the sample. The LDI is expected
to be a multi-photon process to achieve the necessary
energy for ionization. The threshold of ionization was
defined as the point at which a spectrum was observed
with a signal-to-noise level of 10.
3
. Results and discussions
3
.1. Solutions
As illustrated in Eq. (1) for the radical cation of 14,
loss of hydrogen from ions of 14 or 15 yields a stable
carbocation, 17
Here, we report the results of a LDI MS analysis
of tri-9-anthrylborane photolysis in toluene, cyclohex-
ene, and tetrahydrofuran solutions. We expect pre-
dominately products from air autooxidation and
hydrolysis of the initial organo-boron products. Prod-
ucts containing oxygen are important because they
identify fragments originally bonded to boron. The
oxidation of organo-boranes by molecular oxygen pro-
ceeds by a free radical mechanism through peroxide
intermediates [21] such as RBOR(OOR) to yield boro-
ð1Þ
nic acid esters of the general formula RB(OR) , where
2
R equals alkyl, or aryl. These molecules may then
readily hydrolyze to alcohols and boronic acids. Inter-
pretation of the data is aided by the natural isotope
Masses can also be assigned to AnthOH, m/z 194,
and Anth-Anth-OH, m/z 370. The appearance of m/z
530 for dianthrylanthracene, Anth-Anth-Anth, is signif-
icant, suggesting rearrangement of an Anth-Anth-
B(X)Anth species where X is H, OH, or alkyl by the
same mechanism which produces dianthryl.
1
0
11
abundance of
respectively.
B and
B at 19.9% and 80.1%,
The LDI mass spectrum of photolysis products of
TAB in toluene showed masses containing boron at
m/z 312 (C H BO ), and m/z 222 (C H B(OH) )
The LDI mass spectrum of the five-hour photolysis
products of TAB in cyclohexene contained mass 318
(C H BO ) in 10% abundance relative to dianthryl
2
1
17
2
14
9
2
in addition to the predominant mass peak at m/z
54 (C H -C H ). The mass of 312 can be assigned
2
0
19
3
3
1
4
9
14
9
to any or all of the compounds 8 through 10 below,
derivable from borylene insertion into C–H or C–C
bonds.
100
354.1
2
.2E+4
AnthBðOCH
3
ÞðOC
6
H
5
Þ
AnthBðOHÞðOCH
2
C
6
H
5
Þ
8
9
AnthBðOHÞðOC
6
H
4
CH
3
Þ
10
3
55.1
The mass spectra of the photolysis products of
TAB in toluene also contain a significant series whose
masses and assigned formulas are: m/z 388,
2
88.1
3
02.1
258.1
178.1
3
70.1
318.1
434.2
558.2
C H (OH) ; m/z 387, C H (OH) ; m/z 386,
2
8
18
2
28 17
2
0
100
200
300
400
500
600
C H O(OH); m/z 385, C H O(OH); and m/z 384,
2
8
17
28 16
m/z
C H O . Isomeric structures of reasonable represen-
2
8
16
2
Fig. 1. LDI TOF MS of the photolysis products of TAB, m/z 542, in
cyclohexene after 16 h. Product ions are observed at: m/z 558,
tative initial product boranes, 11 and 12, and their
oxidation products, 13 to 16, are indicated below.
The boranes 11and 12 could be products either of
AnthB: geminate recombination with 9,9 -dianthryl,
or a photochemical rearrangement of TAB.
C
C
42
H
H
27BO; m/z 302, C20 19BO
H
2 3
; m/z 318, C20H10BO ; m/z 288,
20
16
O
2
and m/z 258, C20H18 a cyclohexenyl-anthracene. Calibration
0
masses are observed for anthracene at m/z 178 and dianthryl, the base
peak at m/z 354.

966
B.G. Ramsey, M.E. Bier / Journal of Organometallic Chemistry 690 (2005) 962–971
and the most abundant mass was 542 from TAB.
A photolysis reaction run for 16 h under the same con-
ditions, resulted in the mass spectrum shown in Fig. 1.
Because arylboronic acids and their esters (ArB(OR)2,
where R = H, Alkyl, Aryl) are not oxidized by molecular
oxygen under normal conditions, we assigned the origin
of m/z 318 (C H BO ) to structure 18 rather than
results in THF and other solvents may be the result of
the ability THF to coordinate strongly with organo-
borane intermediates and boron containing products.
These results are in agreement with those of Pachaly
and West for the reaction of (C H ) SiB: with THF,
6
5 3
where insertion of the borylene into the THF ring was
observed [23].
2
0
19
3
AnthOB(OR) . Structure 18 is also in keeping with the
2
The observed results of the solution photolysis of
TAB in toluene, THF, and cyclohexene, suggest that
the principal photolysis pathway of TAB proceeds with
the formation of an intermediate characterized by the
appearance of the new UV maxima. Known photo-
chemical reaction mechanisms such as that of anthra-
cene photodimerization [24] or the di-p-methane
rearrangement [25] combined with allowed 1,3 or 1,5
sigmatropic rearrangements could produce any of the
intermediate molecules, 11, 12, and/or 21, the internal
photodimerization product of TAB. This intermediate
free radical mechanism proposed for the oxidation of
alkylboranes by molecular oxygen. The mass at m/z
3
02 (C H BO ) is assigned to structure 19.
20 19 2
0
then, either thermally or photochemically, yields 9,9 -
Given the extraordinary resistance of TAB to oxida-
tion by molecular oxygen, assigning the mass 558
C H BO) to Anth BOAnth does not seem reasonable.
dianthryl and anthrylborylene (AnthB:) within a solvent
cage. Geminate recombination offers an alternative path
to 11 or 12 and the products in toluene (masses 388 to
384), and 20, m/z 558, in cyclohexene. Borylene reac-
tions with solvent produce the previously discussed
products. Borylene trapping by cyclohexene is faster
than by toluene.
(
4
2
27
2
We therefore prefer a structure such as 20 derived from
0
oxidation of the result of AnthB: insertion into 9,9 -
dianthryl or equivalent photochemical rearrangement.
Although we do not see any product corresponding to
that of 6 or 7 from AnthB: addition to cyclohexene, a
related intramolecular mechanism beginning with boron
carbon cleavage in the initial boracyclopropane product
can account for the appearance of m/z 258 (C H ), a
2
0
18
cyclohexenyl-anthracene, and m/z 288 (C H O ). As
2
2
0
16
in the solvent toluene, masses are observed in low abun-
dance for Anth-Anth-OH, AnthOH, anthracene, and
dianthrylanthacene (Anth-Anth-Anth). These results in
cyclohexene are in agreement with the reported isolation
utilizing gas chromatography of aqueous solutions by
Ramsey and Anjo [1] of 1,2-cis-cyclohexanediol from
the Brown [21] hydrogen peroxide work up of cyclohex-
ene solutions of tri-1-naphthylboron (TNB), photolyzed
in soft glass ampoules. The later inability of Calhoun
and Schuster [2] to find the reported cis-cyclohexanediol
product, by extraction from water may be accounted for
by the absence of the addition of mannitol required to
free the glycol [22].
Photolysis of 5 mg of TAB in THF was carried out
for three days under the GE Grow Light, after which
time the 474 nm CT band of TAB was reduced to a
shoulder on the side of a new maximum at 436 nm.
LDI mass spectra of the products contain m/z 292, a re-
sult of borylene insertion followed by oxidation, [An-
thB + THF + 2O], small amounts of 558, 20, and the
Each of the structures 11, 12, and 21 also possess a
chromophoric unit, AnthBR , which would be expected
to exhibit a UV maximum to the blue of the TAB max-
imum, accounting for the appearance of the new max-
ima at 416 and 438 nm during photolysis in
cyclohexene and 436 nm in THF.
Ring opening at a boron–carbon bond of either inter-
mediate 12, or 21, and subsequent reactions with solvent
or traces of water would also produce the necessary pre-
cursors to the trianthryl and hydroxydianthryl, observed
among the photolysis products of TAB in toluene and
cyclohexene as shown in Scheme 1.
As in the photolysis of tri-1-naphthylboron, there is a
free radical component to the solution photochemistry
of TAB. In small abundance, mass numbers are observed
for products of the addition of solvent radicals to
dinaphthyl where the solvent is: toluene, m/z 444; cyclo-
hexene, m/z 434; THF, m/z 424, and addition to anthra-
cene in THF, m/z 258. The neutral solvent radicals are
2
mass 222 of anthrylboronic acid (AnthB(OH) ). As in
2
other solvents, except for the base peak of dianthryl at
m/z 354, product masses are all less than 20% relative
abundance. Athracene itself is a more abundant product
in THF than in other solvents. Differences between

B.G. Ramsey, M.E. Bier / Journal of Organometallic Chemistry 690 (2005) 962–971
967
Anth
H
Anth B
H
hυ
Anth
Anth
B
B
Anth
Anth
O
2
H O
2
OH
hυ
HOB:
+
Anth
B
Anth
Anth
Anth
Scheme 1.
the products of hydrogen abstraction by either triplet
borylene, or dianthrylboryl and anthryl radicals.
is the absence of m/z 177 from the loss of hydrogen.
At a laser power of 20% above threshold, m/z 177 and
other ions of lower mass begin to appear in the spectrum
of anthracene. The ion of m/z 376 reaches 25%
abundance.
In common with TAB, the intense charge transfer
maxima of TXyB (316.8 nm) and TMB (331.3 nm) in
hexane disappear on irradiation from the Hanovia UV
source through a PyrexÒ filter. There is no evidence of
The ionization energy of TAB is identified, through
KoopmansÕ theorem [28], with the energy of the highest
occupied p orbital of TAB, which will be largely local-
ized on the three anthracene rings. The energy of this
orbital is determined by the competing electron releasing
inductive effect of boron, which would raise the orbital
energy, and mixing of the vacant boron 2p function into
the anthracene highest occupied p orbital, which would
lower the orbital energy, increasing the ionization en-
1
,3,5-trimethylbenzene (mesitylene) or 1,3-dimethyl ben-
zene (m-xylene) in the UV spectra of photolyzed solu-
tions. Instead the spectra are characteristic of the
hindered biphenyls rather than substituted benzenes
[
26]. The spectra exhibit intense rising end absorption
from 300 to 210 nm with one to two inflection points be-
tween 230 and 300 nm.
2
3
.2. Mass spectrometry of triarylborane solids
ergy. Assuming sp hybridization of the boron, mixing
of the vacant boron 2p orbital into the anthracene p
orbitals is limited by both the energy gap between the
functions and the fact that for steric reasons the anthra-
cene rings are rotated well out of the plane of the boron
to carbon bonds reducing the overlap of the vacant bor-
on 2p with the anthracene p orbital. The C13 NMR
chemical shifts of the 1 (129.3 ppm), 3 (124.9 ppm), 4
(126.1ppm), and 10 (131.2ppm) carbons of TAB are
all up field with respect to the corresponding C13 shifts
in anthracene, 130.1, 125.4, 130.1 and 132.6 ppm. The
chemical shift of carbon 2 (128.5 ppm) in TAB is down-
field from that of anthracene at 125.4 ppm. If we utilize
the relative chemical shifts of the number 4 and 10 car-
bons as a measure of change in p electron density, we
conclude that in the case of TAB, the boron vacant 2p
does not significantly interact with the p HOMO of
anthracene, and that the ionization potential of TAB
should be close to and probably less than that of anthra-
cene. A soft two photon ionization should therefore be
possible in TAB.
These observations in solution led us to compare the
laser desorption mass spectra of TXyB and TMB with
that of TAB to search for evidence of photochemistry
before ionization. We obtained the LDI spectra of the
three triarylboranes over a range of laser powers, vary-
ing from 2% and 5% above the threshold power, to
power levels 30% to 50% above the threshold level where
extensive ion fragmentation was observed. For compar-
ison purposes, the LDI spectra of anthracene and
5
-methylchrysene were obtained over a similar laser
power range. The vertical ionization energy [27] of
anthracene is 7.40 eV. The laser photon energy of 3.68
eV (356 kJ/mol) makes an efficient soft two photon ion-
ization process possible in which ionization occurs out
of a vibrationally excited first electronic excited state
of anthracene. At a laser power 10% above threshold
in our LDI-MS, the masses 178 (100%), 179 and 180
of anthracene, and 376 (4%) are the only masses
observed in greater than one percent abundance in the
range 50–500 u. For the later purposes of discussion
of the LDI spectra of TAB, the important observation
Two mass spectra of TAB are shown at two different
laser intensities of 2100 and 2500 in Fig. 2(a) and (b),

968
B.G. Ramsey, M.E. Bier / Journal of Organometallic Chemistry 690 (2005) 962–971
1
00
542
The ready cleavage of hydrogen from anthracene
rings in the LDI ionization of TAB requires a facilitat-
ing mechanism. One possible mechanism would cer-
tainly be photochemical generation of AnthB: which
then inserts into a C–H bond of TAB. This would be fol-
lowed by loss of AnthBH with the net result of removing
one hydrogen from TAB. At a laser intensity of 2500,
m/z 189 at 5% relative abundance (not shown) and its
corresponding isotopic peaks were identified as
C H B, AnthBH. The peak near m/z 384 in the LDI
1
449
(
a)
Laser intensity 2100
541
543
3
65
3
64
383
544
0
3
00
360
420
480
540
600
m/z
1
4
10
ionization of TAB (especially in Fig. 2(b)) appears to
be that of a metastable ion, as it is broad and unresolved
from m/z 382 to 385. The empirical formula C H B is
542
100
3.0E+4
(
b)
Laser intensity 2500
28
15 3
541
365
suggested because this keeps the anthracene rings intact
and is consistent with borylene additions to the anth-
racenyl rings of TAB. In keeping with the postulate that
543
364
540
the ion of Anth B results from photon induced fragmen-
2
384
3
63
5
44
tation of the parent ion, Anth B, the abundance of mass
3
3
52
539
557
443
365 increases to 70% with an increase in laser intensity
from 2000 to 2500 as shown in Fig. 2(b). This is not
the pattern observed for TXyB or TMB.
573
0
300
360
420
480
540
600
m/z
The ion at m/z 905 (not shown in Fig. 2(b)) reaches an
abundance of 20% at a laser intensity of 2500, and is
identified as a product of the addition of either Anth-
AnthBH to TAB with subsequent loss of one hydrogen
each from boron and an anthryl ring, or C–H insertion
of Anth-AnthB: into TAB with loss of H from boron.
The latter explanation finds support in the appearance
in the spectrum of masses at 352, 353, and 354 derivable
from dianthryl radical (Anth-Anth-). Addition of
Fig. 2. LDI TOF MS of tri-9-anthrylborane (TAB) at (a) low, 2100
and (b) high, 2500, laser power. At the higher power, increased overall
signal is observed with increased relative abundance of the fragments.
Loss of at least one hydrogen atom accounts for the change in ion
ratios observed at the distribution at m/z 365 and m/z 542. Metastable
ion production is observed by the broad peak at m/z 384.
respectively. The threshold laser intensity, 2000 in our
experiments, for ionization of TAB is found to be the
same as that for anthracene. Mass spectra with
S/N > 10 are obtained at laser intensities 5% above this
at 2100. In the representative mass spectrum in Fig. 2(a),
the following products were observed with indicated
chemical formula and mass: anthracene, C H , 178
Anth B, radical or ion, to TAB requires a subsequent
2
loss of not one, but two hydrogens from anthryl rings
to reach a m/z 905, which seems less likely.
The LDI mass spectrum of TXyB, C H B, at a laser
24 27
1
4
10
(
3
not shown); C H B, 364; C H B, 365; C H B
28 15 3
intensity 2200, just above threshold, is shown in Fig.
3(a). We make the following formula and mass assign-
ments: C H B, 205; C H B, 220; C H B, 221;
2
8
17
28 18
84; C H B 541; C H B, TAB, 542; C H B , 905
42 26 42 27 70 43 2
(
not shown). The overlapping mass spectral peaks of
1
5
14
16 17
16 18
the parent ion of TAB, 542, and TAB after loss of one
hydrogen, 541, were analyzed by fitting the observed
peaks between 540 and 544 to the sum of theoretical
spectra for TAB and [TAB-H] in varying ratios. Be-
tween a laser intensity range 2000–2200, the calculated
ratio of TAB/[TAB-H] ion abundance was typically
about two. The relative abundances of m/z 541 and
C H B, 311; C H B, 325; C H B, 326. The abun-
23 24 24 26 24 27
dance ratio of C H B, m/z 326 to C H B, m/z 325
27
2
4
24 26
was calculated to be 2.5 in a typical mass spectrum.
With an increase in laser intensity to 2700, abundances
increase sharply as shown in Fig. 3(b) and new masses
appear at C H B, 131(7) and C H B , 441 (5) (not
9
12
32 35 2
shown in Fig. 3(b)).
The appearance of the Ar B ions of TXyB and TMB
+
5
42 can also be fit by a single species C H B . This
2
7
19
2
2
+
in much greater abundance than the Ar B precursor
would require some mechanism in which a carbon is lost
from one of the anthracene rings, perhaps as the result
of borylene insertion into an anthracene ring. The mass
peaks in the spectrum of TAB at m/z 365 and 364 at low
laser intensities equate to a four to one mixture of
C H B and C H B. This interpretation would be
3
ion at m/z 326, at low laser intensity and the subsequent
increased relative abundance of the precursor ion as
+
compared to Ar B with increasing photon intensity is
2
unexpected as it is the opposite of the effect observed
with TAB. This difference in the cases of TXyB and
TMB is attributed to initial photochemical fragmenta-
tion of neutral Ar B in it lowest excited state to Ar B
2
8
18
28 17
consistent with the simple loss of one anthracene from
the TAB ions of mass 542 and 541 to form (Anth) B.
2
3
2
The structure of Anth-AnthBH is an isomeric alterna-
tive whose origin would be a photochemical product
such as 12.
and Ar neutral radicals, followed by ionization of the
Ar B radical. Ionization of TXyB and TMB are neces-
sarily an inefficient three photon process which occurs
2

B.G. Ramsey, M.E. Bier / Journal of Organometallic Chemistry 690 (2005) 962–971
969
1
00
3963
ions at masses 240 and 239, but not 237. At a laser inten-
sity of 3000, mass 239 (70) is the second most abundant
ion present and mass 237 has an abundance of 15%. We
conclude, therefore, that the loss of a methyl group from
the benzene rings of TXyB requires some mechanism
other than simple fragmentation from an exited state
of TXyB or its ion.
(
a)
220 221
Laser intensity 2200
325 326
219
222
311
10
327
205
3
The presence, although in low abundance, of
XyBCH (131) in the mass spectra, suggest XyB: inser-
0
1
50
200
250
300
350
400
3
m/z
tion into a methyl to carbon bond of TXyB in a melt
or solid phase photochemical reaction followed by loss
of XyBCH . The steric requirements, however, around
220
100 (b)
Laser intensity 2700
5
.4E+4
3
2
21
22
3
25
the ortho positions of TXyB are unfavorable. Alterna-
tively, we suggest a mechanism such as photochemical
coupling across the 2,2 positions of the benzene rings
219
326
3
11
2
0
2
05
of TXyB to yield 22, photoionization with loss of methyl
to form 23 (mass 311), loss of xylyl, 1,3 sigmatropic
migration of methyl to boron, and finally loss of hydro-
gen from methyl with ring expansion to 24 (mass 205),
containing the aromatic tropylium ring, (Scheme 2).
The methyl migration is symmetry allowed suprafacial
if photochemical, and antarafacial or suprafacial with
inversion if from a ground state.
3
27
2
23
310
297
300
2
04
235
2
48
0
150
200
250
350
400
m/z
Fig. 3. LDI TOF MS of tri-(2,6-dimethylpheny)borane (TXyB) at (a)
low, 2200 and (b) high, 2700, laser power. At the higher power, the
overall signal and fragmentation is increased, but with the unexpected
Ionization of the neutral dixylylboryl radical Xy B, a
2
+
increase in the relative abundance of Ar
3
B
at m/z 326. This relative
product of photochemical cleavage of TXyB, readily ac-
counts for the appearance of mass 221 among the prod-
ucts. The appearance of mass 220 as the most abundant
ion in the spectrum again, however, suggest an energet-
ically favorable loss of hydrogen to form the ion 25 con-
taining the aromatic tropylium ion structure, Eq. (2)
increase in the precursor ion over the fragmentation products is
explained by the improved ionization efficiency of the parent at higher
laser powers. The loss of at least one hydrogen atom accounts for the
change in ion ratios observed at the distribution at m/z 220. The onset
of metastable ion production is observed by the broad peak at m/z 232.
from a molecular Rydberg, rather than lowest excited,
state of the molecule. With increasing laser intensity,
ionization of Ar B becomes more competitive with this
3
fragmentation (see Fig. 3(a) and (b)).
Since the loss of methyl, mass 15, from the parent
TXyB, 326, to give the ion of mass 311 was unexpected,
the LDI spectrum of 5-methylchrysene was measured as
a convenient model. At a threshold laser power of 2600,
the m/z (% relative abundance) observed are 241 (12),
ð2Þ
The low abundance of masses in the LDI mass spec-
trum of TXyB at 441 and 546 can be regarded as addi-
tion products of XyB: or (Xy) B to TXyB, respectively,
followed by the loss of one hydrogen, either from boron
242 (100), 248 (8), and 255(10). Increasing the laser
intensity to 2800 produces the appearance of additional
2
Xyl
B
Xyl
B
hυ
hυ
+
CH3
B
+
e-
3
2
2
23
Xyl
B
B
B
+
+
+
H
2
4
Scheme 2.

970
B.G. Ramsey, M.E. Bier / Journal of Organometallic Chemistry 690 (2005) 962–971
or the benzene ring. Alternatively, the loss of one xylyl
from the ions of mass 546 produces an ion of mass
hBH-C H -BAnth would then carry away one hydro-
14 8 2
gen from TAB.
4
41. We ran an LDI mass spectrum on a mixture of
Ionizations of trimesitylborane, TMB, and tri-(2,6-
dimethylphenyl)borane, TXyB, occur by a three photon
process out of high lying molecular Rydberg states. This
necessarily inefficient process must compete with photo-
chemical fragmentation of the first electronic excited
states of neutral TXyB and TMB into aryl and diarylbo-
TXyB and TAB, and observed cross product ions of
mass 763 (Xy B-TAB) and 689 (Anth B-TXyB). These
bimolecular reactions may be taking place in a melt
prior to vaporization rather than in the gas phase.
2
2
0
The additional presence of a methyl group in the 4, 4
00
and 4 positions of TMB leaves the LDI mass spectrum
of TMB unchanged with respect to that of TXyB, in that
ion masses observed for TMB have a one to one
correspondence to those of TXyB except for an addition
of 14 u.
ryl radicals, which then undergo subsequent photoioni-
+
zation to (Aryl) B ions. Ions exhibiting loss of methyl
2
groups are attributed to rearomatization, by methyl
loss, after ortho photochemical cross coupling of ben-
zene rings in neutral TXyB and TMB. The appearance
in low abundance of m/z 131 in spectra of TXyB and
m/z 145 in TMB spectra, corresponding to ArylBMe of-
fer an alternative borylene insertion mechanism for
methyl loss. A large abundance of ions with masses
one less than a parent ion is attributed to facile loss of
hydrogen from methyl followed by ring expansion to
an aromatic tropylium ion structure.
4
. Conclusions
LDI mass spectrometry serves as a useful method for
the selective analysis of products from a complex reac-
tion mixture of the photolysis products of tri-9-
anthrylborane. LDI mass spectrometry also proves to
be a unique and convenient method for obtaining aryl-
borane mass spectra with prominent molecular ion
peaks for the purposes of molecular weight and struc-
ture determination.
Photolysis of tri-9-anthrylborane (TAB), in solvents
cyclohexene, toluene, and tetrahydrofuran, produces
9
goes addition or insertion reactions with solvent mole-
cules. The products are similar to, but not always
identical with, products reported in other ‘‘borylene’’
reactions. In the formation of dianthryl and other biar-
yls during photolysis, new bond formation occurs across
Acknowledgments
We thank Dr. Manimalha Balasubramani for the
acquisition of some of the MS data while in MEBÕs
lab. We also thank NSF (CHE-9808188) and Carnegie
Mellon University for support in the purchase of the
MALDI TOF MS.
0
,9 -dianthryl and 9-anthrylborylene which then under-
References
[
[
[
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2
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4
9
[
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(
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