Proton sponge phosphines: Electrospray-active ligands

Article abstract of DOI:10.1039/b609561e

Attachment of a proton sponge to a phosphine ligand renders neutral complexes of the ligand highly amenable to analysis by electrospray ionisation mass spectrometry (ESI-MS). The ligand 1,8-bis(dimethylamino) naphthyldiphenylphosphine (3) is extremely eff

Full text of DOI:10.1039/b609561e

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PAPER
www.rsc.org/dalton | Dalton Transactions
Proton sponge phosphines: electrospray-active ligands†‡
Nicola J. Farrer,a,b Robert McDonaldc and J. Scott McIndoe*b
Received 5th July 2006, Accepted 11th August 2006
First published as an Advance Article on the web 24th August 2006
DOI: 10.1039/b609561e
Attachment of a proton sponge to a phosphine ligand renders neutral complexes of the ligand highly
amenable to analysis by electrospray ionisation mass spectrometry (ESI-MS). The ligand
1,8-bis(dimethylamino)naphthyldiphenylphosphine (3) is extremely efficient and highly selective in
forming exclusively [M + H]+ ions, which may be detected at very low concentration. Ionisation
efficiency of 3 in the presence of H+ approached 100%. The bis-substituted ligand bis{1,8-
bis(dimethylamino)naphthyl}phenylphosphine (4) was also prepared and characterised, as were
5
Fe(CO)4- (5c), Mn(g -C5H4Me)(CO)2- (6) and W(CO)5- (7) complexes of 3. Compounds 3,
3·HBr·EtOH, 4 and 5c were all structurally characterised.
these ligands; similarly, cobalt carbonyl clusters with {l3-Si(p-
Introduction
C6H4OMe)} and {l3-Si(p-C6H4NMe2)} ligands provided ESI
mass spectra by protonation.9 However, aryl amines and aryl
ethers are not especially basic and the efficiency with which these
sites are protonated is low. As such, these ligands are not well
suited to the demands of direct ESI-MS analysis of complexes
under non-ideal circumstances, e.g. in a catalytic reaction, a
complex mixture, or in the presence of other compounds that
provide very strong spectra, such as a complex dissolved in
an ionic liquid. The ideal functional group would have high
basicity, be selective for a single ion, and be non-nucleophilic (to
minimise interference in reactions). The group should also not
greatly affect the electronic properties of the electrospray-inactive
ligand it replaces. Aromatic proton sponges fit these requirements
rather well.10 We selected 1,8-bis(dimethylamino)naphthalene (1),
Electrospray ionisation mass spectrometry (ESI-MS) is an increas-
ingly common tool for the direct analysis of catalytically active
species.1 The majority of analyses of this type have employed
catalysts which themselves carry a charge, i.e. are cations or anions.
These species are transported directly into the gas phase with
very high efficiency and hence are ideally suited to analysis by
ESI-MS. High m/z relative to ions derived from solvent and
substrates also assists the efficient detection of catalysts based
on organometallic complexes. However, a sizable proportion of
organometallic catalysts are neutral complexes, and hence rely on
one of the various ionisation pathways that generate detectable
ions.2 Numerous such pathways exist, most well-known being
protonation to form [M + H]+ ions, and related ions such as [M +
Na]+, [M + K]+ or [M + NH4]+ (depending on which ions are added
R
introduced by Alder11 and sold by Aldrich as “Proton Spongeꢀ”;
or the presence of adventitious ions). Loss of halide from metal
it is not only the best-known compound of its type but is
inexpensive and straightforward to derivatise. Its pKaH of 12.1
(in water) is a million-fold higher than most aromatic amines, due
to relief of steric strain in the neutral base upon protonation.12
1 is non-nucleophilic; it may be recovered unchanged after
four days at reflux with ethyl iodide in acetonitrile,11 and it
has been utilised in organic chemistry as a non-nucleophilic
base.13
+
halide complexes, LnMXm, to form [LnMXm−1
]
ions is another
common ionisation pathway.3 Complexes with acidic protons may
undergo loss of H+ to generate [M − H]− ions.4 Particularly
electron-rich complexes may undergo electrochemical oxidation
to generate [M]+ radical cations, most notably those based on
ferrocene.5 In certain cases, addition of an ionisation agent can
result in highly efficient production of charged derivatives, such
as the addition of alkoxy ions, RO−, to metal carbonyl complexes
to generate [M + OR]− ions,6 or of Ag+ to metal–metal bonded
species to form [M + Ag]+ ions.7
Some examples of reactivity of 1 apart from routine protonation
include, somewhat surprisingly, its role as a hydride donor in
its reaction with mer-RhCl3(dmso)3 or [RuCl(dppb)]2(l-Cl)3,14
with fluoroalkyl complexes of iridium,15 or with B(C6F5)3 to
16
Henderson and co-workers introduced the idea of using the
commercially available phosphines P(p-C6H4OMe)3 and P(p-
C6H4NMe2)3 in place of triphenylphosphine as “electrospray-
friendly” ligands.8 This approach allowed the observance of
[M + H]+ and [M + Na]+ ions for most of the complexes of
form the 1,1,3-trimethyl-2,3-dihydroperimidinium cation (TMP+).
There is also a single example of a metal complex coordinating
(directly) to 1 (via the amino groups); the reaction with Pd(hfac)2
immediately generates a poorly-characterised charge-transfer
product, which after standing for a week forms the cationic
complex [Pd(hfac)(1)]+.17 The hfac ligand may be substituted
for b-diketones and one of these complexes was structurally
characterised; coordination causes severe distortion of the proton
aDepartment of Chemistry, The University of Cambridge, Lensfield Road,
Cambridge, UK CB2 1EW
bDepartment of Chemistry, University of Victoria, P. O. Box 3065, Victoria,
BC, Canada V8W 3V6. E-mail: mcindoe@uvic.ca
˚
˚
sponge, the N · · · N distance opening to 2.94 A from 2.51 A. The
proton sponge ligand is easily displaced in this complex, even by
water. 1 can also act as a weak carbon nucleophile, but only in the
presence of exceptionally reactive electrophiles.18
cX-Ray Crystallography Laboratory, Department of Chemistry, University
of Alberta, Edmonton, AB, Canada T6G 2G2
† This paper is dedicated to the memory of Dr Alex Hopkins (1975–2006).
‡ The HTML version of this article has been enhanced with colour images.
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An additional advantage of a proton sponge-substituted ligand
is that the proton sponge confers pH-dependent solubility prop-
erties: in neutral or basic media, the sponge is soluble in organic
solvents; in acidic media, it carries a charge and is soluble in polar
solvents, including water (depending on the counterion involved).
These characteristics are passed on to the ligand and hence also
to metal complexes of the ligand.
Herein we report the syntheses of {1,8-bis(dimethylamino)-
naphthalen-2-yl}diphenylphosphine (3) and bis{1,8-bis(dimethyl-
amino)naphthalen-2-yl}phenylphosphine (4), metal complexes of
ligand 3 and a comparison of its performance as an ESI-MS
handle in organometallic complexes with the PPh3 ligand and
the “electrospray-friendly” ligand P(p-C6H4OMe)3.
white precipitate (6.62 g, 94%) was isolated by suction filtration,
washed with cold EtOH and dried under vacuum. 1H NMR
(300 MHz, CD3CN) dH (1·HBF4): 18.67 (s, 1H+) 8.05 (d, 2H,
3
3
3JH2–H3 8, H2) 7.90 (d, 2H, JH4–H3 8, H4) 7.71 (dd, 2H, JH3–H4 8,
3JH3–H2 8, H3), 3.11 (d, 12H, J 3, N(CH3)).
{1,8-Bis(dimethylamino)naphthalen-2-yl} bromide, 2. A solu-
tion of 1 (10 g, 46.7 mmol) in dry THF (40 ml) was cooled
to −78 ◦C. A solution of N-bromosuccinimide (NBS) (8.5 g,
48 mmol) in dry THF (280 ml) was cooled to −78 ◦C and added
dropwise to the stirring solution. The solution was stirred and
allowed to warm to room temperature overnight. The solvent was
removed under reduced pressure and the oil repeatedly filtered to
remove the succinimide precipitate and washed with portions of
cold dry ether. The ether was removed from the filtrate and the
resultant orange oil vacuum distilled. 2 came over at 152–160 ◦C
(∼1 mm Hg) as a yellow oil (12.13 g, 41.4 mmol, 89%). 1H NMR
(400 MHz, CDCl3) dH (2) : 7.50 (d, 1H, 2J 8.7, H3), 7.38 (dd, 1H, 2J
7.9, 3J 1.2 Hz, H5), 7.30 (dd, 1H, 2J 7.5, 2J 7.9, H6), 7.29 (d, 1H, 2J
8.7, H4), 7.06 (dd, 1H, 2J 7.4, 3J 1.2, H7), 3.00 (s, 6H, (N(CH3)2)2
Experimental
Materials
Dry solvents were obtained by distillation or from a solvent
purification system. Unless indicated, solvents were HPLC grade.
Reagents were purchased from Strem and Aldrich and used
1
on C1), 2.75 (s, 6H, (N(CH3)2)2 on C8). 13C{ H} NMR (100 MHz,
CDCl3) dC (2): 151.6 (C1), 146.8 (C8), 136.8 (C10), 131.1 (C3), 126.6
(C9), 125.7 (C6), 124.9 (C4), 122.8 (C5), 120.8 (C2), 115.0 (C7), 43.4
(2C, N(CH3)2 on C8), 45.7 (2C, N(CH3)2 on C1). ESI-MS (MeOH)
m/z: 293.1 [2 + H]+.
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without further purification except for Proton Spongeꢀ (1),
which was recrystallised from hot MeOH before use. 5a was
prepared using a published method. Unless indicated otherwise,
reactions were performed under nitrogen using standard Schlenk
techniques. Electrospray ionisation mass spectra were collected
using either a Micromass Quattro LC or a Micromass QTof
micro instrument. Relative intensities (%) were determined by
{1,8-Bis(dimethylamino)naphthalen-2-yl}diphenylphosphine, 3.
Dry THF (150 ml) was added to 2 (12.13 g, 41.3 mmol) and the
resulting solution degassed and cooled to −78 ◦C. nBuLi (25.8 ml
of a 1.6 M solution in hexanes, 41.3 mmol) was added dropwise.
R
integration (using Origin 7.5ꢀ). Capillary voltage was set at
◦
2900 V, source and desolvation gas temperatures were at 80 and
150 ◦C, respectively. Samples were infused via syringe pump at 5–
10 lL min−1. NMR spectra were recorded on one of the following
Bruker spectrometers: DPX-400, DRX-500, AV-500, AC-300 and
AMX-360. Chemical shifts are quoted in d (ppm) using internal
references, where appropriate, of CDCl3 (1H d 7.26 ppm), CD3CN
(1H d 1.94 ppm), acetone (1H d 2.05 ppm) and external references
of TMS (1H, 13C) and 85% aqueous H3PO4 (31P). All coupling
constants (J) are given in Hz. Where required, assignments were
After stirring for 30 min at −78 C PPh2Cl (9.5 ml, 43.4 mmol,
5% excess) was added via syringe. After addition was complete
the reaction mixture was stirred at −78 ◦C for 2 h and allowed to
warm to room temperature overnight (a small amount of white
solid 3·HBr precipitated which could be isolated by filtration).
The solvent was removed and addition of minimal CH2Cl2 to the
residue followed by addition of hexane precipitated 3 which was
recrystallised twice to give yellow crystals (9.2 g, 23.1 mmol, 56%).
Crystallisation could also be effected from hot minimal ether or
1
31
1
1
determined through use of H{ P}, H-NOESY, H-COSY, 13C-
1
◦
hot minimal MeCN. Mp (3) 108–110 C. H NMR (500 MHz,
DEPT, 1H–13C HSQC and 1H–13C HMBC NMR experiments. 13
C
3
3
CDCl3) dH (3): 7.45 (dd, 1H, JH5–H6 8.0, JH5–H7 1.5, H5), 7.40 (d,
1H, 3JH4–H3 8.5, H4), 7.33 (dd, 1H, 3JH6–H7 7.5, 3JH6–H5 8.0, H6), 7.27–
7.31 (m, 10H, 2 Ph), 7.18 (dd, 1H, 3JH7–H6 7.5, 3JH7–H5 1.5, H7), 6.88
(dd, 1H, 3JH3–P 2.5, 3JH3–H4 8.5, H3), 2.80 (d, 6H, 4JH–P 1.0, N(CH3)2
assignments were consolidated by running 13C NMR experiments
on both the AV-500 and AC-300 instruments and determining
31P–13C coupling constants by comparing the spectra obtained
on the different instruments. Melting points were recorded on a
Gallenkamp Melting Point Apparatus and are uncorrected. IR
spectra were recorded using a solution cell in a Perkin-Elmer
1000 FTIR spectrometer. Microanalyses were carried out by the
Analytical Laboratory in the Cambridge Chemical Laboratory.
1
on C1), 2.73 (s, 6H, N(CH3)2 on C8). 13C{ H} NMR (126 MHz,
CDCl3) dC (3): 153.5 (d, 2JC–P 23, C1), 152.5 (d, 4JC–P 1, C8),139.4
(d, 1JC–P 13, C11) 138.4 (C10), 135.5 (1JC–P 6, C2), 134.0 (d, 2JC–P 20,
C12), 130.6 (C3), 128.5 (d, 3JC–P 6, C13), 128.4 (C14), 126.6 (d, 3JC–P
4, C9), 126.2 (C6), 125.1 (C4), 124.1 (C5), 116.1 (C7), 46.9 (N(CH3)2
1
on C8), 44.4 (d, 4JC–P 8, N(CH3)2 on C1). 31P{ H} NMR (202 MHz,
Preparations
CDCl3) dP (3): −10.8, dP (3(O)): 26.6. ESI-MS (MeOH) m/z: 399.5
[3 + H]+. Anal. Calc. for C26N2H27P: C, 78.37; H, 6.83; N, 7.03; P,
7.77. Found: C, 77.99; H, 6.83; N, 7.02; P 7.72%.
R
Proton Spongeꢀ, 1. 1 (10 g) was recrystallised from hot
1
methanol (50 ml) (9.29 g, 93%). H NMR (300 MHz, CD3CN)
The HBr salt 3·HBr was obtained as detailed above from the
crude reaction mixture and crystallised from hot EtOH. 3·HBr
crystallised with one molecule of EtOH in the unit cell, Mp (3·HBr)
dH (1): 7.35 (dd, 2H, 3JH4–H3 8, 4JH4–H2 2, H4) 7.30 (dd, 2H, 3JH3–H4
7, 3JH3–H2 7, H3) 6.97 (dd, 2H, 3JH2–H3 7, 4JH2–H4 2, H2), 2.77 (s, 12H,
N(CH3)).
◦
1
189–191 C. 31P{ H} NMR (202 MHz, CDCl3) dP (3·HBr): −17.7,
dP (3(O)·HBr): 32.6. Anal. Calc. for C28H40BrN2O3P: (3·EtOH): C,
64.00; H, 6.52; N, 5.33; P, 5.89. Found C, 63.52; H, 6.46; N, 5.12;
P, 5.88%.
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Proton Spongeꢀ hydrofluoroborate, 1·HBF4. 1 (5 g) was
dissolved in ethanol (100%, 60 ml). HBF4 (aq) (3.05 ml, 48%,
23.4 mmol) was added dropwise to the solution with stirring. The
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1
31P{ H} NMR (202 MHz, CDCl3) dP (4·2HBF4): −26.4, ESI-MS
3·HBF4 showed greater solubility in organic solvents (moder-
ately soluble in MeCN, sparingly soluble in CDCl3) than 3·HBr
and could be obtained by adding 48% HBF4 (aq) to an ethanolic
suspension of crude 3 from the synthesis described earlier. The
solution was heated to reflux under N2 and cooled, the white solid
which precipitated on standing was isolated by suction filtration
and recrystallised from hot EtOH to give 3·HBF4 as a white
crystalline solid (14.8 g, 30.4 mmol, 74%), mp (3·HBF4) 236–
(CH2Cl2) m/z: 535.2 [4 + H]+, 268.1 [4 + 2H]2+
First pKaH of 3: 1H NMR transprotonation experiments19.
Compounds 1, 1·HBF4, 3 and 3·HBF4 were prepared as described
in the previous section.
Equimolar amounts of 1 (13.2 mg, 0.062 mmol) and 3·HBF4
(30.0 mg, 0.062 mmol) were dissolved together in dry, N2-saturated
CD3CN (0.75 ml) and 1H NMR spectra recorded. Equimolar
amounts of 1·HBF4 (7.8 mg, 0.026 mmol) and 3 (10.3 mg,
0.026 mmol) were dissolved together in dry, N2-saturated CD3CN
(0.75 ml) and 1H NMR spectra were recorded. Both experiments
was repeated. The average integral ratios for the coordinated
H+ of the two protonated species over the four experiments
◦
1
238 C. H NMR (500 MHz, CD3CN) dH (3·HBF4): 19.67 (1H,
H+), 8.02 (dd, 1H, J 8, 1, H5), 7.96 (dd, 1H, J 1, J 8, H7), 7.93 (d,
1H, J 9, H4), 7.77 (dd, 1H, J 8, H6), 7.30–7.40 Hz (m, 11H: H3,
2Ph), 3.49 (dd, 6H, J 3, J 0.6, N(CH3)2 on C1), 3.20 (d, 6H, J 3,
1
N(CH3)2 on C8). 13C{ H} NMR (126 MHz, CD3CN) dC (3·HBF4):
150.4 (d, J 24, C1), 144.9 (s, C8), 137.2 (s, C10), 136.4 (d, J 11, C2),
135.4 (d, J 28, C11), 134.9 (s, C13), 134.3 (d, J 19, C12), 130.8 (s,
C5), 130.5 (s, C4), 130.4 (s, C14), 130.1 (d, J 7, C3), 129.3 (s, C6),
123.3 (s, C7), 121.7 (d, J 8, C9), 47.1 (s, CH3, N(CH3)2 on C8), 46.1
(d, CH3, J 17, N(CH3)2 on C1), 31P NMR (202 MHz, CD3CN)
dP (3·HBF4): −17.3, dP (3(O)·HBF4): 32.9. ESI-MS (MeOH) m/z:
399.5 [3 + H]+
ꢀ
ꢀ
were (CD3CN, 360 MHz) dH: 19.7 (3·H, 1), 18.7 (1·H, 0.93)
(1 std. dev. = 0.02). The pKaH of 3 was determined by calculation
using this ratio and the known pKaH of 1 in CD3CN, given as
18.18.20 Analysis of the equilibrium gave Ka2 = Ka1/Keq where Ka1
(known) = [1][H+]/[1·H+], Ka2 = [3][H+]/[3·H+] and Keq (known) =
ꢀ
ꢀ
( 3·H/ 1·H).2
1
First and second pKaH of 4: H NMR transprotonation experi-
ments. Equimolar amounts of 4 (1.18 mg, 0.0022 mmol) and
1·HBF4 (0.67 mg, 0.0022 mmol) were dissolved together in dry,
N2-saturated CD3CN (3 ml), the solution mixed thoroughly and
Di{1,8-bis(dimethylamino)naphthalen-2-yl}phenylphosphine, 4.
Dry THF (35 ml) was added to 2 (3 g, 10.2 mmol), the resulting
solution degassed and cooled to −78 ◦C. nBuLi (6.8 ml of a 1.6 M
solution in hexanes, 10.2 mmol) was added dropwise. After stirring
for 30 min at −78 ◦C PPhCl2 (0.74 ml, 5.4 mmol, 5% excess)
was added via syringe. After addition was complete the reaction
mixture was stirred at −78 ◦C for 2 h and allowed to warm to room
temperature overnight. The solvent was removed under vacuum,
CH3CN was added, the solution filtered and the filtrate reduced.
Dark yellow crystals of 4 were obtained by repeated precipitation
and eventually crystallisation from hot minimal MeCN (0.53 g,
1 mmol, 10%). 1H NMR (500 MHz, CDCl3) dH (4): 7.40 (dd, 1H,
a
1H NMR spectrum recorded of 1 ml of the solution over
several hours (15000 scans). This was repeated and the results
averaged. The average integral ratios for the coordinated H+ of
the two protonated species were (CD3CN, 360 MHz) dH: 19.9 (4·H
ꢀ
ꢀ
and 4·2H, 1), 18.7 (1·H, 0.58) (1 std dev. = 0.04).
Synthesis of iron carbonyl phosphine complexes. CAUTION:
Fe2(CO)9 is pyrophoric.21 These syntheses release Fe(CO)5 and
the initial workups should be carried out in a fumehood.
3
3
3JH5–H6 8.0, JH5–H7 1.5, H5), 7.31 (d, 2H, JH4–H3 8, H4), 7.29 (dd,
2H, JH6–H7 7.5, JH6–H5 8.0, H6), 7.22–7.27 (m, 5H, Ph), 7.14 (dd,
2H, 3JH7–H6 7.5, 3JH7–H5 1.5, H7), 6.78 (dd, 2H, 3JH3–P 2.5, 3JH3–H4 8.5,
H3), 2.83 (s, 12H, N(CH3)2 on C1), 2.72 (s, 6H, N(CH3)2 on C8),
3
3
Fe(CO)4(PPh3), 5a. A solution of PPh3 (0.7 g, 2.52 mmol)
in dry diethyl ether (20 ml) was degassed and added to a
flask containing a magnetic stirring bar and Fe2(CO)9 (0.99 g,
2.52 mmol). The mixture was vigorously refluxed for 90 min,
with stirring. Filtration of the solution gave a brown precipitate
which was washed with THF, the filtrate and washings were
pooled and reduced under vacuum. The reaction formed both 5a
2.70 (s, 6H, N(CH3)2 on C8). 13C{ H} NMR (126 MHz, CDCl3) dC
1
(4): 153.4 (d, 1JC–P 23.6, C1), 152.5 (d, 4JC–P 1.6, C8), 141.8 (d, 1JC–P
17.9, C11), 138.2 (C10), 137.2 (d, 2JC–P 10, C2), 134.3 (d, JC–P 21.6,
C12), 131.3 (s, C3), 128.3 (d, JC–P 5.9, C13), 127.9 (s, C14), 126.4 (d,
5JC–P 3.0, C9), 125.9 (C6), 124.7 (C4), 124.0 (C5), 115.8 (C7), 46.6,
and Fe(CO)3(PPh3)2 as evidenced by 31P{ H} NMR: (146 MHz,
1
47.1 (N(CH3)2 on C8), 44.6 (d, JC–P 8, N(CH3)2 on C1). 31P{ H}
NMR (202 MHz, CDCl3) dP (4): −17.4. ESI-MS (CH2Cl2) m/z:
535.2 [4 + H]+, 268.1 [4 + 2H]2+.
4
1
CH2Cl2, d6-acetone lock) dP (crude): 81.32 (Fe(CO)3(PPh3)2), 70.67
(5a) (both peaks equal intensity). The monosubstituted product
(5a) was crystallised from minimal hot methanol (0.◦42 g, 1 mmol,
◦
1
39%), mp (5a) 189–190 C (decomp.), lit: 201–203 C. H NMR
(500 MHz, CDCl3) dH (5a): 7.46–7.50 (m, 9H, 6H3 and 3H4), 7.41–
7.44 (m, 6H, 6H2); 13C NMR (126 MHz, CDCl3) dC (5a): 213.6 (d,
J 19, CO), 134.2 (d, J 49, C1), 133.4 (d, J 10, C3), 131.1 (d, J 3,
4·2HBF4. Obtained by addition of HBF4 (aq) to a suspension
of 4 in CH3CN.
1H NMR (CD3CN, 500 MHz) dH (4·2HBF4): 19.32 (H+, 2H),
8.04 (dd, J 9, 1, 2H, H5), 7.96 (dd, J 7, 1, 2H, H7), 7.94 (d, J 9,
2H, H4), 7.75 (dd, J 8, 2H, H6), 7.42–7.46 (m, 1H, H14), 7.40 (ddd,
J 8, 7, 2, 2H, H13), 7.28 (dd, 2H, J 9, H3), 7.22 (ddd, J 8, 8, 1,
2H, H12), 3.34 (d, J 1.4, 6H, N(CH3)2 on C1), 3.30 (d, J 1.5, 6H,
N(CH3)2 on C1), 3.21 (d, J 3.4, 6H, N(CH3)2 on C8), 3.19 (d, J
C4), 128.9 (d, J 11, C2); 31P{ H} NMR (202 MHz, CDCl3) dP (5a):
1
72.51; IR : m(CO) (hexane) 2051 (s), 1978, 1945 (s, br).
Fe(CO)4(PC6H4OMe)3, 5b.
A solution of P(C6H4OMe)3
(1.00 g, 2.84 mmol) in dry THF (20 ml) was degassed and added
to a flask containing a magnetic stirring bar and Fe2(CO)9 (1.03 g,
2.84 mmol). The mixture was heated to ∼55 ◦C with stirring, and
monitored over time with 31P NMR (146 MHz, THF, D2O lock).
Conversion of the free ligand (−9.7 ppm) via an intermediate
(23.6 ppm) gave mono and disubstituted iron complexes, giving
rise to resonances at 66.01 ppm, (relative integral = 1, 5b) and
1
3.4, 6H, N(CH3)2 on C8). 13C{ H} NMR (CD3CN, 500 MHz) dC
(4·2HBF4): 149.9 (d, J 24, C1), 144.4 (C8), 137.5 (C10), 136.6 (d, J
6, C9), 135.0 (d, J 21, C12), 134.5 (C3), 133.4 (d, J 28, C11), 131.3
(C5), 131.2 (C14), 130.8 (C4), 130.6 (d, J 8, C13), 129.5 (C6), 123.5
(C7), 122.4 (d, J 9, C2), 47.4 (N(CH3)2 on C8), 47.3 (N(CH3)2 on
C8), 45.7 (d, J 16, N(CH3)2 on C1), 45.0 (d, J 13, N(CH3)2 on C1),
4572 | Dalton Trans., 2006, 4570–4579
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2
3
2
(d, JC–P 10, C3), 129.5 (d, JC–P 6, C9), 128.4 (d, JC–P 10, 4C13),
78.06 ppm (relative integral = 0.4, Fe(CO)3{P(C6H4OMe)3}2),
respectively. The reaction was judged to be complete after 60 min,
when the signal at −9.7 ppm had disappeared.
127.6 (C6), 126.5 (d, JC–P 11.3, C4), 125.9 (s, C5), 118.8 (s, C7),
3
47.8 (s, N(CH3)2 on 2C8), 43.9 (s, N(CH3)2 on 2C1). 31P{ H} NMR
1
(146 MHz, CDCl3) dP (5c): 64.22. IR (hexane) cm−1 : 2046 (sharp),
1971 (m, br), 1944 (s, br), 1935 (s, br). ESI-MS (MeCN, formic
acid) m/z: 399.1 [3 + H]+, 567.1 [5c + H]+.
The brown solution was filtered through cotton wool and the
residue washed with dry THF (20 ml) to give a green-brown
filtrate. The solvent was removed under vacuum, 20 ml of dry
MeCN was added to the residue and the solution filtered to leave
a cream precipitate (crude Fe(CO)3{P(C6H4OMe)3}2, see below
for purification). The MeCN solution was washed repeatedly with
hexane (6 × 10 ml) to remove Fe3(CO)12 and the solvent volume
was reduced on a vacuum line. Crude yellow 5b precipitated from
the cold minimal MeCN. Dissolving the precipitate in CH2Cl2
(5 ml) and filtering into ice-cold MeOH (10 ml) before leaving for
12 h in the fridge gave yellow crystals of 5b (0.27 g, 18%, 0.5 mmol),
mp (5b) 151–153 ◦C.
ESI-MS comparison of complexes 5a–5c. The metal com-
plex 5 (0.004 mmol) was dissolved in 0.8 ml CH2Cl2 and
0.2 ml (0.02 equiv) of
CH2Cl2) of the ionic liquid (tetradecyltrihexyl phosphonium
bis(trifluromethylsulfonyl)amide was added. The sample was
serially diluted in CH2Cl2 and a drop of acid (diluted in MeOH)
was added (HBF4 for 5a and 5b, HCl for 5c).
a solution (0.0035 g in 10 ml
ESI-MS (5a; CH2Cl2–HBF4) 483 (100%) [PC32H68]+, (5b;
CH2Cl2–HBF4) 521 (23%) [5b + H]+, 483 (100%) [PC32H68]+, 369
(51%) [PO(C6H4OMe)3H]+, 353 (23%) [P(C6H4OMe)3 + H]+, (5c;
CH2Cl2–HCl) 567 (100%) [5c + H]+, 483 (20%) [PC32H68]+, 415
10% [3O + H]+, 399 5% [3 + H]+.
1H NMR (500 MHz, CDCl3) dH (5b): 3.82 (s, 9H, 3CH3), 6.91
(dd, 6H, 3JH3–H2 8.8, 4JH3–P 1.8, H3), 7.38 (dd, 6H, 3JH2–H3 8.8, 3JH2–P
8.0, H2), 13C NMR (125 MHz, CDCl3) dC (5b): 55.5 (s, CH3),
114.3 (d, 3JC3–P 11.3, C3), 126.0 (d, 1JC1–P 55.3, C1), 134.8 (d, 3JC3–P
1
1
11.3, C2), 161.6 (d, JC4–P 2.5 C4), 213.9 (d, JOC–P 18.9, 4CO).
Mn(g5-MeC5H4)(CO)2(3), (6). A large sublimation apparatus
cooled with dry ice was charged with a magnetic stirrer, dry THF
(50 ml) and Mn(g -MeC5H4)(CO)3 (0.215 g, 0.98 mmol) so that
the solution was in contact with the cold finger. The solution was
degassed, placed under nitrogen and the apparatus placed in a cold
water-bath. The solution was irradiated (UV) for 4 h with stirring.
During the irradiation the internal temperature of the solution
was maintained between −32 and +40 ◦C and over the course
of the irradiation the pale yellow solution deepened in colour to
31P{ H} NMR: (202 MHz, CDCl3) dP (5b): 67.06 IR : m(CO)
1
(50 : 50 hexane–CH2Cl2) 2047 (sh), 1972, 1938 (s, br). ESI-MS
(MeOH) 542 [5b + Na]+, ESI-MS (CH2Cl2–HBF4) 521.1 (87%)
[5b + H+], 369.2 (59%) [PO(C6H4OMe)3 + H]+, 353.2 (100%)
[P(C6H4OMe)3 + H]+.
5
The cream-coloured residue of Fe(CO)3{P(C6H4OMe)3}2 was
dissolved in minimal CH2Cl2 and precipitated from the pale
green solution by addition of hexane. Dissolving in CH2Cl2
(5 ml), filtering into ice-cold MeOH (10 ml) and leaving for
3 h at 4 ◦C gave Fe(CO)3{P(C6H4OMe)3}2 as flaky pale yellow
crystals (13%, 0.313 g, 0.37mmol). mp (Fe(CO)3{P(C6H4OMe)3}2)
5
wine-red, indicating formation of Mn(g -MeC5H4)(CO)2(THF).
The apparatus was removed from the water bath and cooled to
−78 ◦C. A chilled degassed solution of 3 (0.39 g, 0.98 mmol)
in THF (8 ml) was slowly added via syri◦nge to the apparatus.
The resulting solution was stirred at −78 C for 2.5 h and then
217–218 ◦C. 31P{ H} NMR (146 MHz, CH2Cl2, d6-acetone
1
lock) dP: 74.8. ESI-MS (CH2Cl2–HBF4) m/z: 845.3 (100%)
[Fe(CO)3{P(C6H4OMe)3}2 + H]+, 353.2 (7%) [P(C6H4OMe)3 +
H]+.
◦
at 0 C overnight. The now-orange solution was stirred at room
temperature for 24 h to give a yellow solution. Yellow platelet
crystals of 6 were obtained by diffusion of pentane into a CH2Cl2
solution of 6 (0.17 g, 30%, 0.3 mmol), mp (6): 170–171 ◦C. 6
readily decomposed in air or in solution if exposed to air, to give
Fe(CO)4(3) (5c). A solution of 3 (1 g, 2.52 mmol) in dry
diethyl ether (20 ml) was degassed and added to a flask containing
a magnetic stirring bar and Fe2(CO)9 (0.99 g, 2.52 mmol).
The mixture was vigorously refluxed for 90 min, with stirring.
The reaction formed both Fe(CO)4(3) (5c) and Fe(CO)3(3)2 as
1
a paramagnetic brown compound. H NMR (500 MHz, CDCl3)
dH (6): 1.94 (s, 3H, CH3 on Cp), 2.22 (s, 6H, N(CH3)2 on C8), 2.58
(s, 6H, N(CH3)2 on C1), 4.00, 4.07, (4H, Cp), 7.13–7.65 (m, 15H,
1
evidenced by 31P{ H} NMR (146 MHz, CH2Cl2, d6-acetone lock)
Ar); 31P{ H} NMR (202 MHz, CDCl3) dP (6): 92.6; IR m(CO)
1
dP (crude): 83.47 (Fe{CO}3(3)2), 67.85 (5c). Suction filtration
of the solution through a glass frit isolated a cream-coloured
precipitate from the red supernatant. The precipitate was rinsed
(hexane) cm−1: 2014 (w), 1937 (s), 1877 (s); ESI-MS (CH2Cl2–
formic acid) m/z: 589.2 [6 + H)]+, 399.1 [3 + H]+; ESI-MS/MS
(589, 6 + H+); 589.1, 399.1.
◦
with small portions of cold (0 C) diethyl ether. The precipitate
was rinsed with CH2Cl2 to leave a brown paramagnetic residue, the
orange filtrate was collected, saturated with nitrogen and reduced
under vacuum until the product began to precipitate from the
dichloromethane.
W(CO)5(3), (7). W(CO)6 (253 mg, 0.72 mmol), 3 (287 mg,
0.72 mmol) and Me3NO·2H2O (0.24 mg, 2.16 mg) were dissolved
in CH2Cl2 (25 ml) and stirred at room temperature for 3 h.
Additional Me3NO·2H2O was added (0.24 mg, 2.16 mg) and
the solution refluxed overnight to give a transparent yellow
solution and a black oil. The reaction was monitored by IR and
TLC. The yellow solution was decanted and the solvent removed
by evaporation, the residue was purified by preparative TLC (5 : 1
hexane–thf plus a drop of triethylamine) and the product, 7, was
obtained by precipitation from CH2Cl2–pentane in 30% yield.
ESI-MS (CH2Cl2–formic acid) 723 [7 + H]+, 399 [3 + H]+.
IR m(CO) (CH2Cl2–hexane) (cm−1) 2070 (s), 1978, 1920 (br),
1860 (w).
The monosubstituted product, 5c, was obtained as fine yellow
crystals from CH2Cl2–hexane (0.36 g, 27%, 0.7 mmol), mp (5c)
190–191 ◦C (decomp.). 1H NMR (500 MHz, CDCl3) dH (5c): 7.60
2
3
(dd, 4H, J 10, J 8, 4H12), 7.55 (d, 1H, J 8, H5), 7.51 (d, 1H, J
8.5, H4), 7.43 (m, 7H, 4H13, 2H14 and H6), 7.35 (d, 1H, J 7, H7),
7.06 (dd, 1H, J 8.5, J 10, H3), 2.58 (6H, H8), 2.41 (6H, H1); 13C
NMR (126 MHz, CDCl3) dC (5c): 214.6 (d, 2JC–P 19, 4CO), 154.2
2
1
(d, JC–P 7, C1), 153.4 (C8), 139.8 (C10), 136.6 (d, JC–P 47, C11),
134.1 (d, 2JC–P 10, 4C12), 133.3 (d, 1JC–P 58, C2), 130.4 (2C14), 130.1
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Crystal structure determinations
brominating system proved to be N-bromosuccinimide (NBS) in
THF at −78 ◦C, being regiospecific for monobromination at the 2-
position, producing 1,8-bis(dimethylamino)-2-bromonaphthalene
(2) in good yield.40 Little polybromination was observed under
these conditions, indicating either that the system is under
kinetic control or that the NMe2 group ortho- to the position of
bromination participates in the bromination through an unknown
mechanism.
Syntheses of 3 and 4 from 2 were straightforward via lithiation
followed by treatment with PPh(3−n)Cln (n = 1 or 2, see Scheme 1)
but purification proved to be more complicated. Proton sponge
derivatives adhered strongly to both silica and sand and smeared
on neutral activated alumina. Low loadings of these compounds
could be partially purified by alumina column chromatography,
a general method involved loading the compound in CH2Cl2 and
X-Ray crystallographic data for 3 and 3·HBr·C2H5OH were col-
lected in the X-ray Laboratory of the Cambridge University Chem-
ical Laboratory. Measurements were made on a Nonius Kappa
CCD diffractometer at −93 ◦C with graphite-monochromated
˚
Mo-Karadiation (k = 0.71073 A). Data for 4 and 5c were collected
at −80 ◦C on a Bruker PLATFORM/SMART 1000 CCD
diffractometer with graphite-monochromated Mo-Ka radiation
˚
(k = 0.71073 A), at the X-ray Crystallography Laboratory of the
University of Alberta. The structures were solved by direct meth-
ods, except for 5c (Patterson search/structure expansion)22 and
refined by full-matrix least-squares on F2.23 A Flack parameter24 of
0.14 (11) for 4 indicated a small degree of racemic twinning, which
was accommodated during the refinement. ORTEP diagrams were
generated using ORTEP 325 and POV-Ray 3.5.26
Crystal data and selected details of structure determinations are
given in Table 1.
CCDC reference numbers 611411–611414.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b609561e
Results and discussion
Several groups including those of Alder,27 Staab,28 Pozharskii29
and Lloyd-Jones30 have investigated and derivatised 1. Nu-
merous kinetic,31 structural,32 spectroscopic33 and theoretical
investigations34 of 1 and its derivatives have also been carried
out. We investigated a variety of bromination methods as a route
to functionalisation of 1; nearly all gave mixtures of mono and
dibrominated products; HBr (48%, aq.) in AcOH–DMSO,35 Br2
in CCl436 or H2SO4,37 TBABr3 in CH2Cl238 and 2,4,4,6-tetrabromo-
2,5-cyclohexadien-1-one in CH2Cl2.39 In our hands, the best
Scheme 1 Syntheses of 2–4 from Proton SpongeꢀR (1).
Table 1 Crystallographic experimental details for 3, 3·HBr·C2H5OH, 4 and 5c
3
3·HBr·C2H5OH
4
5c
Formula
Mr
C26H27N2P
398.47
C28H33BrN2OP
524.44
C34H39N4P
534.66
C30H27FeN2O4P
566.36
Crystal dimensions/mm
Crystal system
Space group
0.46 × 0.21 × 0.12
Monoclinic
P21/n
17.1220(3)
10.1292(2)
25.3141(5)
93.8090(10)
4380.58(14)
8
0.46 × 0.10 × 0.05
Monoclinic
P21/c
16.0283(3)
7.18140(10)
22.5380(6)
98.9630(10)
2562.57(9)
4
0.64 × 0.24 × 0.15
Orthorhombic
Pna21
15.4223(12)
10.8193(8)
17.8969(14)
90
2986.3(4)
4
1.189
0.62 × 0.41 × 0.37
Monoclinic
P21/n
10.5700(9)
15.0412(13)
17.6505(16)
104.2580(11)
2719.7(4)
4
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
Dc/g cm−3
l/mm−1
1.208
0.140
1.359
1.690
1.383
0.652
0.121
T/◦C
−93(2)
−93(2)
−80
−80
h Range/◦
Total data
hkl Ranges
3.58 to 27.51
32696
3.66 to 27.51
23406
2.20 to 25.85
22819
2.46 to 26.37
16008
−22 to 22, −13 to
13, −32 to 32
10020
−20 to 20, −9 to 9,
−29 to 29
5838
−19 to 19, −13 to
13, −22 to 22
6154
−13 to 13, −18 to
18, −22 to 21
5553
Independent reflections
Rint
0.0831
0.0613
0.0504
0.0224
Range transmission factors
Data/restraints/parameters
Goodness-of-fit (S)
0.984–0.848
10020/0/523
1.023
0.0547
0.1137
0.990–0.884
5838/0/299
1.038
0.0546
0.1170
0.9821–0.9265
6154/0/353
1.023
0.0503
0.1374
0.7945–0.6881
5553/0/343
1.021
0.0368
0.1085
2
R1 [Fo ≥ 2r(Fo2)]
wR2 [Fo ≥ 3r(Fo2)]
2
−3
˚
Largest diff. peak, hole/e A
0.265, −0.343
0.684, −0.542
0.234, −0.248
0.549, −0.595
4574 | Dalton Trans., 2006, 4570–4579
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using 19 : 1 hexane–THF with trace triethylamine (TEA) to elute.
Distillation proved to be the method of choice to purify multigram
quantities of 2 but could not be easily used to separate different
(ortho/para) isomers of 2. In general, separation of different pro-
ton sponge derivatives from each other was difficult—for example,
1 and 3 are both soluble in 2—and therefore clean reactions were
important. The nature of the compounds meant that incidental
protonation during workup procedures had to be considered
since this altered the physical properties of the compounds.
Intentional protonation/deprotonation could however be used to
aid purification. Standard organic workuptechniques (extractions,
washing, drying with MgSO4) and chromatography were used
initially but eventually avoided where possible as they greatly
reduced yields. The bulky phosphines 3 and 4 proved to be fairly
air stable when solid, and were stable in deoxygenated solution
under N2. They could be reversibly protonated/deprotonated with
strong acid and base multiple times without decomposition. The
phosphine oxides (which were also yellow) could be removed by
stirring the impure phosphine in minimal EtOH under N2, cooling
on ice and filtering, thus removing the soluble oxide.
Fig. 2 ORTEP plot of 3·HBr (ethanol of crystallisation not shown).
Non-hydrogen atoms are represented by Gaussian ellipsoids at the 50%
probability level. Hydrogen atoms are shown wi◦th arbitrarily small thermal
˚
parameters. Key bond lengths (A) and angles ( ): P1–C2 1.857(3), N1–C1
X-Ray crystal structures of both 3 and its hydrobromide, 3·HBr
(along with an ethanol of crystallisation) were obtained (Fig. 1
and 2). Protonation of 3 to give 3·HBr significantly increases the
planarity of the naphthalene; the average deviation from the best-
1.459(3), N2–C8 1.468(4); C2–P1–C31 100.79(13), C2–P1–C31 101.70(14),
C21–P1–C31 104.64(14).
141) and as the nitrogen lone pairs coordinate to the proton there
is less involvement with the naphthalene p system resulting in
longer C(1)–N(1) and C(8)–N(2) bonds. The average length of
˚
fit plane defined by the ten naphthalene carbon atoms is 0.102 A
˚
for 3 but only 0.007 A for 3·HBr. The deviation from planarity
is also manifested by the N–C(1)–C(8)–N torsion angles, 37.4◦
for 3 but just 1.2◦ for 3·HBr. The twisting of the naphthalene is
consistent with co-repulsion of the lone pairs, relief of this strain
being the suggested driving force for protonation.12 The position
of the nitrogen lone pairs can be estimated from the orientation of
the NMe2 methyl groups. Upon protonation the N · · · N distance
˚
˚
these bonds is 1.46 A in 3·HBr and 1.43 A in 3. The sum of angles
around N(1) and N(2) in 3 are 355◦ (∼98% sp2 character) and 338◦
(∼94% sp2 character), respectively. In 3·HBr the nitrogen atoms
become marginally more sp3 like, as the angles reduce to 343◦
(95%) and 337◦ (93%). The anisotropic displacement parameters
for N(1) and N(2) are smaller in 3·HBr than in 3, reflecting the
increased bonding and reduced freedom of the nitrogen atoms.
The diproton sponge phosphine ligand 4 was also examined
structurally (Fig. 3). One of the proton sponge groups on 4 has
similar geometry to its counterpart in 3; an N–C1–C8–N torsion
angle of 36.6◦, an average deviation from the best-fit plane defined
˚
˚
is reduced from 2.90 to 2.56 A (cf . 2.55–2.62 A in protonated
˚
by the ten naphthalene carbon atoms of 0.098 A, and an N–N
˚
distance of 2.91 A. However, the other proton sponge group
Fig. 1 ORTEP plot of 3 (one of the two independent molecules in the unit
cell; bond lengths and angles do not vary significantly between the two).
Non-hydrogen atoms are represented by Gaussian ellipsoids at the 50%
probability level. Hydrogen atoms are shown wi◦th arbitrarily small thermal
Fig. 3 ORTEP plot of 4. Non-hydrogen atoms are represented by
Gaussian ellipsoids at the 50% probability level. Hydrogen atoms are
˚
shown with arbitrarily small thermal parameters. Key bond lengths (A)
˚
parameters. Key bond lengths (A) and angles ( ): P1–C2 1.842(2), N1–C1
and angles (◦): P–C12 1.843(3), P–C22 1.850(3), N1–C11 1.413(5), N2–C19
1.432(4), N3–C21 1.423(4), N4–C29 1.410(4); C12–P–C32 100.45(12),
C12–P–C31 100.67(12), C22–P–C31 101.30(12).
1.428(2), N2–C8 1.430(3); C2–P1–C21 100.54(9), C2–P1–C31 100.62(9),
C21–P1–C31 104.15(10).
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attached to the phosphorus (containing N1 and N2) is much
less distorted from planarity, with the respective torsion angle,
◦
˚
deviation from planarity and N–N distance being 16.55 , 0.057 A
˚
and 2.84 A. The reason for the difference is not immediately
obvious, but presumably can be attributed to crystal packing
forces and suggests a reasonable degree of flexibility of the proton
sponge unit.
Ligands 3 and 4 both display pH-dependent solubility. In their
unprotonated form they dissolve readily in organic solvents such
as CH2Cl2 or THF, but addition of acid greatly increases solubility
in polar solvents; 3·HBF4 is water-soluble, for example. Neutral-
isation with a strong base such as NaOH results in the ligand
moving back into the organic phase. Ligands that confer water-
solubility to their complexes, such as the well-known 1,3,5-triaza-
7-phosphaadamantane (PTA),42 have numerous applications in
aqueous phase or biphasic homogeneous catalysis.
Fig. 4 1H NMR spectra of an equimolar mixture of 1 and 3·HBF4 (an
identical spectrum is obtained for an equimolar mixture of 1·HBF4 and 3)
in the low-field region.
Transprotonation experiments
The high basicity of proton sponges has made determining their
pKa values challenging, the pKa of the solvent in which the study
is conducted being a constraining factor. Temperature-jump43 or
NMR methods44 have been used by other groups. We adopted
the latter approach, albeit less accurate, due to the very clear
signal of the coordinated proton in the 1H NMR spectra at high
chemical shifts (∼20 ppm) for all derivatives of 1. Solubility of
all four compounds (the two protonated/unprotonated pairs) in
the deuterated solvent was a critical issue since any insolubility
would affect the equilibrium and therefore the determined value.
HBF4 and CD3CN proved to be the best combination. Equimolar
amounts of 1 and 3·HBF4 were dissolved in CD3CN and the
1H NMR spectrum was recorded. The same experiment was
conducted with equimolar amounts of 1·HBF4 and 3. In both
cases, equilibrium between 3·HBF4 and 1·HBF4 was quickly
reached and the ratio of these species remained roughly constant
4 is demonstrated by the integrals to be less than two-thirds, this
implies that the second pKaH of 4 is significantly less than the
first pKaH of 1. This is not unexpected, as the second pKaH of 4
corresponds to the protonation of a formally charged compound,
4·HBF4, to form 4·2HBF4.
ESI-MS of the proton sponge derivatives
Throughout this work we have observed that ESI-MS of proton
sponge derivatives generally show no sign of cationisation by
species other than H+, even in the presence of excess Li+, Na+,
K+ or NH4 . The exception is the spectrum of 5c, which like 5a
+
and 5b, provides an [M + Na]+ signal, though this is certainly due
to interactions with the carbonyl ligands rather than the with the
proton sponge.6b ESI-MS of a solution of a 1 : 1 molar ratio of
tetradecyltrihexylphosphonium bis(trifluromethylsulfonyl)amide
and the ligand 3 showed that the ionisation efficiency of 3
approaches that of the formally charged phosphonium ion (in
CH2Cl2, CH3CN or MeOH in presence of a H+ source). This
has important implications with the use of the ligand to identify
catalytic intermediates; reactions typically employ catalysts at a
1–10 mol% concentration1a and so the concentration of catalytic
intermediates can be expected to be very much lower. Therefore the
ionisation efficiency of the ligand must be as high as possible for
the ligand to reveal these low concentration reactive intermediates
in the ESI mass spectrum. Even without added H+, the ligand 3
provides a very strong signal from a dichloromethane solution,
approximately 50% of the intensity of the phosphonium ion
(Fig. 5).
1
from within 5 min of sample mixing. H NMR signals in the
low-field region corresponded to the proton coordinated by the
different proton sponges, d 19.7 ppm corresponding to 3·HBF4
and d 18.7 ppm to 1·HBF4. The ratios of the protonated species
were averaged over 4 samples and the first pKaH of 3 (CD3CN) was
calculated as 18.24 0.02 (1 std) units at 297 K, demonstrating a
slight increase in basicity when compared to the value for 1 (18.18
in CH3CN).20 Fig. 4 shows the relative ratios of the two peaks.
The slightly higher pKaH is in agreement with the observation
that proton sponges with “buttressing” substituents in the 2- and
7-positions demonstrate increased pKaH values as the lone pairs
are forced closer together in the unprotonated state, the relief of
strain upon protonation being correspondingly higher.12,45
After determining that the equilibrium set up was identical from
either starting point, the experiment was repeated using equimolar
quantities of 4 and 1·HBF4. The low solubility of 4 in CD3CN
necessitated very dilute solutions to ensure total dissolution and
Metal complexes
1
no precipitation of any species during the experiment, H NMR
Several organometallic complexes incorporating 3 as a ligand were
prepared by standard methods (Scheme 2).
experiments were run overnight (15000 scans). 4·HBF4 showed a
single peak in the 1H NMR spectra at 19.9 ppm (relative integral
1). The proton coordinated by 1 gave a peak at 18.7 ppm (relative
integral 0.6). Due to the structural similarity between 3 and 4 it
is anticipated that their first pKaH will be approximately equal.
Since the combined coordination by both proton sponge sites of
The preparations of 5c, 6 and 7 were adapted from syn-
5
theses of Fe(CO)4(PPh3),46 Mn(g -C5H4Me)(CO)2(PPh3),47 and
W(CO)5(PPh3),48 respectively. For the sake of comparison, the
known complexes Fe(CO)4(PR3) (R = Ph, 5a; R = p-C6H4OMe,
5b) were also prepared.
4576 | Dalton Trans., 2006, 4570–4579
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r-donation).52 Steric effects would also be expected to encourage
the largest ligand in the complex to occupy the equatorial position
(with only two groups at 90◦ cf . three for substituents in an axial
position). The orientation of the N(1) methyl groups suggests that
there may be a weak interaction between the N(1) lone pair and
˚
the Fe centre, but in fact the intramolecular distance of 3.58 A is
substantially longer than the sum of van der Waals radii.
Complexes 5a, 5b and 5c exhibited carbonyl stretches in the
IR region in the order of PPh3 > P(C6H4OMe)3 ∼ 3 (2051,
2047, 2046 cm−1 as highest-frequency Aꢀ band) indicating that
the net electron density donated to the iron by 3 is greater than
for the other ligands, with back-donation to the CO ligands in this
complex being correspondingly greater, weakening the CO bond.
Just as is the case for the other unprotonated sponges, the 1,8-
bis(dimethylamino)naphthyl group is significantly distorted from
planarity, with an N–C1–C8–N torsion angle of 28.1◦, an average
deviation from the best-fit plane defined by the ten naphthalene
Fig. 5 Positive-ion ESI-MS of an equimolar mixture of 3 and an internal
reference, [P(C6H13)3(C14H29)]+, in CH2Cl2. The dotted line indicates the
change in intensity of the [3 + H]+ signal observed when excess H+ (a drop
of formic acid) is deliberately added to the solution.
˚
˚
carbon atoms of 0.078 A, and an N–N distance of 2.948 A. These
parameters are middle-of-the-range compared to 3 and 4, with the
exception of the N–N distance, which is significantly longer than
the other examples. Again, this appears to point to a reasonable
degree of flexibility in the 1,8-bis(dimethylamino)naphthyl group
and to the participation of crystal packing forces in the structure.
The phosphine ligand in 5c has a cone angle53 of 168◦ taking
into account the surfaces of the van der Waals spheres in the
substituents,54 with measurements taken directly from the crystal
structure. The cone angle of PPh3 in the triphenylphosphine
analogue Fe(CO)4PPh3 (5a) is 151◦ when calculated by the same
method.55
Scheme 2 Syntheses of 5c, 6 and 7 from 3.
5c readily crystallised and an X-ray crystal structure was
obtained of this complex (Fig. 6). It revealed a slightly distorted
trigonal-bipyramidal geometry about iron with the phosphine
in an axial position, as is seen in the analogous compound
Fe(CO)4(PPh3) (5a).49 Although it is often assumed that in trigo-
nal bipyramidal organometallic complexes the better p-acceptor
ligand adopts an equatorial position,50 evidence points to the r-
donation also being important51 with the weaker r-donor ligand
favouring the equatorial position (where CO < PPh3 in terms of
Comparison of electrospray activity of 5a, 5b and 5c
Using [P(C6H13)3(C14H29)]+ as an internal reference or in competi-
tion experiments it was shown that the ionisation efficiency of 5c
is approximately 20x that of 5b in acidic CH2Cl2, despite 5c having
just one site for protonation compared to three for 5c. 5a was not
detectable under these conditions. The 20x improvement in signal
intensity for 5c over 5b is probably conservative; the mass spectra
of 5c were greatly superior under nearly all conditions but less
pronounced under the relatively high concentrations used in the
competition and internal reference experiments. It certainly ap-
pears that the primary advantage of the proton sponge phosphine
will be under conditions that are marginal, and those in which the
speciation is complicated. Other circumstances in which ESI-MS
analysis is challenging and where proton sponge phosphines may
provide useful handles include the characterisation of complexes
in ionic liquids56 or in non-polar solvents such as toluene or
hexane (in which lipophilic ionic liquids are required to assist
the formation of a stable spray).57
The proton sponge ligand is also more selective, always pro-
viding only [M + H]+ ions except in neutral or basic solutions
containing added Na+, which presumably associates with the
complex via an interaction with the carbonyl ligands. Both 5b
and 5c showed the appearance of the free ligand in all spectra;
the presence of phosphine oxide suggests that the ligand is not
appearing as a fragment generated by the ionisation process, but
rather is dissociated to a reasonable degree in solution (Fig. 7).
Its appearance suggests a degree of decomposition; this is not
unusual in ESI mass spectra, as the concentration of sample used
Fig. 6 ORTEP plot of 5c. Non-hydrogen atoms are represented by
Gaussian ellipsoids at the 50% probability level. Hydrogen atoms are
˚
shown with arbitrarily small thermal parameters. Key bond lengths (A)
and angles (◦): Fe–P 2.2739(6), Fe–C1 1.791(2), Fe–C4 1.776(3), P–C12
1.8416(19), N1–C11 1.428(2), N2–C19 1.429(3); Fe–P–C12 117.46(6),
C31–P–C41 101.38(9), C12–P–C41 103.95(9).
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Conclusions
In order to detect neutral complexes by ESI-MS under challenging
conditions of low concentration or in the presence of abundant
competing species-circumstances common when wishing to study
catalytic systems, for example—the complex must possess func-
tionality capable of selective ionisation with high efficiency. Proton
sponges confer exactly these properties to a ligand, and the ligand
in turn confers them to the complexes to which they are bound.
Absolute selectivity for H+ means simple spectra uncomplicated
by multiple signals derived from different ionisation mechanisms.
Complexes incorporating proton sponge phosphines provide
strong [M + H]+ ions even in solutions free of added H+ or protic
solvents, making the ligands ideal for facilitating direct ESI-MS
analysis of catalytic reactions involving neutral complexes and
phosphine ligands.
Fig. 7 Positive-ion ESI-MS of 5c in CH2Cl2–trace formic acid. The inset
shows the experimental and calculated isotope patterns. Free ligand 3 and
free oxidised ligand 3a were always present in spectra of metal complexes
to at least some degree.
Acknowledgements
is very low (micromolar) and so the presence of small amounts
of air and moisture in the solvent becomes significant. The 31P
NMR spectrum, collected at much higher concentration, showed
insignificant levels of decomposition (in the form of free phosphine
or phosphine oxide).
The syntheses of the manganese and tungsten complexes 6
and 7 further demonstrated the ability of this proton sponge
derivative 3 to act as a conventional phosphine ligand, with
bonding via phosphorus and the –NMe2 groups of the proton
sponge uninvolved in bonding to the metal. Both complexes
displayed strong [M + H]+ ions in their ESI mass spectra. MS/MS
studies on these compounds (and of 5c) revealed that the primary
fragmentation process was loss of the protonated ligand from the
metal core (see Fig. 8). For 6, a small amount of [6 − 2CO + H]+ is
present, indicating competition between CO and phosphine loss;
this process is not observed at all for 5c or 7.
N. J. F. thanks the EPSRC for a studentship and Professor Brian
F. G. Johnson for support. J. S. M. thanks Natural Sciences and
Engineering Research Council (NSERC) of Canada, the Canada
Foundation for Innovation (CFI) and the British Columbia
Knowledge Development Fund (BCKDF), and the University of
Victoria for instrumentation and operational funding.
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