1,8-Bis(dimethylamino)-2-(4-methoxyphenyl)naphthalene: An electrospray-active analogue for η6-coordinating ligands

Article abstract of DOI:10.1016/j.jorganchem.2012.07.009

1,8-Bis(dimethylamino)-2-(4-methoxyphenyl)naphthalene (1) was synthesized for use as an η⁶-coordinating ligand with a chargeable tag for ESI-MS analysis. The pK_a of the proton sponge moiety was essentially unaffected by addition of the anisole group (pK_a = 18.2 ± 0.7 at 297 K in CD₃CN). Cr(η⁶-1)(CO)₃ (2) was prepared to demonstrate the utility of (1) as an ESI tag and 2 was easily visible by ESI(+)-MS. A single crystal structure of the complex showed η⁶-binding of the ligand as expected with the charged moiety being far removed from the metal centre. The gas-phase reactivity of 2 was found to match that of the analogous uncharged chromium carbonyl complex Cr(C ₆H₅OCH₃)(CO)₃ indicating that 1 has potential as a chargeable tag for the study of organometallic complexes and their reactions by ESI-MS.

Full text of DOI:10.1016/j.jorganchem.2012.07.009

Journal of Organometallic Chemistry 716 (2012) 252e257
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
1,8-Bis(dimethylamino)-2-(4-methoxyphenyl)naphthalene:
An electrospray-active analogue for h
6-coordinating ligands
,
,
*
Krista L. Vikse a 1, Samantha Kwok a, Robert McDonald b, Allen G. Oliverc, J. Scott McIndoe a
a Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, BC V8W3V6, Canada
b X-Ray Crystallography Laboratory, Department of Chemistry, University of Alberta, Edmonton, AB, T6G 2G2, Canada
c Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA
a r t i c l e i n f o
a b s t r a c t
1,8-Bis(dimethylamino)-2-(4-methoxyphenyl)naphthalene (1) was synthesized for use as an h6
-
Article history:
Received 8 May 2012
Received in revised form
2 July 2012
coordinating ligand with a chargeable tag for ESI-MS analysis. The pKa of the proton sponge moiety
was essentially unaffected by addition of the anisole group (pKa ¼ 18.2 ꢀ 0.7 at 297 K in CD3CN).
Cr(h
6-1)(CO)3 (2) was prepared to demonstrate the utility of (1) as an ESI tag and 2 was easily visible
Accepted 6 July 2012
by ESI(þ)-MS. A single crystal structure of the complex showed
h
6-binding of the ligand as expected
with the charged moiety being far removed from the metal centre. The gas-phase reactivity of 2 was
found to match that of the analogous uncharged chromium carbonyl complex Cr(C6H5OCH3)(CO)3
indicating that 1 has potential as a chargeable tag for the study of organometallic complexes and their
reactions by ESI-MS.
Keywords:
Proton SpongeÒ
Electrospray ionization
Mass spectrometry
Ó 2012 Elsevier B.V. All rights reserved.
(p-arene)chromium complex
Chargeable tag
1. Introduction
already charged in solution (or contain an easily charged site) can
be detected. While this last point allows for very low detection
While electrospray ionization mass spectrometry (ESI-MS) is
perhaps most widely known for its ability to manipulate and
analyze large biomolecules, the technique is also steadily gaining
recognition as a valuable tool for the study of organometallic
complexes and their reactivity. The first experiments in this area
were by Berman in 1991 in which ESI-MS was used to study envi-
ronmentally relevant organoarsenic ions [1]. Since then, ESI-MS has
been used to identify and monitor organometallic ions that play
key roles in a variety of catalytic transformations including oxida-
tion [2e5], hydrogenation [6e8], hydrosilylation [9,10] and
carbonecarbon bond forming reactions [11e20]. ESI is especially
well-suited to the study of organometallic catalysis because it
allows charged species to be observed (1) at very low concentra-
tions, (2) in complex mixtures and (3) in polar or non-polar
solvents [21]. In addition, electrospray ionization (ESI) is a “soft
ionization” technique meaning that little or no fragmentation of
fragile organometallic ions is observed, and only species that are
limits (most common solvents are neutral and therefore invisible to
ESI-MS), it also causes a problem when the organometallic species
of interest is not charged. In some cases it is possible to study these
formally neutral complexes by exploiting facile ionization path-
ways that are commonly available to organometallic complexes; for
example, protonation of a basic site, loss of a halide, or oxidation of
a low valent metal. However, many interesting species do not have
an easily accessible ionization pathway, or when they do spectra
may be severely complicated by multiple competing pathways.
To address this problem, charged “tags” have been employed to
make otherwise neutral organometallic complexes amenable to ESI-
MS. Quaternary ammonium [22e25], pyridinium [26], phospho-
nium [27,28] or sulfonate [7,29,30] bearing ligands are common, and
tags are often derived from ligands designed for water solubility.
Another option is to use chargeable tags. Chargeable tags are
neutral molecules that contain a functional group of interest, and at
a secondary site bind either and acid or a base to become charged.
In addition to the benefits conferred by charged tags, chargeable
tags (1) minimize any effect the charged group may have on reac-
tivity (since the charged tag is in equilibrium with its neutral
counterpart), and (2) can be designed so that their solubility is pH
dependant, thus making them potentially useful for catalyst
recovery. The first to use such a tag was Canary in 1994 when he
investigated the intermediates involved in the Suzuki reaction by
Abbreviation: PS, Proton SpongeÒ
ionization.
; EDESI, energy-dependent electrospray
* Corresponding author. Tel.: þ1 250 721 7181; fax: þ1 250 721 7147.
E-mail address: mcindoe@uvic.ca (J.S. McIndoe).
Present address: School of Chemistry, The University of Melbourne, Victoria
1
3010, Australia.
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.07.009

K.L. Vikse et al. / Journal of Organometallic Chemistry 716 (2012) 252e257
253
employing pyridyl bromide as a substrate [31]. The substrate was
easily charged through protonation of the ring nitrogen and its
complexes were readily observable by ESI-MS.
atmosphere,
{1,8-bis(dimethylamino)naphthalene-2-yl}bromide
(5.00 g, 0.017 mol) and 4-methoxyphenylboronic acid (2.74 g,
0.018 mol) were dissolved in n-propanol (30 mL) with stirring.
Distilled water (6 mL), sodium bicarbonate (10 mL, 2 M), palladiu-
m(II) acetate (0.034 g, 0.051 mmol), and triphenylphosphine (0.21 g,
0.8 mmol) were added consecutively and the mixture was refluxed
overnight. The reaction mixturewascooledto room temperature and
quenched by the dropwise addition of deionized water (21 mL) with
stirring in air. The mixture was extracted twice with ethylacetate and
the organic layer was washed with sodium bicarbonate (5% w/v, 2ꢃ)
and brine (2ꢃ), and dried with sodium sulfate and activated charcoal
(3 g). The mixture was filtered through a 3 cm bed of celite and the
filtrate was reduced by rotary evaporation. Hexanes were added to
yield the product as a white ppt in 41% yield (2.27 g). 1H NMR
(300 MHz, CDCl3) dH 1: 7.50e6.97 (m, aromatic protons, 9H), 3.88 (s,
OCH3, 3H), 2.78 (s, N(CH3)2, 6H), 2.63 (s, N(CH3)2, 6H). 13C NMR
(400 MHz, CDCl3) dC 1: 158.3, 137.3, 130.5, 130.2, 125.5, 123.1, 122.5,
113.7 (aromatic carbons); 55.5 (OCH3); 45.4 (N(CH3)2); 45.1
(N(CH3)2). ESI-MS (MeOH) m/z: 321.1983([1 þ H]þ, model C21H24ON2
321.1967). mp: 105e107 ꢁC.
We have developed a number of chargeable ligands derived
from the strong base 1,8-bis(dimethylamino)naphthalene also
known as “Proton SpongeÒ” (PS) that serve as analogues for
commonly used organometallic ligands and are especially suited to
act as chargeable tags [32,33]. First studied by Alder and coworkers,
Proton SpongeÒ is a very strong, non-nucleophilic base (pKa ¼ 12.34
in H2O) which acts to selectively bind Hþ by forming intramolecular
hydrogen bonds with both nitrogen centres in the molecule to give
the charged [N/H/N]þ moiety. The high basicity of Proton
SpongeÒ is thermodynamic in nature and arises from steric clash-
ing of the methyl groups on the two nitrogen atoms which causes
the nitrogen lone pairs to point toward each other. The lone
pairelone pair repulsion in turn leads to a twisted naphthalene
backbone. Protonation of the neutral molecule returns the system
to a more optimal geometry and forms strong intramolecular
hydrogen bonds between the proton and both nitrogen lone pairs.
Steric crowding of the basic site by the methyl groups is severe
and gives Proton SpongeÒ its non-nucleophilic tendencies: only
proton can fit into the small pocket between nitrogen atoms. The
overall effect is that Proton SpongeÒ is approximately six orders of
magnitude more basic than typical aromatic amines, and can
selectively trap adventitious protons to become charged.
2.3. Preparation of 1,8-bis(dimethylamino)-2-(4-methoxyphenyl)
naphthalene hydrofluoroborate ([1H]þ[BF4]ꢂ)
1 (0.0020 g, 4.9
mmol) was dissolved in ethanol and tetra-
Arenes are used extensively as
h
6-ligands in organometallic
fluoroboronic acid (0.004 mL, 48% in water) was added dropwise.
Slow evaporation of solvent yielded colourless crystals (98%,
0.0039 g). 1H NMR (300 MHz, CDCl3) dH [1H]þ: 19.4 (s, Hþ, 1H),
8.00e6.97 (m, aromatic protons, 9H), 3.79 (s, OCH3, 3H), 3.04 (s,
N(CH3)2, 6H), 2.79 (s, N(CH3)2, 6H).
chemistry. Their ability to change hapticity and expose an active
metal centre makes them excellent ligands for catalyst precursors
[34]. They have been used in hydrogenation and hydration reac-
tions, olefin metathesis, cycloaddition and polymerization reac-
tions [34,35]. For this reason the electrospray-active analogue of
methoxybenzene was selected as a target compound. Herein we
report the synthesis of the novel ligand 1,8-bis(dimethylamino)-2-
(4-methoxyphenyl)naphthalene (1) and demonstrate its usefulness
as an electrospray tag by investigating the unimolecular gas-phase
reactivity of its chromium carbonyl complex (Cr(CO)3(1)) (2).
2.4. pKa determination of 1: 1H transprotonation experiments [36]
Equimolar amounts of [PS$H]þ[BF4]ꢂ (0.0040 g, 0.013 mmol)
and 1 (0.0042 g, 0.013 mmol) were dissolved together in dry CD3CN
(0.7 mL), allowed to stand overnight, and a 1H NMR spectrum was
recorded. The experiment was repeated four separate times. The
average integrations for the coordinated proton of the two
protonated species were (CD3CN, 300 MHz) dH: 19.49 (1$Hþ, !1.00),
18.67 (PS$Hþ, !1.01) (Std. Dev. ¼ 0.04, % RSD ¼ 4). The first pKa of 1
(CD3CN) was determined by calculation using the ratio of the
integrations of the two peaks and the known pKa of PS (18.18 in
CH3CN) [37] Analysis of the equilibrium gave Ka1 ¼ KaPS/Keq where
KaPS (known) ¼ [PS][Hþ]/[PS$Hþ] ¼ 1.514 ꢃ 1018, Ka1 ¼ [1][Hþ]/
2. Materials and methods
2.1. General information
Dry solvents were obtained from a solvent purification system. All
solventswere HPLCgrade. ReagentswerepurchasedfromAldrichand
used without further purification except for Proton SpongeÒ which
was recrystallized from hot methanol before use. Reactions per-
formed under nitrogen were carried out using standard Schlenk
techniques. All electrospray mass spectra were collected using
a Micromass Q-TOF micro instrument. Capillary voltage was set to
2900 V, source and desolvation temperatures were at 80 ꢁC and
150 ꢁC, respectively. Samples were infused via syringe pump at
[1$Hþ] and Keq (exptl.)
¼
[1$Hþ][PS]/[1][PS$Hþ]
¼
(!1$H/
!PS$H)2 ¼ 0.98 [38]. Ka1 ¼ 1.514 ꢃ 1018/0.98 ¼ (1.54 ꢀ 0.06) ꢃ 1018
pKa of 1 ¼ 18.2 ꢀ 0.7 in CD3CN.
.
2.5. Preparation of chromium(1) tricarbonyl (2)
10
trometer. Chemical shifts are quoted in ppm using internal references
ofCHCl3 (1H 7.26ppm)orCH3CN(1H
1.94 ppm) where appropriate.
m
L minꢂ1. NMR spectra were recorded on a Bruker AC-300 spec-
Under a nitrogen atmosphere, dry THF (0.13 mL) and dibuty-
lether (2.5 mL) were added to 1 (0.1005 g, 0.314 mmol) and chro-
mium hexacarbonyl (0.0521 g, 0.233 mmol). The mixture was
refluxed for 24 h. The air-sensitive solution was quickly filtered in
air and the solvent was removed by rotary evaporation to yield
a yellow oil (28%, 0.030 g). Product was recrystallized under inert
d
d
Melting points were recorded on a Gallenkamp Melting Point Appa-
ratus and are uncorrected. IR spectra were recorded using a solution
cell in a Perkin Elmer Spectrum 1000 FT-IR spectrometer. X-Ray
crystallographic data was collected by Dr. Robert MacDonald
(Universityof Alberta)andDr. AllenOliver(UniversityofNotreDame).
See Supplementary information for details.
2.2. Preparation of 1,8-bis(dimethylamino)-2-(4-methoxyphenyl)
naphthalene (1)
Scheme 1. Synthesis of 1. a) NBS/THF/ꢂ78 ꢁC. b) Pd(OAc)2/PPh3/NaCO3H/n-propanol/
water. Reflux 12 h.
{1,8-Bis(dimethylamino)naphthalene-2-yl}bromide was synthe-
sized according to related literature [33]. Under
a nitrogen

254
K.L. Vikse et al. / Journal of Organometallic Chemistry 716 (2012) 252e257
Fig. 3. ESI(þ)-MS of 1 ([1 þ H]þ ¼ m/z 321) in dichloromethane spiked with formic
Fig. 1. Single crystal X-ray structure of 1,8-bis(dimethylamino)-2-(4-methoxyphenyl)
naphthalene (1). Side- (top) and edge-on (bottom) perspectives of the naphthalene
rings.
acid.
coupled with 4-methoxyphenylboronic acid via a Suzuki cross-
coupling reaction (Scheme 1).
atmosphere from a solution of toluene by evaporation. 1H NMR
(300 MHz, CDCl3) dH 2: 7.47e7.05 (m, aromatic protons, 5H), 5.74
(d, anisole aromatic protons, 2H), 5.20 (d, anisole aromatic protons,
2H), 3.78 (s, OCH3, 3H), 2.74 (s, N(CH3)2, 6H), 2.73 (s, N(CH3)2, 6H).
nCO bands: 1964 cmꢂ1, 1890 cmꢂ1. ESI-MS (MeOH) m/z: 457.1224
([2 þ H]þ, model C24H24O4N2Cr 457.1220). mp: 167e169 ꢁC.
After aqueous workup, the white crystalline product was iso-
lated in 41% yield, and a single crystal structure of 1 was obtained
(Fig. 1). In its unprotonated form the Proton SpongeÒ moiety
deviates from planarity as expected due to mutual repulsion of
the lone pairs on each nitrogen atom. This can be seen in the torsion
angles of ꢂ8.78ꢁ for N(1)eC(11)eC(20)eC(19) and ꢂ9.63ꢁ for N(2)e
C(19)eC(20)eC(11). The anisole group lies slightly off perpendic-
ular to the plane of the naphthalene ring (torsion angle: C(13)e
C(12)eC(21)eC(22) ¼ 107.50ꢁ). We expected metal coordination
to occur on the side of the arene ring that is furthest from the
methyl groups on Proton SpongeÒ.
3. Results and discussion
3.1. Synthesis and characterization of 1
1 was synthesized via bromination of Proton SpongeÒ at the two
position as previously described by Farrer [33] to give {1,8-
bis(dimethylamino)naphthalene-2-yl} bromide which was then
The approximate pKa of 1 was determined by NMR trans-
protonation experiments [39]. A deuterated acetonitrile solution
Fig. 2. : 1H NMR spectrum (18e20 ppm) of an equimolar mixture of [PS$H]þ and 1 in deuterated acetonitrile.

K.L. Vikse et al. / Journal of Organometallic Chemistry 716 (2012) 252e257
255
to that of Proton SpongeÒ (18.2 ꢀ 0.7 at 297 K in CD3CN). Fig. 2
shows the relative intensities of the two signals for one of the
NMR experiments.
The ESI mass spectrum of 1 in dichloromethane (and spiked
with formic acid) is dominated by the [M þ H]þ peak as expected.
There are no dimers, no coordination of sodium or potassium ions,
and no doubly-charged species. The complete dominance of
a single ionization pathway and the excellent signal intensity
exhibited by 1 demonstrates its potential as an ESI tag (Fig. 3).
An energy dependent ESI (EDESI) experiment [40] was per-
formed on 1 in which the cone voltage in the ESI source was raised
in a stepwise manner to induce fragmentation. Spectra collected at
each cone voltage are compiled into a two-dimensional contour
plot featuring the mass-to-charge ratio on the horizontal axis and
the cone voltage on the vertical axis. A single spectrum showing
data summed over all collision voltages is shown above the contour
plot. Using this format the fragmentation behaviour of 1 at all
collision voltages can be viewed in one compact image. The
resulting contour plot shows that the compound begins to frag-
ment at a cone voltage of approximately 45 V with loss of a 15 Da
fragment. This is consistent with loss of one methyl group as
a neutral radical (Fig. 4): a fairly high-energy process consistent
with the elevated cone voltage.
Fig. 4. EDESI of 1.
containing equimolar amounts of unprotonated 1 and protonated
Proton SpongeÒ ([PS$H]þ[BF4]ꢂ) was allowed to come to equilib-
rium overnight and a 1H NMR experiment was performed. The
solubility of 1, PS, [1$H]þ[BF4]ꢂ and [PS$H]þ[BF4]ꢂ in acetonitrile
were all carefully tested beforehand since any insolubility would
lead to inaccurate signal intensities and therefore an inaccurate pKa
value. The high chemical shift for the coordinated proton in both
compounds appears in the low-field region of the spectrum at
around 19 ppm where there are no interfering signals providing us
with an ideal NMR handle. Two peaks were seen: one at
3.2. Application of 1 as a chargeable tag for ESI-MS analysis
To further test the suitability of 1 as an ESI probe, the h
6-coor-
dinated chromium tricarbonyl complex of 1 was synthesized and
studied by ESI-MS. Chromium hexacarbonyl in THF and dibutyl
ether was reacted with 1 under reflux for 24 h [41]. After workup
and recrystallization, yellow rod-like crystals were obtained and
the crystal structure of chromium(1) tricarbonyl (2) was solved
(Fig. 5).
d
19.49 ppm corresponding to [1$H]þ and the other at
d 18.67 ppm
corresponding to [PS$H]þ. The size of the integrations for the NMR
signals corresponding to [1$H]þ and [PS$H]þ were averaged over 4
separate experiments. Using the average ratio of the integrations
and the reported pKa of PS (18.18 in CH3CN) [37], the Ka of 1 was
calculated to be (1.50 ꢀ 0.06) ꢃ 1018. The pKa is approximately equal
The crystal structure shows the expected h
6-coordination of the
arene ligand where the metal centre binds to the face of the arene
that is pointing away from the nitrogen groups of the proton
Fig. 5. Single crystal X-ray structure of chromium(1) tricarbonyl (2).

256
K.L. Vikse et al. / Journal of Organometallic Chemistry 716 (2012) 252e257
sponge moiety. This is ideal since the portion of the complex
bearing the charge is far-removed from the reactive chromium
centre and is unlikely to interfere with reactivity. The COeCreCO
angles of 88.1ꢁ, 88.7ꢁ and 90.3ꢁ are typical for a chromium tri-
carbonyl complex (87e92ꢁ) [42] indicating that steric perturbation
of the complex due to addition of the chargeable group is minimal.
Solution phase data for 2 is consistent with the crystal structure;
the proton NMR spectrum displays signals consistent with h6
-
coordination of the ligand and the IR spectrum displays CO
stretches at 1964 cmꢂ1 and 1890 cmꢂ1 (compared to 1978 cmꢂ1 and
1908 cmꢂ1 for Cr( 6-anisole)(CO)3) [43].
h
The ESI(þ)-MS of 2 shows that the proton sponge tag behaves as
expected. The spectrum is clean and dominated by the [2 þ H]þ
signal (Fig. 6). No competing ionization processes are active. Some
uncoordinated ligand is observed (m/z 321) perhaps indicating that
the anisole ligand is labile in solution or that the areneeCr inter-
action is relatively weak and some fragmentation of the compound
occurs during the ESI process.
Fig. 7. EDESI(þ)-MS of 2 in dichloromethane spiked with formic acid. Scan time ¼ 2 s.
CO dissociation (373 m/z) and ligand fragmentation (357 m/z) pathways are observed.
Further information regarding the structure and reactivity of 2
may be obtained through an EDESI experiment. In this case the
experiment reveals that there is a threshold cone voltage of
approximately 27 V at which all three CO groups are lost simulta-
neously (Fig. 7). While perhaps unintuitive, these results are
consistent with earlier EI-MS studies by Dyson and McGrady in
which the dominant fragmentation pathway for a number of
similar chromium carbonyl complexes was concerted loss of all
three carbonyl groups [44]. They also noted much smaller peaks
corresponding to the di- and mono-carbonyl species, and on closer
examination these peaks are observable in our spectrum as well:
[Cr(1)(CO)2 þ H]þ ¼ m/z 429 and [Cr(1)(CO) þ H]þ ¼ m/z 401. Since
the fragmentation we observed matches with that found in the
literature, we are further convinced that 1 is a suitable ESI-active
probe.
distinct types of methyl group present, and the propensity for CeH
(and even CeN or CeC) activation by the Cr will be high following
loss of all 3 CO ligands. Given the non-involvement of the deuteron,
CeN and CeH activation of the closest NMe2 group and reductive
elimination of CH4 seems the most probable pathway. What is
important to note is that use of this ligand as an ESI tag is not
advisable at high collision energies; however, this advice generally
At higher cone voltages (>40 V) decomposition of the ligand
occurs with loss of methane (16 Da). In an attempt to further
understand this decomposition process CID experiments were
performed on [2 þ H]þ and deuterium labelled [2 þ D]þ. Loss of
16 Da is observed as the major decomposition pathway (after CO
dissociation) in the presence and in the absence of deuterons
(Fig. 8). This outcome suggests that the fragmentation process does
not involve loss of the acidic proton, or a loss of 17 Da would be
expected when deuterium was incorporated. It is unclear what the
mechanism of this fragmentation might be e there are three
Fig. 8. (Top) ESI(þ)-MS/MS of 2 in methanol spiked with formic acid. (Bottom) ESI(þ)-
MS/MS of 2 in d1-methanol spiked with formic acid.
Fig. 6. ESI(þ)-MS of 2 in methanol ([2 þ H]þ ¼ m/z 457).

K.L. Vikse et al. / Journal of Organometallic Chemistry 716 (2012) 252e257
257
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4. Conclusions
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1,8-Bis(dimethylamino)-2-(4-methoxyphenyl)naphthalene (1)
possesses all of the qualities of a useful chargeable tag for analysis by
ESI-MS: the ligand readily coordinates to chromium in an h
6 fashion;
the proton sponge moiety selectively and efficiently binds proton to
become charged; and the charged site is far-removed from the active
site of the metal so that reactivity of the chromium complex was
found to be unaltered. Having proven its suitability as an ESI tag this
ligand can now be employed in the ESI-MS study of organometallic
reactions of neutral complexes with arene ancillary ligands.
Acknowledgements
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KLV thanks the University of Victoria for a Pacific Century
Fellowship. SK thanks NSERC for an Undergraduate Student
Research Award. JSM thanks NSERC for operational funding
(Discovery and Discovery Accelerator Supplement), and CFI, BCKDF
and the University of Victoria for instrumental support.
Appendix A. Supplementary material
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Supplementary material associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
jorganchem.2012.07.009.
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