Through-Space Shielding Effects of Metal-Complexed Phenyl Rings

Article abstract of DOI:10.1021/acs.joc.6b01858

Several naphthalene compounds containing a methyl group in a 1,8-relationship to a metal-complexed phenyl ring bearing various substituents have been synthesized. The through-space shielding effects of the phenyl ring, as a function of substituent and com

Full text of DOI:10.1021/acs.joc.6b01858

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Article
Through-Space Shielding Effects of Metal-Complexed Phenyl Rings
William Herbert Miles, Michael J. Robinson, Samantha G. Lessard, and Dasan M. Thamattoor
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b01858 • Publication Date (Web): 18 Oct 2016
Downloaded from http://pubs.acs.org on October 22, 2016
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Through-Space Shielding Effects of Metal-
Complexed Phenyl Rings
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William H. Miles,†* Michael J. Robinson,† Samantha G. Lessard,‡ and Dasan M. Thamattoor‡*
†Department of Chemistry, Lafayette College, Easton, PA 18042
‡Department of Chemistry, Colby College, Waterville, ME 04901
milesw@lafayette.edu, dmthamat@colby.edu
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to
the journal that you are submitting your paper to)
X
M
CH3
Set 1: M = Cr(CO)3 and X = H; CH3; OCH3; N(CH3)2; F; CO2CH3
Set 2: M = Cr(CO)3; Mn+(CO)3 BF4 ; RuCp+ PF6 and X = CH3
Several naphthalene compounds containing a methyl group in a 1,8-relationship to a metal-
complexed phenyl ring bearing various substituents have been synthesized. The through-space
shielding effects of the phenyl ring, as a function of substituent and complexing metal species,
1
were monitored by observing the H NMR signal of the methyl group located in the shielding
zone of the ring. In all cases, the methyl signal was slightly more downfield in the complexes
than in the uncomplexed analogs. A comparison of available crystal structures, however, show
that the phenyl ring is slightly closer to the naphthyl methyl group in the complexes than in their
metal-free counterparts. X-ray structures and DFT calculations also reveal a slight elongation in
the average length of the carbon-carbon bonds of the phenyl ring upon complexation. The effect
of substituents on the signal of the naphthyl methyl group is small but discernible in the
uncomplexed derivatives, and consistent with our previous report. A similar trend is absent in the
corresponding metal complexes, as exemplified by the chromium series, and the effect of the
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metal appears to be more dominant than that of the substituents. These observations were found
to be in line with NMR shift calculations.
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Introduction
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Following the development of a ring current model in the 1930s to describe the diamagnetic
anisotropy of aromatic molecules,1 a number of workers have investigated the interplay among
ring current, aromaticity, and NMR chemical shifts in such systems.2 It is now well known that
the magnetic field induced by a diatropic ring current in aromatic species causes the peripheral
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1
(outside) protons to be deshielded and appear downfield in the H NMR spectrum, whereas
protons positioned within the ring are shielded and show an upfield chemical shift. Likewise,
protons that are located opposite the face of an aromatic ring are also in the shielding zone and
show chemical shifts that are upfield in the NMR spectrum.3
Although the influence of substituents on the chemical shifts of phenyl ring protons have been
investigated in detail,4 much less is known about how substituents affect the through-space
shielding of aromatic rings.5 In this regard, we recently reported a detailed study on the influence
of substituents on the through-space shielding of phenyl rings based on our investigation of ten
naphthalene derivatives (1a-j).6 The chemical shift of the methyl group in 1 was carefully
monitored as the substituent X was varied over a broad range of electronic effects.
X
1a: X = H
1b: X = p-CH3
1f: X = p-Cl
1g: X = p-NH2
CH3
1c: X = p-OCH3 1h: X = p-CN
1d: X = p-F
1e: X = o-F
1i: X = p-CHO
1j: X = p-NO2
We concluded that although the perturbation of the chemical shift of the methyl signal was
1
quite small in 1, a trend could still be discerned. In the H NMR, the methyl signal was slightly
upfield when X was an electron-withdrawing compared to when X was electron donating. We
hypothesized that the electron withdrawing group reduced the electron density on the phenyl ring
which led to slight polarization of the C-H bonds of the methyl group so that the hydrogens were
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somewhat shielded (Figure 1a). An electron donating group, on the other hand, would have just
the opposite effect (Figure 1b). Our computational studies were consistent with the experimental
observations.6
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EWG
EDG
+
-
δ
δ
+
-
δ
δ
H
H
(a)
(b)
H2C
H2C
Figure 1. Proposed polarization of methyl C-H bonds in 1 dependent on the nature of the
substituent X.6
The notion of polarization discussed above, and pictorially depicted in Figure 1, finds analogy
in a computational report that examined the shielding of the proximal hydrogen of H2 located
above the face of a benzene ring complexed with a metal (alkali or alkaline) cation on the other
face (Figure 2).7 That work suggested that complexation of the benzene ring with a metal cation
had a substantial synergistic influence on shielding in that “…the decreased p electron density on
the side of the complexed benzene ring opposite the cation polarizes the covalent bond of
diatomic hydrogen, increasing the electron density near the proximal hydrogen, with the
consequence that the proximal hydrogen becomes more shielded.”7
H
-
H
δ
+
δ
M+
Figure 2. The cation-p complex is calculated to cause polarization of H2 leading to enhanced
shielding of the hydrogen proximal to the ring.7
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The consequences of complexing an aromatic ring with a metal containing fragment, however,
remain a controversial subject especially with respect to the question of aromaticity. The
pronounced change in reactivity between aromatic compounds and their corresponding transition
metal complexes (e.g. Cr(CO)3, Mn(CO)3+, RuCp+ systems) is well established, in which the
metal dramatically facilitates the addition of nucleophiles to the aromatic ring.8 The significant
upfield shifts in the 1H and 13C NMR of nuclei bonded to the aromatic ring upon complexation of
the metal has led some to propose that aromaticity is reduced in the complexed compounds.9 In
examining cyclophanes and other compounds where there are protons in the shielding region of
the aromatic system, complexation usually leads to a downfield shift of those protons in the
complexed compounds relative to the corresponding uncomplexed compounds, suggesting that
the ring current is attenuated by complexation.10 Many computational studies have suggested that
aromaticity is diminished with coordination with a transition metal, with one going so far as to
suggest that the aromatic ring becomes antiaromatic in η6-C6H6Cr(CO)3.11 Other studies, both
experimental and computational, however, have argued that aromaticity is either unchanged12 or
increased with complexation.13
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In light of the foregoing reports, we considered transition metal complexes of 1 as new probe
systems to experimentally investigate the effects of complexation, and the influence of
substituents, on shielding by the phenyl ring. Accordingly, we synthesized eight compounds
(Figure 3) where the metal is complexed to the phenyl ring on the side opposite to that facing the
naphthyl methyl group. Six of these complexes are formally derived from the precursors 1a-d
and the remaining two were obtained from new compounds 1k and 1l. Herein we report on the
chemical shift perturbations of the methyl signal and other pertinent physical properties in these
complexes.
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X
X
1a-Cr: M= Cr(CO)3; X = H
1b-Cr: M= Cr(CO)3; X = CH3
1b-Mn: M= Mn+(CO)3 BF4 ; X = CH3
1b-Ru: M= RuCp+ PF6 ; X = CH3
1c-Cr: M= Cr(CO)3; X = OCH3
1d-Cr: M= Cr(CO)3; X = F
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M
CH3
CH3
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1k: X = N(CH3)2
1l: X = CO2CH3
1k-Cr: M= Cr(CO)3; X = N(CH3)2
1l-Cr: M= Cr(CO)3; X = CO2CH3
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Figure 3. The eight metal complexes of compounds 1(a-d,k,l)-Cr, 1b-Mn, and 1b-Ru employed
in this study.
Results and Discussion
Syntheses of η6-arene complexes: The syntheses of 1a-d using the Suzuki–Miyaura reaction
have been previously described6 and new compounds 1k and 1l were prepared in an analogous
manner (Scheme 1). The direct complexation of the aryl ring of 1(a-c) and 1k by chromium
using classical conditions8a for the coordination of aromatic rings by the Cr(CO)3 moiety
(Cr(CO)6, O(n-Bu)2, THF, 140-150 °C; Scheme 1) gave chromium complexes 1(a-c)-Cr and 1k-
Cr (20-87% yield). Complexes 1(a-c)-Cr and 1k-Cr were readily purified, removing variable
amounts of side products tentatively identified as the chromium complexes resulting from the
complexation of the naphthyl ring. The very poor yields for the direct complexation of the
electron-poor aromatic rings of 1d and 1l by the Cr(CO)3 moiety necessitated an alternative
approach to the synthesis of 1d-Cr and 1l-Cr. Several studies have shown the very high
reactivity of η6-chloroarylchromium complexes (much higher than the corresponding
uncomplexed chloroarene compounds) with arylboronic acids using the Suzuki–Miyaura
reaction.14 The required boronic acid 2 was prepared by lithium-halogen exchange of 1-bromo-8-
methylnaphthalene with methyllithium, followed by trimethyl borate and hydrolysis of the
intermediate boronic ester (40% yield). The Suzuki–Miyaura coupling of 2 with η6-(4-
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XC6H4Cl)Cr(CO)3 complexes (X=F, CO2CH3)15 gave the desired chromium complexes 1d-Cr
and 1l-Cr in 22% and 48% yield, respectively.
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X
X
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Cr(CO)3
Cr(CO)6
THF, (n-Bu)2O
140-150 °C
CH3 Br
CH3
CH3
4-XC6H4B(OH)2
Suzuki-Miyaura
(27-55%)
(0-87%)
1(a-c)-Cr
1k-Cr
1
1a: X=H
1b: X=CH3
1c: X=OCH3 1l: X=CO2Me
1d: X=F
1k: X=NMe2
1. CH3Li
2. B(OCH3)3
3. H2O, HCl
(40%)
X (F, CO2Me)
Cr(CO)3
X
Cr(CO)3
CH3 B(OH)2
CH3
Cl
Pd2(dba)3
SPhos
K3PO4
2
1d-Cr
1l-Cr
(22-48%)
Scheme 1. Synthesis of 1(a-d,k,l) and their chromium complexes.
In our initial attempts to synthesize manganese arene complex 1b-Mn under standard
conditions (BrMn(CO)5, AgBF4, CH2Cl2, 1h, 40 °C; 1b, 4h, 40 °C)16 a yellow air-sensitive solid
1
was isolated that was difficult to characterize or purify. The H NMR in CD2Cl2 gave broad
1
signals that pointed to the coordination of the naphthalene ring to give complex 3. When the H
NMR was conducted in CD3CN or (CD3)2CO, the only signals observed were for the
uncomplexed 1b.17 Suspecting that under these conditions we were observing the kinetic
product(s), 3 was dissolved in ClCH2CH2Cl, additional 1b was added, and reaction mixture was
heated to 80 °C for 20 h. The 1H NMR of the resulting product was consistent with the structure
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of the desired complex 1b-Mn. Based on these observations, a simple one-pot reaction
(BrMn(CO)5, AgBF4, ClCH2CH2Cl, 1h, 40 °C; 1b, 20h, 80 °C) was developed (Scheme 2) to
give 1b-Mn in 34% yield.
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CH3
CH3
+
-
Mn(CO)3 BF4
1. AgBF4, CH2Cl2 40 °C; 1h
2. 1b, 80 °C, 20 h
CH3
CH3
BrMn(CO)5
(34%)
+
-
Mn(CO)3 BF4
1b-Mn
3
Scheme 2. Synthesis of 1b-Mn.
In contrast to the difficulty in isolating the kinetic product of the complexation of 1b by
Mn(CO)3+, the η6-naphthalene complexes 4a and 4b formed by the complexation of 1b by
RuCp(CH3CN)3PF618 were easily isolated and identified (Scheme 3). These complexes, which
rapidly formed as a 2:1 mixture in acetone, exhibited clear evidence of coordination by the
naphthalene ring, with signals at δ 6.25 (t) and 6.10 (d) for 4a and at δ 6.35 (t) and 6.01 (d) for
4b. Further purification of the major product by recrystallization allowed the conclusive
assignment of the major product as 4a, the result of coordination of the methyl-substituted ring
of the naphthyl moiety. In addition to the downfield shift exhibited by the C8 methyl group (δ =
2.09) relative to the parent compound 1b (δ = 2.02), an NOE experiment showed the close
proximity of the C-7 proton at δ 6.10 with the methyl group of the coordinated naphthyl ring at δ
2.09. Further, the non-equivalency of the protons on the 4-methylphenyl ring strongly suggests
that there is restricted rotation about the C1/aryl bond. As in the case of manganese arene
complex 1b-Mn, the synthesis of the ruthenium arene complex 1b-Ru required more forcing
conditions (Scheme 3). The thermodynamic product 1b-Ru was formed under refluxing
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conditions in 3-pentanone (but not in 1,2-dichloroethane) in 77% yield. These results are very
similar to Mann’s studies19 for the complexation of rubrene (5,6,11,12-tetraphenyltetracene) by
RuCp+, in which there was rapid complexation by the polyaromatic tetracene ring at room
temperature (kinetic product), with longer reaction times and higher temperatures leading to
complexation by one of the phenyl rings (thermodynamic product).
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Scheme 3. Synthesis of 4a, 4b, and 1b-Ru.
IR Spectroscopy: The IR stretches for the carbonyl ligands of transition metal arene complexes
have been correlated with the electron donating and withdrawing ability of substituents on the
coordinated arene ring.20 There are very clear trends for the (CO)3Cr(arene) series, with electron-
donating substituents leading to lower energy bands and electron-withdrawing substituents
leading to higher energy bands for the CO ligand stretches (Table 1). Complexes 1(a-d)-Cr and
1(k,l)-Cr exhibited comparable trends, with electron-donating substituents (as in 1(b,c)-Cr and
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1k-Cr) on the arene ring leading to lower energy bands for the CO ligand stretch and electron-
withdrawing substituents (as in 1d-Cr and 1l-Cr) leading to higher energy bands for the CO
ligand stretch relative to the parent complex 1a-Cr (Table 1).
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Table 1. IR stretches for the carbonyl ligands of 1(a-d,k,l)-Cr and η6-(XC6H5)Cr(CO)3
complexes.a
Substituent Compound CO stretches
Change from
Compound
CO stretches Change from
on phenylb
(cm-1)
1a-Cr (cm-1)c
(cm-1)d
C6H6Cr(CO)3 (cm-1)e
C6H6Cr(CO)3
H
1a-Cr
1b-Cr
1c-Cr
1d-Cr
1k-Cr
1l-Cr
1969, 1892
1965, 1887
1966, 1886
1978, 1904
1953, 1870
1978, 1908
0
1974, 1894
0
CH3C6H5Cr(CO)3
CH3OC6H5Cr(CO)3
FC6H5Cr(CO)3
CH3
-4, -5
-3, -6
9, 12
-16, -22
9, 16
1969, 1889 -5, -5
1973, 1891 -1, -3
1982, 1903 8, 9
OCH3
F
(CH3)2NC6H5Cr(CO)3
CH3CO2C6H5Cr(CO)3
N(CH3)2
1956, 1868 -18, -26
CO2CH3
1985, 1911 11, 17
a Recorded in CH2Cl2; b For Cr-complexes of the 1 series, these refer to X in Figure 3; c Complex minus 1a-Cr;
d From reference 20; e Complex minus C6H6Cr(CO)3.
.13C NMR Spectroscopy: In a similar manner, the CNMR shifts of the carbonyl ligand of η6-
13
arene complexes 1-Cr have been correlated to the electron-donating and withdrawing ability of
substituents of the coordinated arene ring.20 In a series of η6-(XC6H5)Cr(CO)3 complexes, the 13C
NMR resonance of the carbon of the carbonyl ligand for the more electron-donating substituents
on the arene ring led to a downfield shift, while the more electron-withdrawing substituents on
the arene ring led to an upfield shift relative to the parent complex, C6H6Cr(CO)3 (Table 2).
These results were rationalized in terms of the degree of back donation of the chromium to the
carbonyl ligand. A similar trend was observed for complexes 1(b,c,k)-Cr, with the electron-
donating substituents (X = CH3, OCH3, N(CH3)2) on the arene ring leading to downfield shifts of
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the carbonyl ligand with respect to the unsubstituted complex 1a-Cr, while the complexes 1(d,l)-
Cr with electron-withdrawing substituents (X=F, CO2Me) on the arene ring led to upfield shifts
of the carbonyl ligand with respect to 1a-Cr (Table 2).
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Table 2. 13C chemical shifts for the carbonyl ligands of 1-Cr and η6-(4-XC6H5)Cr(CO)3
Complexes.
Substituent Compound 13C shift for
Change from
Compound
13C shift for Change from
on phenyla
H
CO (ppm)b
233.36
233.74
233.58
232.16
235.01
231.98
1a-Cr (ppm)c
CO (ppm)d
234.00
234.29
234.25
232.98
235.52
231.98
C6H6Cr(CO)3 (ppm)e
C6H6Cr(CO)3
1a-Cr
1b-Cr
1c-Cr
1d-Cr
1k-Cr
1l-Cr
0
0
CH3C6H5Cr(CO)3
CH3OC6H5Cr(CO)3
FC6H5Cr(CO)3
CH3
0.38
0.22
-1.20
1.65
-1.38
0.29
0.25
-1.02
1.52
-2.02
OCH3
F
(CH3)2NC6H5Cr(CO)3
CH3CO2C6H5Cr(CO)3
N(CH3)2
CO2CH3
aFor Cr-complexes of the 1 series, these refer to X in Figure 3; b Recorded in CD2Cl2; c Complex minus 1a-Cr;
d From reference 20; e Complex minus C6H6Cr(CO)3.
X-ray crystallographic studies: Single-crystal X-ray diffraction experiments were performed
on all eight complexes and their structures are shown in Figure 4. Of the formally uncomplexed
precursors 1, the structure of 1a, which is a liquid at ambient temperature, was not determined
while those of 1b-d have been previously published by us6 and are not shown here. The amine
1k and ester 1l are new, however, and their structures are shown in Figure 5.
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1a-Cr
1b-Cr
1c-Cr
1d-Cr
1k-Cr
1l-Cr
1b-Mn*
1b-Ru*
Figure 4. Single-crystal x-ray structures of 1(a-d)-Cr (top) and 1(k,l)-Cr, 1b-Mn, and 1b-Ru
(bottom). Thermal ellipsoids are displayed at a 50% probability level. *For clarity, counter
anions are not shown.
1k
1l
Figure 5. Single-crystal x-ray structures of 1k and 1l. Thermal ellipsoids are displayed at a 50%
probability level.
The salient crystal structure data for the uncomplexed compounds 1(b-d, k, and l) and
their metal complexes are collected in Table 3. These data indicate that, in the solid state, the
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phenyl ring is substantially twisted away from the naphthyl ring. Furthermore, the length of the
carbon-carbon bond connecting the phenyl ring to the naphthyl ring, which is comparable to the
corresponding bonds in 1,8-diarylnaphthalenes,21 does not indicate any kind of conjugation
between the two ring systems. Consistent with previous observations on related systems,9a,b,22 the
average length of the carbon-carbon bonds of the phenyl ring increases in the complexes but
there is no indication of any length alternation among them. In any case, these changes increase
the perimeter and surface area of the phenyl ring in the complexes with implications for reduced
electron density. Two other interesting geometric distortions are also revealed by the X-ray
structures. The first is that the distance between the naphthyl methyl group and the centroid of
the phenyl ring appears to decrease slightly upon complexation. This is evident by comparing
1(b-d, k, l) to their corresponding metal complexes. This observation is particularly significant in
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1
the context of comparing the H NMR signals displayed by the naphthyl methyl groups in the
complexed and uncomplexed compounds (vide infra). Second, phenyl rings bearing strong p
donors show an appreciable deviation from planarity in their chromium complexes as can be
seen from the structures of 1c-Cr, 1d-Cr, and 1k-Cr. In these instances, the phenyl ring is
slightly bowl-shaped with the convex face closer to the metal. Similar observations have been
reported for the h6-chromium tricarbonyl complexes of substituted benzenes.23
Table 3. Selected crystallographic data for the metal complexes and some of their metal-free
precursors. Refer to structure below for parameters used in the table.
X
C4
Q
H3C1 C3
Q = centroid of
phenyl ring
C2
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Compound
Phenyl-naphthyl
interplanar angle bond
Length of C2-C3
C1 to Q
distance
C3-Q-C4
angle
Average C-C bond
length of phenyl ring
1a-Cra
1bb
66.11°
89.40°
77.06°
66.96°
77.09°
69.08°
79.87°
67.78°
62.45°
82.66°
85.10°
81.86°
74.54°
1.50 Å
1.50 Å
1.49 Å
1.50 Å
1.51 Å
1.49 Å
1.50 Å
1.49 Å
1.49 Å
1.49 Å
1.50 Å
1.50 Å
1.50 Å
1.41 Å
1.38 Å
1.41 Å
1.41 Å
1.41 Å
1.39 Å
1.41 Å
1.38 Å
1.41 Å
1.39 Å
1.41 Å
1.39 Å
1.41 Å
3.44 Å
3.53 Å
3.34 Å
3.40 Å
3.40 Å
3.51 Å
3.26 Å
3.50 Å
3.35 Å
3.42 Å
3.34 Å
3.49 Å
3.38 Å
178.64°
179.86°
178.91°
177.88°
178.72°
179.56°
176.51°
179.51°
177.19°
179.67°
176.58°
179.62°
179.14°
1b-Cra
1b-Mna
1b-Rua
1cb
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1c-Cra
1db
1d-Cra
1ka
1k-Cra
1la
1l-Cra
a) This work. b) From reference 6.
1H NMR Spectroscopy: All H NMR spectra of 1 and, 1(a-d,k,l)-Cr, 1b-Mn, and 1b-Ru were
recorded in CD2Cl2 at 400 MHz. The methyl signal for 1-methylnaphthalene appears at δ 2.691,
with a significant upfield shift with the introduction of the phenyl ring (complexed or
uncomplexed) at the 8-position (Table 4). The chemical shifts for the methyl protons of 1 in
CD2Cl2 follow the same trends observed in the previous study in CDCl3,6 in which there is a
small but significant downfield shift for the more electron-rich substituents relative to X = H. In
the case of 1l, bearing an electron withdrawing ester group, the upfield shift is negligibly small
compared to what we have previously noticed for other much stronger electron withdrawing
groups.6 For the chemical shifts of the methyl protons of the chromium arene complexes 1(a-
d,k,l)-Cr, there were two distinct trends: there was a significant downfield shift upon
complexation of the phenyl ring (~0.25 ppm) and a smaller downfield shift associated with the
more electron-rich substituents on 1(c,k)-Cr relative to 1a-Cr. The 1H NMR of naphthyl methyl
group of the η6-manganese complex 1b-Mn exhibited a downfield shift relative to 1b (2.20 vs.
2.02 ppm) and was upfield relative the analogous chromium complex 1b-Cr (2.20 vs. 2.26 ppm).
Complexes 1b-Mn and 1b-Cr are isoelectronic but complex 1b-Mn is cationic while 1b-Cr is
1
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neutral. The 1H NMR of naphthyl methyl group of the cationic η6-ruthenium complex 1b-Ru was
also downfield relative to 1b (2.10 vs. 2.02 ppm) but not as much as either 1b-Cr and 1b-Mn.
Table 4. 1H Chemical Shift Data for the Naphthyl Methyl Signal for 1 (a-d,k,l) and 1(a-d,k,l)-
Cr in CD2Cl2.
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Substituent Compound
on phenyla
Naphthyl
Change from Compound
Naphthyl
Change from
Change from
CH3 (ppm) 1a (ppm)b
CH3 (ppm) 1a-Cr (ppm)c
precursor (ppm)d
H
1a
1b
1c
1d
1k
1l
1.996
2.017
2.030
2.009
2.073
1.994
0
1a-Cr
1b-Cr
1c-Cr
1d-Cr
1k-Cr
1l-Cr
2.265
2.264
2.283
2.263
2.345
0
0.269
0.247
0.253
0.254
0.272
CH3
0.021
0.034
0.013
0.077
-0.002
-0.001
0.018
-0.002
0.080
-0.006
OCH3
F
N(CH3)2
CO2CH3
2.258
0.264
a For the 1 series, these refer to X in Figure 3; Compound minus 1a; Complex minus 1a-Cr3; Complex minus
b
e
d
corresponding uncomplexed precursor in second column.
Computational Results: Structures of the complexes and their metal-free precursors were
modeled using hybrid density functional methods such as B3LYP24 and M0625 using several
basis sets as shown in Table 5. Although the chromium complexes could be,22a and were, fully
modeled using the 6-311G (2d,p) basis set, the LANL2DZ (Los Alamos National Laboratory 2-
double zeta) basis set, with relativistic effective core potentials (recp) implemented on the
transition metals,26 were also used to model all of the complexes. Where possible, the available
crystal structures were used as the starting point for geometry optimizations.
The calculations are in general agreement with the observed trends. Critical structural
parameters such as the C2-C3 bond length (see reference figure in Table 5), the increase in
average C-C bond length of the phenyl ring upon complexation, the C1 to Q distance, and even
the deviation from planarity of complexes containing p-donor substituents on the phenyl ring are
reproduced closely. Only in one case (M06/6-311G(d,p), LANL2DZ) was a minor discrepancy
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observed in the C1 to Q distance of the charged complexes 1b-Mn and 1b-Ru relative to the
uncomplexed precursor 1b.
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Table 5. Selected computational data for the metal complexes and their metal-free precursors.
Refer to structure in Table 3 for parameters used in this table.
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Compound Model
Phenyl-naphthyl
Length of
C2-C3 bond
C1 to Q
distance
C3-Q-C4
angle
Average C-C bond
length of phenyl ring
Chemistry interplanar angle
1a
A
74.22°
73.88°
69.42°
88.54°
84.96°
85.48°
73.30°
72.87°
69.55°
78.90°
76.66°
67.37°
55.51°
56.90°
58.48°
60.48°
72.46°
72.15°
69.55°
82.37°
80.72°
73.63°
73.47°
73.21°
69.32°
89.18°
90.00°
76.88°
71.59°
71.32°
67.87°
89.89°
89.88°
89.17°
71.34°
71.08°
68.05°
62.85°
62.58°
63.85°
1.50 Å
1.50 Å
1.48 Å
1.50 Å
1.50 Å
1.49 Å
1.50 Å
1.50 Å
1.48 Å
1.50 Å
1.50 Å
1.49 Å
1.49 Å
1.48 Å
1.49 Å
1.48 Å
1.49 Å
1.50 Å
1.49 Å
1.50 Å
1.50 Å
1.49 Å
1.50 Å
1.50 Å
1.48 Å
1.50 Å
1.50 Å
1.49 Å
1.49 Å
1.49 Å
1.48 Å
1.50 Å
1.50 Å
1.49 Å
1.50 Å
1.50 Å
1.48 Å
1.50 Å
1.50 Å
1.49 Å
1.39 Å
1.40 Å
1.39 Å
1.41 Å
1.41 Å
1.41 Å
1.39 Å
1.40 Å
1.39 Å
1.41 Å
1.41 Å
1.41 Å
1.41 Å
1.41 Å
1.42 Å
1.42 Å
1.39 Å
1.40 Å
1.39 Å
1.41 Å
1.41 Å
1.41 Å
1.39 Å
1.39 Å
1.39 Å
1.41 Å
1.41 Å
1.41 Å
1.40 Å
1.40 Å
1.39 Å
1.42 Å
1.42 Å
1.41 Å
1.39 Å
1.40 Å
1.39 Å
1.41 Å
1.41 Å
1.41 Å
3.55 Å
3.55 Å
3.46 Å
3.39 Å
3.38 Å
3.33 Å
3.55 Å
3.55 Å
3.45 Å
3.39 Å
3.40 Å
3.39 Å
3.53 Å
3.47 Å
3.51 Å
3.45 Å
3.55 Å
3.55 Å
3.45 Å
3.38 Å
3.38 Å
3.36 Å
3.55 Å
3.55 Å
3.45 Å
3.37 Å
3.37 Å
3.33 Å
3.56 Å
3.56 Å
3.48 Å
3.36 Å
3.36 Å
3.31 Å
3.56 Å
3.56 Å
3.51 Å
3.46 Å
3.46 Å
3.41 Å
179.56°
179.55°
179.23°
178.24°
178.82°
179.18°
179.67°
179.64°
179.28°
178.23°
178.51°
179.16°
177.64°
178.35°
178.00°
178.71°
179.54°
179.52°
179.28°
176.77°
176.93°
177.60°
179.56°
179.54°
179.26°
177.57°
177.81°
178.16°
179.08°
179.05°
179.02°
175.50°
175.51°
175.89°
179.60°
179.58°
179.47°
178.32°
178.66°
179.51°
B
C
1a-Cr
1b
A
B(recp)
C(recp)
A
B
C
1b-Cr
A
B(recp)
C(recp)
B(recp)
C(recp)
B(recp)
C(recp)
A
1b-Mn
1b-Ru
1c
B
C
1c-Cr
1d
A
B(recp)
C(recp)
A
B
C
1d-Cr
1k
A
B(recp)
C(recp)
A
B
C
1k-Cr
1l
A
B(recp)
C(recp)
A
B
C
1l-Cr
A
B(recp)
C(recp)
A = B3LYP/6-311G(2d,p); B = B3LYP/6-311G(d,p); B(recp) = B3LYP/6-311G(d,p), LANL2DZ (transition
metals); C = M06/6-311G(d,p); C(recp) = M06/6-311G(d,p), LANL2DZ (transition metals).
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All of the chromium complexes and their uncomplexed precursors, which were optimized
at the same B3LYP/6-311G(2d,p) level of theory, were then subjected to an NMR calculation
using Cramer’s WP04 functional27 with cc-pVDZ as per the procedure of Bally and Rablen.28
These calculations simulated dichloromethane as the solvent medium for comparison with
experimental data. In particular, the chemical shifts of the three hydrogens of the naphthyl
methyl group were averaged and compared to the chemical shift of TMS which was also
computed in dichloromethane at the same level of theory. The differences between the computed
chemical shifts of the naphthyl methyl group in the complexed and uncomplexed pairs of
compounds are shown in Table 6. These results are consistent with the experimental observation
that complexation of 1a-d, k, l with the chromium tricarbonyl moiety causes a slight downfield
shift in the methyl signal.
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Table 6. Differences in the computed and experimentally observed chemical shifts of the
naphthyl methyl group for the chromium complexes and their uncomplexed precursors.
Compound
pairs
dCH3 complex - dCH3 precursor
(Calculated at GIAO/WP04/cc-pVDZ)
dCH3 complex - dCH3 precursor
(Experimental)
1a and 1a-Cr + 0.07 ppm
1b and 1b- + 0.07 ppm
Cr
+ 0.27 ppm
+ 0.25 ppm
1c and 1c-Cr + 0.05 ppm
1d and 1d- + 0.05 ppm
Cr
+ 0.25 ppm
+ 0.25 ppm
1k and 1k- + 0.06 ppm
Cr
+ 0.27 ppm
+ 0.26 ppm
1l and 1l-Cr + 0.10 ppm
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As can be discerned from the foregoing description, the various spectroscopic and structural data
for the chromium complexes in this study are comparable to the corresponding data for typical
η6-(arene)Cr(CO)3. The substituents on the phenyl ring of 1(a-d,k,l)-Cr affect the CO ligands in
the same way as typical η6-(arene)Cr(CO)3 complexes, as demonstrated by the lower energy
stretches and downfield 13C shifts of the carbonyl ligand observed for the more electron-donating
substituents. Similarly, X-ray structures indicate that deformations of the phenyl ring in 1(a-
d,k,l)-Cr, such as elongation of the carbon-carbon bonds and deviation from planarity, are also
comparable to those observed for other η6-(arene)Cr(CO)3. Further, the structural descriptors are
closely aligned with the results of computational studies.
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In considering the consequences for the proton chemical shift of the naphthyl methyl group
upon coordination of the metal on the opposite face of the phenyl ring, two factors favor the
upfield shift of the methyl group: the well documented8 electron-withdrawing nature of the
Cr(CO)3 moiety and the shortening of the distance between the centroid of the phenyl ring and
the methyl group. The previous study6 showed a small but distinct trend for the more electron-
withdrawing groups on the uncoordinated phenyl ring to affect an upfield shift, perhaps through
the polarization of the CH bonds of the methyl group. The crystallographic data of the complexes
1(a-d,k,l)-Cr, however, show the methyl group deeper in the shielding zone of the phenyl ring
than in the corresponding uncomplexed compounds 1(a-d,k,l). Yet, the chemical shift of the
naphthyl methyl group in 1(a-d,k,l) moves 0.25-0.27 ppm downfield upon complexation with
Cr(CO)3. As can be seen from Table 6, this trend is consistent with NMR calculations using
modern methods and basis sets.
An early report, from 1978, seemed to indicate that the chemical shift of the protons in the
shielding zone of the phenyl ring show a very slight upfield shift (0.05 ppm) upon complexing
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the ring with Cr(CO)3.29 Several subsequent studies that focused on Cr(CO)3 complexes of
various cyclophanes, however, showed that just the opposite was true. In these cyclophane
complexes, the protons in the shielding zone of the phenyl ring move downfield upon
complexation.30 Our results are also in line with the observations for such cyclophane systems.
Although the theoretical work of Schleyer, Sorenson, and coworkers has argued against such a
notion,12 several investigators have been inclined to attribute the downfield shift of the protons in
the shielding zone of the Cr(CO)3 complexes to an “attenuation” of aromaticity in the phenyl ring
upon complexation.9a,30a The structural distortions that we have noted above for 1(a-d,k,l)-Cr,
evident from experiment and theory, also appear to support the idea of reduced aromaticity in the
complexed phenyl ring. The lengthening of the carbon-carbon bonds of the phenyl ring in 1(a-
d,k,l)-Cr likely has a slight detrimental effect on conjugation. In addition, the increased surface
area of the phenyl ring in these complexes and the electron withdrawing effect of the metal
moiety also contributes to a slightly diminished p electron density of the phenyl ring.
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The naphthyl methyl signal in the cationic complexes 1b-Mn and 1b-Ru, are also slightly
downfield compared to the uncomplexed 1b but, as already mentioned, the shift is not as
pronounced as in 1b-Cr. In this regard, it is noteworthy that the distance of the methyl carbon
from the phenyl centroid in 1b-Cr is shorter than the corresponding distances in 1b-Mn and 1b-
Ru. Similar downfield shifts of protons in the shielding zone of phenyl rings of [10]-cyclophanes
complexed with cationic Mn and Ru moieties have been also noted by Kreindlin et al.31
Conclusions: In this study, a series of naphthalene compounds in which a metal-complexed
phenyl ring, bearing various substituents, is located in a 1,8-relationship to a methyl group have
been synthesized. The through-space shielding effects of the phenyl ring, as a function of
1
substituent and complexing metal species, were monitored by observing the H NMR chemical
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shift of the methyl group whose protons are pointed toward the face of the ring. In all cases, it
was observed that the chemical shift of the methyl signal was slightly more downfield in the
metal complexes than in the uncomplexed analogs. Furthermore, the effect of the metal fragment
on the methyl signal appears to be much more pronounced compared to that of just the phenyl
substituent in the uncomplexed analogs. X-ray structures of the complexes reveal a slight
decrease in the distance between the naphthyl methyl group and the centroid of the phenyl ring
upon complexation. There is, however, a significant elongation of the average bond lengths of
the carbon-carbon bonds of the phenyl ring as compared to the formally uncomplexed
precursors. The structural features determined experimentally, and the trends in the observed
chemical shifts of the naphthyl methyl signal, are also consistent with DFT calculations. To the
extent that shielding effects - as evaluated by NMR chemical shifts - diagnose the strength of
ring current in aromatic compounds, the findings reported herein indicate an attenuation of that
current in phenyl rings upon complexation with metals. The electron withdrawing effect of the
metal fragment, along with an increase in the perimeter and surface area of the phenyl ring, have
implications for the slightly diminished shielding of the naphthyl methyl group in the complexes.
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Experimental Section
General Experimental Procedures. All reactions were carried out under Ar. All glassware
was dried in the oven (110 °C) before use. IR spectra were obtained on a FT-IR using NaCl
1
liquid cells.. The H and 13C NMR spectra were recorded at 400 MHz and 100 MHz,
1
respectively, in the indicated solvents. All chemicals shifts in the H NMR are reported in ppm
13
relative to TMS (δ = 0.00 ppm) and in the C NMR are reported in ppm relative to CDCl3
(d=77.16 ppm), CD2Cl2 (d=53.84 ppm), (CD3)2CO(d=29.84 ppm for the methyl carbon), or TMS
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1
(d=0.00 ppm). For the H NMR spectra of 1(a-d,k,l), 1(a-d,k,l)-Cr, 1b-Mn and 1b-Ru, the
concentration of the compounds was 0.01-0.03 M. Flash chromatography was performed on 60
Å silica gel (40-75 µm). Anhydrous dibutyl ether, diethyl ether and THF were used without
further purification. Methylene chloride was dried over powdered 4 Å molecular sieves. High
resolution mass spectra were obtained on a Direct Analysis in Real Time - Time-Of-Flight
(DART-TOF) mass spectrometer.
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1-Bromo-8-methylnaphthalene. To a solution of 1,8-dibromonaphthalene (11.44 g, 40 mmol)
in THF (200 mL) was added CH3Li (26 mL, 42 mmol; 1.6 M in Et2O) at 0° C over 7 min. After
stirring for an additional 30 min at 0° C, CH3I (10.0 mL, 22.8 g, 161 mmol) was added dropwise.
The reaction mixture was allowed to warm to 22° C and stirred for an additional 3 h. The
reaction mixture was quenched with brine (100 mL), the organic phase separated and dried over
Na2SO4, and concentrated on the rotary evaporator. Column chromatography (silica gel;
petroleum ether) gave 1-bromo-8-methylnaphthalene (6.85 g) in greater than 90% purity. Further
purification by recrystallization from IPA gave 1-bromo-8-methylnaphthalene (5.71 g, 65%
yield; contaminated with ~ 5% 1,8-dimethylnaphthalene) as a crystalline, white solid: mp 73-
77°C (lit. mp 77-78 °C).32 1H NMR (CDCl3) d 7.83 (dd, J = 1.3, 7.5 Hz, 1H), 7.77 (dd, J = 1.3,
8.2 Hz, 1H), 7.71 (m, 1H), 7.37-7.34 (m, 2H), 7.29 (t, J = 7.7 Hz, 1H), 3.12 (s, 3H).
General procedure for the Susuki-Muyari coupling of 1-bromo-8-methylnaphthalene
with arylboronic acids. Synthesis of 1a-d and 1l. These compounds were prepared according to
the literature procedure6 or by the following procedure. To a mixture of 1-bromo-8-
methylnaphthalene (1.20 g, 5.43 mmol), 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl
(SPhos; 0.180 g, 0.438 mmol), arylboronic acid (8.0 mmol), K3PO4 (3.45 g, 16.3 mmol), and
Pd2(dba)3 (0.98 g, 0.107 mmol) was added toluene (10 mL) in a Schlenk flask. The reaction
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mixture was degassed with argon for 5 min and then refluxed for 16-20 h. After cooling to
ambient temperature, the reaction mixture was diluted with Et2O (100 mL), filtered through
Celite, and concentrated on the rotary evaporator. The crude product was purified by flash
chromatography (silica gel; hexanes→5% EtOAc in hexanes). Compound 1k was prepared by an
alternative method (see below) since the yield was very low with the above procedure. Yields
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and characterization for new compounds 1k and 1l are reported below, as are the H NMR
spectra for the other known 1-methyl-8-arylnaphthalene compounds.
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1-Methyl-8-phenylnaphthalene (1a).6 H NMR (CD2Cl2) d 7.86 (dd, J = 1.5, 8.1 Hz, 1H),
7.77 (br d, J = 8.0 Hz, 1H), 7.49-7.32 (m, 7H), 7.29 (dd, J = 1.5, 7.0 Hz, 1H), 7.23 (br d, J = 7.0
Hz, 1H), 2.00 (s, 3H).
1-Methyl-8-(4-methylphenyl)naphthalene (1b).6 1H NMR (CD2Cl2) d 7.84 (dd, J = 1.5, 8.1
Hz, 1H), 7.76 (br d, J = 8.4 Hz, 1H), 7.43 (dd, J = 7.0, 8.1 Hz, 1H), 7.36 (dd, J = 7.0, 8.0 Hz,
1H), 7.27 (dd, J = 1.5, 7.0 Hz, 1H), 7.23-7.21 (m, 5H), 2.43 (s, 3H), 2.02 (s, 3H).
1-(4-Methoxyphenyl)-8-methylnaphthalene (1c).6 1H NMR (CD2Cl2) d 7.83 (dd, J = 1.5, 8.1
Hz, 1H), 7.76 (br d, J = 8.2 Hz, 1H), 7.43 (dd, J = 7.0, 8.0 Hz, 1H), 7.36 (dd, J = 7.0, 8.1 Hz,
1H), 7.28 (dd, J = 1.5, 7.0 Hz, 1H), 7.25-7.21 (m, 3H), 6.93 (m, 2H), 3.86 (s, 3H), 2.03 (s, 3H).
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1-(4-Fluorophenyl)-8-methylnaphthalene (1d).6 H NMR (CD2Cl2) d 7.86 (dd, J = 1.5, 8.4
Hz, 1H), 7.77 (br d, J = 8.0 Hz, 1H), 7.44 (dd, J = 7.1, 8.3 Hz, 1H), 7.37 (dd, J = 7.3, 8.1 Hz,
1H), 7.34-7.27 (m, 3H), 7.24 (br d, J = 7.0 Hz, 1H), 7.10 (m, 2H), 2.01 (s, 3H).
Methyl 4-(8-methylnaphthalen-1-yl)benzoate (1l). White, crystalline solid: 0.150 g (28%
yield based on 0.423 g (1.91 mmol) of 1-bromo-8-methylnaphthalene); mp 129-131° C. IR
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(CH2Cl2) 1719, 1610 cm-1; H NMR (CD2Cl2) d 8.06 (m , 2H), 7.89 (dd, J = 1.3, 8.2 Hz, 1H),
7.79 (br d, J = 8.0 Hz, 1H), 7.48-7.42 (m, 3H), 7.39 (dd, J = 7.3, 8.0 Hz, 1H), 7.29 (dd, J = 1.4,
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7.0 Hz, 1H), 7.26 (br d, J = 7.0 Hz, 1H), 3.93 (s, 3H), 1.99 (s, 3H); 13C NMR (CD2Cl2) d 167.2,
150.2, 139.6, 135.3, 130.9, 130.2, 130.0 (2), 129.6, 129.5, 129.0 (2), 127.7, 126.0, 124.5, 52.3,
25.3.
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N,N-dimethyl-4-(8-methylnaphthalen-1-yl)aniline (1k). To a degassed solution of Pd(PPh3)4
(0.250 g, 0.216 mmol) and 1-bromo-8-methylnaphthalene (0.884 g, 4.00 mmol) in DME (30 mL)
was added a slurry of 4-(N,N-dimethylamino)phenylboronic acid (1.00 g, 6.06 mmol) in EtOH (9
mL), followed by Na2CO3 (1.50 g, 14.2 mmol) in degassed water (11 mL). The reaction mixture
was heated to reflux for 20 h. The reaction mixture was diluted with Et2O (100 mL), the organic
phase was separated, the aqueous phase was extracted with additional Et2O (100 mL), and the
combined organic phases were washed with 1 M NaOH (100 mL). The organic phase was then
extracted with 1 M HCl (2 x 100 mL). The aqueous phase was neutralized with 6 M NaOH and
extracted with Et2O (2 x 100 mL). The organic phase was concentrated on the rotary evaporator
to give crude product, which was purified by flash chromatography (silica gel; hexanes→50%
CH2Cl2 in hexanes) to give 1k as an off-colored solid (0.50 g; ~30% (4-(Me2N)C6H4)2). Further
recrystallization from petroleum ether gave 1k as a crystalline off-white solid (0.280 g, 27%
1
yield): mp 133-135° C. IR (CH2Cl2) 1611 cm-1; H NMR (CD2Cl2) d 7.80 (dd, J = 1.5, 8.0 Hz,
1H), 7.75 (br d, J = 8.1 Hz, 1H), 7.42 (dd, J = 7.3, 8.1 Hz, 1H), 7.34 (dd, J = 7.3, 8.1 Hz, 1H),
7.28 (dd, J = 1.5, 7.0 Hz, 1H), 7.26 (br d, J = 7.0 Hz, 1H), 7.15 (m, 2H), 6.76 (m, 2H), 3.00 (s,
6H), 2.07 (s, 3H). 13C NMR (CDCl3) d 149.6, 140.9, 136.0, 135.2, 133.4, 131.7, 130.4, 129.9,
129.6, 128.4, 127.4, 125.4, 124.5, 111.7, 40.8, 25.3.
General Procedure for the Direct Complexation of 1 using Cr(CO)6. Synthesis of 4(a-c)-
Cr and 1k-Cr. A mixture of 1 (1.70 mmol) and Cr(CO)6 (0.440 g, 2.00 mmol) in (n-Bu)2O (12
mL) and THF (1.0 mL) was purged with Ar. The reaction mixture was then heated in an oil bath
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to 140-150 °C for 44 h. The solvent was removed by vacuum to give the crude product. The
crude product was purified by flash chromatography (silica gel; for 1(a-c)-Cr, hexanes → 50%
Et2O in hexanes; for 1k-Cr, hexanes → 50% CH2Cl2 in hexanes) to give 1-Cr, in some cases
contaminated by η6-naphthalene complexed products. Further purification was accomplished by
washing with hexanes.
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(1-Methyl-8-(η6-phenyl)naphthalene)Cr(CO)3 (1a-Cr). Yellow, crystalline solid; 0.151 g
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(20% yield based on 0.46 g (2.1 mmol) of 1a). IR (CH2Cl2) 1969, 1892 cm-1; H NMR (CD2Cl2)
d 7.88 (dd, J = 1.3, 8.2 Hz, 1H), 7.78 (br d, J = 8.1 Hz, 1H), 7.75 (dd, J = 1.4, 7.0 Hz, 1H), 7.47
(dd, J = 7.3, 8.1 Hz, 1H), 7.39 (t, J = 7.5 Hz, 1H), 7.29 (br d, J = 7.0 Hz, 1H), 5.84 (dd, J = 0.9,
6.8 Hz, 2H), 5.65 (br t, J = 5.9 Hz, 1H), 5.37 (t, J = 6.4 Hz, 2H), 2.27 (s, 3H); 13C NMR (CD2Cl2)
d 233.4, 135.2, 134.33, 134.31, 134.1, 132.1, 131.0, 130.8, 128.1, 126.1, 124.9, 119.6, 99.8, 96.4,
88.8, 27.2.
1-Methyl-8-(η6-(4-methylphenyl))naphthalene)Cr(CO)3 (1b-Cr). Yellow, crystalline solid;
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0.177 g (31% yield based on 0.369 g (1.59 mmol) of 1b). IR (CH2Cl2) 1965, 1887 cm-1; H NMR
(CD2Cl2) d 7.87 (dd, J = 1.1, 8.1 Hz, 1H), 7.77 (br d, J = 8.0 Hz, 1H), 7.72 (dd, J = 1.3, 7.1 Hz,
1H), 7.44 (dd, J = 7.4, 8.0 Hz, 1H), 7.38 (br t, J = 7.5 Hz, 1H), 7.28 (br d, J = 7.0 Hz, 1H), 5.86
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(d, J = 6.6 Hz, 2H), 5.23 (d, J = 6.6 Hz, 2H), 2.29 (s, 3H), 2.26 (s, 3H); C NMR (CD2Cl2) d
233.7, 135.1, 134.4 (2), 134.1, 132.2, 130.7 (2), 128.1, 126.1, 125.0, 117.5, 112.3, 100.5, 89.8,
27.3, 20.8. 1D NOE experiments confirmed the assignment of the 3H signal at d 2.29 ppm as the
methyl group on the naphthalene ring and the 3H signal at d 2.26 ppm as the methyl group on the
phenyl ring.
1-(η6-(4-Methoxyphenyl))-8-methylnaphthalene Cr(CO)3 (1c-Cr). Yellow, crystalline solid;
1
0.470 g (73% yield based on 0.416 g (1.68 mmol) of 1c). IR (CH2Cl2) 1966, 1886 cm-1; H NMR
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(CD2Cl2) d 7.87 (d, J = 7.7 Hz, 1H), 7.77 (d, J = 8.1 Hz, 1H), 7.65 (d, J = 7.3 Hz, 1H), 7.42 (t, J
= 7.7 Hz, 1H), 7.38 (t, J = 7.5 Hz, 1H), 7.28 (d, J = 7.0 Hz, 1H), 5.91 (d, J = 7.0 Hz, 2H), 5.21
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(d, J = 7.0 Hz, 2H), 3.80 (s, 3H), 2.28 (s, 3H); C NMR (CD2Cl2) d 233.6, 144.5, 135.1, 134.4,
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133.7, 132.3, 130.9, 130.8, 128.2, 126.0, 125.0, 114.0, 100.2, 75.7, 56.2, 27.4.
(η6-N,N-dimethyl-4-(8-methylnaphthalen-1-yl)aniline)Cr(CO)3
(1k-Cr).
Yellow,
crystalline solid; 0.357 g (87% yield based on 0.271 g (1.04 mmol) of 1k). IR (CH2Cl2) 1953,
1870 cm-1; 1H NMR (CD2Cl2) d 7.85 (dd, J = 1.2, 8.4 Hz, 1H), 7.76 (br d, J = 8.1 Hz, 1H), 7.68
(dd, J = 1.5, 7.0 Hz, 1H), 7.44-7.35 (m, 2H), 7.27 (d, J = 7.0 Hz, 1H), 5.92 (d, J = 7.4 Hz, 2H),
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4.89 (d, J = 7.3 Hz, 2H), 2.97 (s, 6H), 2.35 (s, 3H); C NMR (CD2Cl2) d 235.0, 136.3, 135.1,
134.8 (2), 134.5, 132.6, 130.7, 130.5, 128.1, 125.9, 125.1, 111.4, 101.9, 72.1, 40.0, 27.5.
(8-Methylnaphthalen-1-yl)boronic acid (2). To a solution of 1-bromo-8-methylnaphthalene
(1.04 g, 4.70 mmol) in Et2O (25 mL) was added n-BuLi (3.5 mL, 1.6 M in hexane, 5.6 mmol) at
over 2 min. The reaction mixture was allowed to warm to 22° C, stirred for 2 h, then cooled to -
78 °C. B(OMe)3 (0.77 mL, 0.72 g, 6.9 mmol) in EtO2 (10 mL) was added in one portion, allowed
to warm to 22 °C, and stirred for 16 h. The reaction mixture was transferred to an Erlenmeyer
flask with additional EtO2 (20 mL), treated with aqueous HCl (2 M, 50 mL), and stirred
vigorously for 16 h. The organic phase was separated, the aqueous extracted with EtO2 (50 mL),
and the combined organic extracts were concentrated on the rotary evaporator. The crude product
was purified by flash chromatography (silica gel; hexanes→50% EtOAc in hexanes) to give 2 as
a white solid (0.35 g, 40% yield): mp 142-145°C. IR (CH2Cl2) 3618, 1602 cm-1; H NMR
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1H), 7.44 (dd, J = 6.8, 8.2 Hz, 1H), 7.39 (t, J = 7.3 Hz, 1H), 7.34 (br d, J = 7.3 Hz, 1H), 4.74 (s,
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2H), 2.79 (s, 3H); 13CNMR ((CD3)2CO) d 136.4, 135.0, 134.8, 130.5, 129.7, 128.3 ,127.9, 126.0,
125.4, 22.1; HRMS (DART-TOF) calcd for C11H11BO2 [M + 1]+ 187.0930, found 187.0934.
1-(η6-(4-Fluorophenyl))-8-methylnaphthaleneCr(CO)3 (1d-Cr). To an argon-purged
solution of boronic acid 2 (0.257 g, 1.38 mmol), η6-(4-FC6H4Cl) Cr(CO)315 (0.305 g, 1.14 mmol)
and SPhos (0.053 g, 0.13 mmol) in toluene (3 mL) was added K3PO4 (1.00 g, 4.71 mmol) and
Pd2(dba)3 (0.027 g, 0.29 mmol). The reaction mixture was heated to 80 °C for 2 h and 90° C for 3
h, cooled to ambient temperature, poured into Et2O (50 mL) and EtOAc (25 mL), filtered
through a plug of silica gel, and concentrated on the rotary evaporator. Flash chromatography
(silica gel; hexanes→30% toluene in hexanes) gave 1d-Cr (0.098 g, 22% yield) as a yellow
solid. IR (CH2Cl2) 1978, 1904 cm-1; 1H NMR (CDCl3) d 7.87 (d, J = 7.7 Hz, 1H), 7.78 (br d, J =
8.0 Hz, 1H), 7.58 (d, J = 6.6 Hz, 1H), 7.44-7.37 (m, 2H), 7.29 (d, J = 6.6 Hz, 1H), 5.89 (dd, J =
2.9, 6.6 Hz, 2H), 5.44 (dd, J = 5.1, 6.2 Hz, 2H), 2.26 (s, 3H); 13C NMR (CD2Cl2) d 232.1, 147.6
(JC-F = 265.1 Hz), 134.1, 134.2, 133.1 (2), 132.3, 131.0 (2), 128.2, 126.2, 124.9, 115.2, 98.5 (JC-F
= 10.7 Hz), 76.8 (JC-F = 20.5 Hz), 27.5.
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Methyl η6-(4-(8-methylnaphthalen-1-yl))benzoateCr(CO)3 (1l-Cr). Using the procedure
described in the synthesis of 1d-Cr but employing η6-(4-MeCO2C6H4Cl)Cr(CO)315 instead of η6-
(4-FC6H4Cl)Cr(CO)3, complex 1l-Cr was isolated as an orange, crystalline solid (0.180 g, 48%
yield based on 0.28 g (0.92 mmol) of η6-(4-MeCO2C6H4Cl)Cr(CO)3). IR (CH2Cl2) 1978, 1908,
1729 cm-1; 1H NMR (CD2Cl2) d 7.88 (dd, J = 1.1, 8.1 Hz, 1H), 7.78 (br d, J = 7.7 Hz, 1H), 7.69
(dd, J = 1.3, 7.1 Hz, 1H), 7.46 (dd, J = 7.3, 8.1 Hz, 1H), 7.41 (t, J = 7.7 Hz, 1H), 7.31 (d, J = 7.0
Hz, 1H), 6.09 (d, J = 7.0 Hz, 2H), 5.84 (d, J = 7.0 Hz, 2H), 3.94 (s, 3H), 2.26 (s, 3H); 13C NMR
(CD2Cl2) d 232.0, 166.5, 135.3, 134.1, 133.7 (2), 132.0, 131.2, 131.1, 128.1, 126.2, 124.7, 121.3,
97.0, 93.2, 90.5, 27.2.
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(1-Methyl-8-(η6-(4-methylphenyl))naphthalene)Mn(CO)3BF4 (1b-Mn). To a solution of
(CO)5MnBr (0.256 g, 0.96 mmol) in 1,2-dichloroethane (10 mL) in a 25 mL Schlenk flask was
added AgBF4 (0.191 g, 0.98 mmol). The reaction mixture was heated to 40 °C for 1 h. To the
reaction mixture was added 1b (0.48 g, 2.1 mmol) and then heated to 80 °C for 20h. After
cooling, the reaction mixture was filtered through a pad of Celite, washing with additional
CH2Cl2 (2 x 5 mL). Et2O (50 mL) was added and the precipitate was filtered to give complex 1b-
Mn (0.150 g, 34% yield) as a yellowish solid. IR (CH2Cl2) 2078, 2029 (sh), 2018 cm-1; 1H NMR
(CD2Cl2) d 8.03 (br d, J = 7.7 Hz, 1H), 7.87 (br d, J = 8.4 Hz, 1H), 7.59-7.48 (m, 3H), 7.41 (br d,
J = 6.6 Hz, 1H), 6.87 (d, J = 8.2 Hz, 2H), 6.48 (d, J = 6.2 Hz, 2H), 2.61 (s, 3H), 2.20 (s, 3H); 13C
NMR (CD2Cl2/CD3CN) d 214.9, 135.2, 133.6, 132.9, 132.8, 131.9, 131.3, 129.0, 128.2, 127.0,
124.6, 124.0, 120.7, 104.4, 96.8, 27.5, 20.0.
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(η6-1,2,3,4,9,10-1-Methyl-8-(4-methylphenyl)naphthalene)Mn(CO)3BF4 (3). To a solution
of (CO)5MnBr (0.310 g, 1.13 mmol) in CH2Cl2 (10 mL) in a 25 mL Schlenk flask was added
AgBF4 (0.225 g, 1.15 mmol). The reaction mixture was heated to 40 °C for 1 h. After cooling to
25 °C, 1b (0.58 g, 2.5 mmol) was added to the reaction mixture and then heated to 40 °C for 4 h.
After cooling, the solution containing 3 was decanted from the solids (AgBr) and the solids were
extracted with additional CH2Cl2 (5 mL). Et2O (70 mL) was added under Ar to the organic
solution and the rapidly filtered to give 3 (0.28 g, 34% yield) as a bright yellow solid that was
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air- and water-sensitive. Further purification invariably led to decomposition of 3. H NMR
(CD2Cl2) d 8.19 (br d, J = 7.7 Hz, 1H), 7.95 (br t, J = 7.7 Hz, 1H), 7.78 (br d, J = 6.2 Hz, 1H),
7.63 (m, 1H), 7.42-7.22 (m, 3H), 7.07 (br d, J = 7.7 Hz, 1H), 6.89 (br s, 1H), 6.56 (br s, 1H),
2.47 (s, 3H), 2.25 (s, 3H).
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1-methyl-8-(η6-(4-methylphenyl)naphthalene))RuCpPF6 (1b-Ru). To a Ar-degassed
solution of 1b (0.255 g, 1.10 mmol) in 3-pentanone (20 mL) was added CpRu(CH3CN)3PF6
(0.434g 1.00 mmol). The reaction mixture was refluxed for 24 h. The solvent was removed on a
rotary evaporator and washed with Et2O (2x25 mL). The crude product was dissolved in CH2Cl2
and filtered through a plug of Celite. After removal of the volatiles, the crude product was
redissolved in CH2Cl2 (10 mL), and Et2O (30 mL) was added. The precipitate was filtered and
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washed with additional Et2O (20 mL) to give 1b-Ru as a tan solid (0.41g, 77% yield). H NMR
(CD2Cl2) d 7.95 (dd, J = 2.2, 7.7 Hz, 1H), 7.81 (br d, J = 8.1 Hz, 1H), 7.51-7.43 (m, 3H), 7.33 (br
d, J = 7.0 Hz, 1H), 6.36 (d, J = 6.2 Hz, 2H), 6.33 (d, J = 6.2 Hz, 2H), 5.42 (s, 5H), 2.47 (s, 3H),
2.10 (s, 3H); 13C NMR (CD2Cl2) d 135.2, 133.6, 133.5, 131.8, 131.6, 131.5, 131.3, 128.1, 126.7,
124.5, 114.9, 102.4, 88.9, 85.3, 82.2, 27.5, 20.5.
(η6-1,2,3,4,9,10-1-Methyl-8-(4-methylphenyl))naphthaleneRuCp PF6 (4a) and (η6-
5,6,7,8,9,10-1-methyl-8-(4-methylphenyl))naphthaleneRuCpPF6 (4b). To a Ar-degassed
solution of 1b (0.150 g, 0.65 mmol) in acetone (10 mL) was added CpRu(CH3CN)3PF6 (0.216 g,
0.497 mmol). The reaction mixture was stirred at 22 °C for 1 h. The solvent was removed on a
rotary evaporator and the crude product was washed with Et2O (2 x 25 mL). The crude product
was dissolved in CH2Cl2 (10 mL), and Et2O (30 mL) was added to give a yellow precipitate that
was primarily complexes 4a and 4b as a 2:1 mixture (0.234 g). The crude product was dissolved
in CH2Cl2 (7 mL), filtered through a Celite plug, and degassed with Ar. Slow diffusion of Et2O
into the CH2Cl2 solution gave 4a·CH2Cl2 and 4b·CH2Cl2 as yellow/orange crystals (2:1 mixture
of 4a·CH2Cl2:4b·CH2Cl2, 0.260 g, 81% yield). Fractional recrystallization (slow diffusion of
Et2O into the CH2Cl2 solution) gave 4a·CH2Cl2 in greater than 95% purity. When 4a·CH2Cl2 is
dissolved in (CH3)2CO and the solvent is removed, 4a is obtained free of solvent. For 4a: 1H
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Page 29 of 32
The Journal of Organic Chemistry
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NMR (CD2Cl2) d 7.76 (dd, J = 1.3, 8.6 Hz, 1H), 7.59 (dd, J = 6.8, 8.6 Hz, 1H), 7.50 (dd, J = 1.3,
6.7 Hz, 1H), 7.36 (dm, J = 7.7 Hz, 1H), 7.29 (dd, J = 2.0, 7.9 Hz, 1H), 7.26 (br d, J = 7.7 Hz,
1H), 7.06 (dd, J = 2.0, 7.9 Hz, 1H), 6.91 (d, J = 5.8 Hz, 1H), 6.25 (t, J = 6.1 Hz, 1H), 6.11 (d, J =
5.8 Hz, 1H), 4.93 (s, 5H), 2.45 (s, 3H), 2.09 (s, 3H); 13C NMR (CD2Cl2) d 141.4, 138.9, 138.4,
135.0, 131.1, 130.4, 130.0, 129.9, 129.2, 128.6, 100.0, 97.7, 95.9, 90.6, 86.6, 84.1, 80.6, 25.3,
21.4. Partial 1H NMR (CD2Cl2) for 4b: d 7.64 (br d, J = 8.8 Hz, 1H), 7.52 (dd, J = 6.6, 8.8 Hz,
1H), 7.04 (dd, J = 0.7, 6.2 Hz, 1H), 6.35 (t, J = 6.0 Hz, 1H), 6.02 (dd, J = 0.7, 5.9 Hz, 1H), 5.07
(s, 5H), 2.47 (s, 3H), 1.94 (s, 3H). Anal Calcd for C23H21F6PRu: C, 50.82; H, 3.90. Found: C,
50.62; 3.94.
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General Procedure for X-ray Structure Determination. Crystals suitable for X-ray structure
determination were prepared by the slow evaporation of Et2O (for 1a-Cr and 1b-Cr), CH2Cl2
(for 1c-Cr and 1l-Cr), Et2O/cyclohexane (for 1d-Cr), or hexanes (for 1l); or by recrystallization
from EtOAc (for 1k and 1k-Cr) or CH2Cl2 (for 1b-Mn); or by slow diffusion of diethyl ether
into a solution of methylene chloride solution at 0° C (for 1b-Ru).
X-ray data were acquired at 173 K on a CCD diffractometer employing graphite
monochromated Mo Ka radiation (l = 0.71073 Å). The Apex2 suite of programs33 was used to
process the data and the SAINT software package34 was used to integrate the frames with a
narrow-frame algorithm. The multi-scan method (SADABS)35 was used to correct the data for
absorption effects. The SHELXTL software package36 was used to perform structure solution by
direct methods, and refinement by full-matrix least-squares on F2. All nonhydrogen atoms were
refined anisotropically with suggested weighting factors and the hydrogens were calculated on a
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