Cationic half-sandwich quinolinophaneoxazoline-based (η6-p- cymene)ruthenium(ii) complexes exhibiting different chirality types: Synthesis and structural determination by complementary spectroscopic methods

Article abstract of DOI:10.1039/c3dt52291a

Novel, optically pure 2-{4-methyl[2]paracyclo[2](5,8)quinolinophan-2-yl}-4- aryl/alkyloxazolines (QUIPHANOX) exhibiting both planar and central chirality have been prepared by reacting (R_p)- and (S_p)-2-cyano-4- methyl[2]paracyclo[2](

Full text of DOI:10.1039/c3dt52291a

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Cationic half-sandwich quinolinophaneoxazoline-
based (η6-p-cymene)ruthenium(II) complexes
exhibiting different chirality types: synthesis and
structural determination by complementary
spectroscopic methods†‡
Cite this: Dalton Trans., 2014, 43,
1636
Renzo Ruzziconi,*a Gianfranco Bellachioma,a Gianluca Ciancaleoni,a Susan Lepri,a
Stefano Superchi,*b Riccardo Zanasi*c and Guglielmo Monacoc
Novel,
optically
pure
2-{4-methyl[2]paracyclo[2](5,8)quinolinophan-2-yl}-4-aryl/alkyloxazolines
(QUIPHANOX) exhibiting both planar and central chirality have been prepared by reacting (Rp)- and (Sp)-
2-cyano-4-methyl[2]paracyclo[2](5,8)quinolinophane with 2-aryl/alkyl-2-aminoethanols. The reaction of
each of the above N,N-ligands with [Ru(η6-p-cymene)Cl2]2 in methanol, in the presence of either NH4PF6
or NaBPh4, gave the corresponding half-sandwich [(η6-p-cymene)Ru(QUIPHANOX)Cl]+Y−, (Y− = PF6
,
−
BPh4−) as stable complex salts exhibiting planar and carbon- and metal-centered chirality. The unknown
absolute configuration (AC) at the metal was determined by 1H NMR and by theoretical calculations of
electronic circular dichroism (ECD) spectra and subsequently confirmed by crystallographic X-ray analysis.
(Rp) and (Sp)-ligands afforded (RRu)- and (SRu)-configured complexes, respectively, independent of the AC
at the chiral carbon of the oxazoline moiety.
Received 21st August 2013,
Accepted 18th October 2013
DOI: 10.1039/c3dt52291a
www.rsc.org/dalton
systematic investigation of the correlation between the catalytic
activity and stereoselectivity of chiral organometallic com-
Introduction
Nowadays, a plethora of organometallic complexes active in plexes with reference to both the metal and the ligand remains
homogeneous catalysis is available thanks to the great variety a stimulating challenge. In this context, ligands having nitro-
of possible ligand–transition metal combinations. Moreover, gen as the donating heteroatom have raised great interest.1
the presence of chiral ligands makes these species widely Chiral C2-symmetric bisoxazolines, bipyridines, semicorrins
employed in asymmetric synthesis.
and diimino ligands are among the most known and they have
Many heteroatom donors such as phosphorus, nitrogen, been used to prepare chiral organometallic complexes for
oxygen, and sulfur are used to bind transition metals, and a stereoselective reactions.
combination of different heteroatoms has originated a myriad
Owing to a variety of peculiarities, among which are a high
of mono-, bi- and tridentate ligands. Most of the progress in electron transfer ability,2 a high coordinating capability toward
this area was based on empirical approaches; however, a a variety of heteroatoms,3 and a remarkable Lewis acid charac-
ter,4 ruthenium plays a leading role in many stereoselective
syntheses. While we refer the reader to several specialized
books and review articles,5 we wish to focus our attention on a
particular class of cationic half-sandwich ruthenium(II) arene
complexes having sp2 nitrogen as the electron donating
heteroatom.
aDipartimento di Chimica, Università di Perugia, Via Elce di Sotto 8, 06123 Perugia,
Italy. E-mail: ruzzchor@unipg.it
bDipartimento di Scienze, Università della Basilicata, Viale dell’Ateneo Lucano 10,
85100 Potenza, Italy
cDipartimento di Chimica e Biologia, Università di Salerno, via Giovanni Paolo II
Many chiral bidentate ligands having sp2-nitrogen as the
donating heteroatom have been employed to prepare a variety
of ruthenium complexes; bis(oxazolines),6 bipyridines,7 and
salen-type ligands8 have stimulated great interest as chirality
inducers in several ruthenium(II)-catalysed stereoselective
132, Fisciano, 84084 (SA), Italy
†We dedicate the present work in memory of our friend Professor Carlo Rosini,
who prematurely passed away.
‡Electronic supplementary information (ESI) available: NMR spectra of ligands
1–5 and complexes 6–10, ECD spectra of complexes 6–10, Cartesian coordinates
of DFT-optimized structures 6–10 and crystallographic data of (Sp,R,SRu)-7b.
CCDC 902347. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt52291a
syntheses.
Nevertheless,
ruthenium
complexes
with
nitrogen ligands exhibiting planar chirality have gained little
1636 | Dalton Trans., 2014, 43, 1636–1650
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consideration and, in general, their study was limited to the
For many years, circular dichroism (CD) analysis emerged
ferrocenyl scaffold.9 On the other hand, owing to their remark- as a valuable tool for structural analysis of chiral molecules
able rigidity, benzenophane-based systems seem suitable for both in natural compound chemistry and in biology.14 Nowa-
the structural determination of a variety of both dissymmetric days, quantum-chemical calculations are used along with the
and asymmetric N,N-type bidentate planar chiral ligands. In experimental CD methods to assign the configuration of
1999, through a complex multistep synthesis, Vögtle and Pfaltz different systems in solution15 including transition metal
were able to prepare both enantiomers of 13-pyridyl[2](1,4)- complexes with chiral ligands having different heteroatom
benzene[2](2.5)pyridinophane and 2-(pyrid-2-yl)[2]paracyclo[2]- donors.16
(5,8)quinolinophane that were stereochemically characterized
In this paper we report the synthesis and the structural
by their ECD spectra and X-ray crystallographic analysis.10 determination of chiral half-sandwich ruthenium(II)(η6-p-
These benzenophane derivatives were used to prepare chiral cymene)quinolinophaneoxazolines complexes 6–10 derived
copper and iridium complexes. To the best of our knowledge, from optically pure asymmetric 4-aryl/alkyl-substituted 2-{4′-
the only example of a cationic half-sandwich ruthenium(II)- methyl[2]paracyclo[2](5′,8′)quinolinophane-2′-yl)}oxazolines
arene complex involving a [2.2]paracyclophane-derived N,N- (QUIPHANOX) 1–5 (Fig. 1). Ligands 1–4 exhibit both planar
ligand was previously reported by us.11 The lack of information and central chirality while ligand 5 exhibits only planar chiral-
about chiral N,N-chelate ruthenium complexes with chiral N,N- ity. Our aim was to investigate the possibility of assigning the
bidentate ligands and our constant interest in the synthesis of AC at the metal directly in solution using complementary
new chiral [2]paracyclo[2](5,8)quinolinophane derivatives12 spectroscopic methods based on 1H NMR and electronic circu-
prompted us to perform a structural investigation of new asym- lar dichroism (ECD) techniques.
metric ruthenium complexes involving nitrogen-donor ligands
exhibiting different types of chirality.
The coordination of ruthenium(II)arene with an optically
pure N,N-bidentate ligand provides half-sandwich complexes
where the metal itself becomes a stereogenic center and, in
Results and discussion
principle, a mixture of two epimers at the metal can be formed The reaction of optically pure (Rp)-4-methyl-[2]paracyclo[2](5,8)-
whose molar ratio depends on their relative thermodynamic quinolinophane-2-carbaldehyde [(Rp)-11]12c with hydroxyl-
stability. This, in turn, is determined by the steric require- amine in refluxing chloroform, followed by in situ oxidation of
ments of the arene ligand. Crystallization usually provides a the resulting oxime (Rp)-12 with SeO2,17 provided (Rp)-2-cyano-
single solid epimer whose structure, and the AC at the metal, 4-methyl[2]paracyclo[2](5,8)quinolinophane [(Rp)-13] in 83%
is usually inferred by X-ray analysis.13 However, equilibration yield. Then, R1,R2-substituted 2-{4′-methyl[2]paracyclo[2](5′,8′)-
might occur in solution leading to a thermodynamic mixture quinolinophan-2′-yl}oxazolines 1–5 were obtained in 64–71%
of the two epimers at the metal. The epimerization rate yield by heating a mixture of nitrile 13 and commercially avail-
strongly depends on the steric requirements of both groups able (R)- or (S)-2-alkyl/aryl-2-aminoethanol or, in the case of
bonded to the chelating ligand and the η6-arene ligand, as well the ligand 10, 2-amino-2-methylpropan-1-ol, in chlorobenzene
as on the temperature, the equilibration times ranging from at 130 °C in the presence of anhydrous zinc chloride as the
few minutes to several days.13
Lewis catalyst (Scheme 1).18
Fig. 1 [Ruthenium(chloro)(η6-p-cymene)(QUIPHANOX)]+Y− salts 6–10 prepared from dichloro (η6-p-cymene)ruthenium(II) dimer and quinolino-
phaneoxazoline (QUIPHANOX) 1–5.
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Scheme 1
Finally, the [(η6-p-cymene)Ru(QUIPHANOX)Cl]+Y− complex
salts 6–10 yielding up to 94% were prepared simply by adding
either sodium tetraphenylborate or sodium hexafluoro-
phosphate to a mixture of dichloro(p-cymene)ruthenium(II)
dimer and each of the ligands 1–5 in methanol solution,
according to a literature protocol.11
In principle, the combination of each of the above optically
pure ligands 1–5 with dichloro(p-cymene)ruthenium(II) dimer
should generate a couple of epimers. Since the configuration
at the metal could be crucial for the stereochemical outcome
of prospective Ru(QUIPHANOX)Cl-catalyzed enantioselective
processes, we thought it essential to assess the relative stability
of the two potential epimers in solution and to determine the
respective AC at the metal (XM).
Scheme 2 Possible diastereoisomers in the complexation of ligands
1–5 with [Ru(p-cymene)Cl2]2 in the presence of either NH4PF6 or
NaB(C6H5)4.
The AC of each ligand 1–5 being known, the problem was
how to assess the relative position of chlorine and arene
ligand in each expected diastereomer and how to establish
the equilibrium constant between them in the given solvent.
In other words, to establish whether chlorine or, if preferred,
η6-arene ligand is situated syn or anti to the substituent at
C-23 of the oxazoline ring referring to N–Ru–N plane
(Scheme 2).
requirements of the alkyl substituents in either the η6-arene
ligand or the oxazoline ring.19 The most stable epimer at the
metal is that in which the η6-arene ligand and the substituent
on C-4 of the oxazoline ring are located on the opposite side of
the N–Ru–N plane.
Since the considerable steric requirements of the η6-p-
cymene ligand make its accommodation in between the
“phane” ring overhanging the quinoline moiety and the substi-
tuent R practically impossible [Scheme 2, structure (a)], the
(RM) AC looks quite predictable for complexes 6 and 8 derived
from ligands (Rp,R)-1 and (Rp,R)-3, respectively [Scheme 2,
structure (b)]. However, no prediction can be made a priori
about the configuration at the metal of the complexes 7 and 9
built with (Rp,S)-configured ligands 2 and 4 [Scheme 2, struc-
In our case, a rough glance at the molecular models seems
to suggest a slight preference for the diastereomer (Rp,S,RRu
)
bearing the R substituent at the C-23 and p-cymene ligand on
the same side with respect to the N–Ru–N plane [Scheme 2,
structure (d)], but caution is required as both the substituent
at C-23 and the “phane” ring overhanging the quinoline
moiety seem to exhibit very similar steric requirements. Never-
theless, we thought that 1H NMR might be a useful tool to
solve this structural problem. Particularly, the nuclear Over-
hauser effect (NOE)20 would be very helpful, even though not
quite conclusive, in establishing the relative position of the
chlorine atom and p-cymene ligands in the complexes 6–10.
i
tures (c) and (d), R = Ph, Pr]. On the other hand, it was found
that the molar ratio between the two possible epimers at the
metal of a chiral half-sandwich ruthenium(II)chloro[2-(pyrid-2-
yl)-4-R-oxazoline] cation is very sensitive to the steric
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To this end, the chemical shifts of all protons in the half-sand- and aliphatic protons of p-cymene are much better resolved,
wich cationic complex [(η6-p-cymene)Ru(QUIPHANOX)Cl]+ 6–10 indicating an enhanced rotational freedom of the arene ligand
were previously unequivocally assigned by means of NOESY (see SI-17, SI-19 and SI-29‡).
experiments.
A rough correlation between the rotational freedom and
A careful analysis of the NOESY spectra points out a sub- NMR signal resolution could also explain the different patterns
stantial interaction of the aromatic protons H-25 and H-26 of exhibited by the two methyl of the isopropyl group in both
the p-cymene ligand with H-23 (R2 = H) in all complexes exhi- (Rp,R,RRu)-6a and (Rp,R,RRu)-6b (SI-17 and SI-19‡). The signal
biting the (R) configuration at the C-23 (Fig. 2(a) and Fig. S1 of one methyl group is a broad singlet, most probably because
and S3 in ESI‡), while no interaction was observed between it is jammed into a crowded neighbourhood that limits its free
the aromatic protons of the arene ligand and the protons rotation around the CH–CH3 bond, while the other, pointing
located above the N–Ru–N plane. This implies that the outside of the quinolinophaneoxazoline ligand, is free to
p-cymene ligand is placed anti to R, therefore, the AC XRu = RRu rotate, so that its signal is a well resolved doublet. Such a
could be assigned to the metal stereogenic center.
hypothesis seems to be supported by the X-ray structure of
In contrast, no NOE contact of the aromatic protons of the (Sp,R,SRu)-7b stereoisomer (see below) in which one methyl of
p-cymene ligand with either H-23 or any protons above the the isopropyl group points toward and the other outside the
N–Ru–N plane was observed in the NOESY spectra of the com- centroid of the cyclophane ring (CH–π interaction, H-centroid
plexes exhibiting (S)-configuration at C-23 of the oxazoline ring distance = 2.95 Å). The same deductions hold true for the aro-
[Fig. 2(b), Fig. S2 and S4 in ESI‡]. Nevertheless, substantial matic protons of the arene ligand.
NOE contacts of the aromatic protons of p-cymene with the
Most likely, complexes salts 6–10 in solution of low polarity
protons of the isopropyl group in complex 9 were noticed solvents, such as CHCl3, are made up of ion pairs,22 with the
[Fig. 2(b) and Fig. S4 in ESI‡]. On this basis, the (Rp,S,RRu) AC anion positioned below the N–Ru–N plane, opposed to the
can be assigned to both complexes (Rp,S,XRu)-7 and (Rp,S,XRu)-9 “phane” moiety and close to the p-cymene ligand. Such a
with a very modest margin of error.
hypothesis is supported by noticeable NOE contacts of the aro-
1
−
Although the H NMR spectrum of the complex (Rp,XRu)-10 matic protons of BPh4 with both the aromatic and aliphatic
appears to be the simplest one, the NOESY analysis shows protons of p-cymene, as well as with the H-23 and H-22t of the
some ambiguities due to the very broad signals of the η6-p- oxazoline ring, but no contact with H-22c (Fig. S3 and S4 in
cymene protons.
ESI‡). The X-ray structure of complex (Sp,R,SRu)-7b, showing
Actually, such signals look unresolved and broadened to the PF6− anion in the same side of the arene ligand and anti to
some extent in the spectra of all the complexes, except for the “phane” moiety (Fig. 6), seems to confirm the above state-
(Rp,R,RRu)-8. Particularly, the p-cymene signals of the com- ment. Though it looks speculative, this could also explain the
plexes 7 and 9 exhibiting the (S) configuration at C-23, and enhanced resolution of the 1H NMR signals of the p-cymene
−
10 are so broadened that they are hardly recognizable in the ligand when the very bulky BPh4 anion is replaced by the
spectrum (see SI-22, SI-32 and SI-35‡). Probably, since the much smaller PF6 [compare the 1H NMR spectra of
−
arene ligand is inserted in between the cyclophane ethylenic (Rp,R,RRu)-6a and (Rp,R,RRu)-6b, SI-17 and SI-19, respectively, in
bridge and the R substituent at C-23, its rotation around the ESI‡].
Ru-p-cymene coordination bond is slowed-down to the NMR
Unfortunately, any attempt to enhance the signal resolution
by registering the NMR spectra at variable temperatures was
time scale causing a broadening of the NMR signal.21
Accordingly, in the spectra of complexes (Rp,R,RRu)-6 and unsuccessful. Raising the temperature up to 353 K, the
(Rp,R,RRu)-8 (R2 = H in Fig. 1) the signals of both the aromatic maximum value compatible with the solvents used, had a neg-
ligible effect, while solubility problems arose at temperatures
below 268 K both in CDCl3 and CD2Cl2.
ECD analysis of complexes [(η6-p-cymene)ruthenium-
(QUIPHANOX)Cl]+Y− 6–10
ECD spectroscopy proved to be one of the most practical tools
to provide information about the stereochemistry of chiral
transition metal complexes.23 In particular, such a technique
allows AC assignments to be performed in solution, thus
avoiding the need for single crystal X-ray analysis. It also
reveals the absolute configuration at a chiral metal center even
in the presence of other chiral moieties within the complex.
For these reasons, ECD has also been extensively used in the
study of chiral metal arenes in which the chirality resides in
the ligand,24 in the arene,25 in the metal,26 or in two of these
Fig. 2 NOE observed in the NOESY spectra of complexes 6–8 (a) and
7–9 (b).
moieties.27,28 We therefore decided to undertake an
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Table 1 Main CD spectral features of ligand 2 and complexes 6–10 (c ∼1 × 10−3 M in THF)
Compound
ECD λ(Δε)
(Rp,S)-2
208(61.7), 248(−23.3), 280(−11.8), 303(7.2), 338(−2.1)
(Rp,R,XM)-6a
(Rp,S,XM)-7a
(Rp,S,XM)-7b
(Rp,R,XM)-8
(Rp,S,XM)-9
(Rp,XM)-10
205(−69.7), 225(22.2), 245(−39.7), 263(17.4), 285(−23.5), 318(72.8), 384(41.3), 437(−29.1)
212(−53.9), 225sh(−23.7), 243(−13.2), 264(56.6), 287(−29.3), 319(83.4), 385(38.8), 430(−16.2)
206(−69.2), 222sh(−33.0), 233(7.9), 243(−11.7), 264(53.6), 285(−21.7), 313(87.8), 381(36.3), 426(−8.5)
205(−77.6), 227(15.7), 243(−24.8), 263(29.8), 288(−21.6), 320(69.6), 383(38.3), 435(−26.4)
224(−23.9), 245(−12.9), 265(40.6), 287(−18.0), 318(66.1), 385(30.8), 432(−11.1)
206(−57.3), 226sh(−12.3), 234(2.6), 244(−18.4), 265(39.5), 288(−19.1), 318(77.2), 383(38.4), 425(−23.5)
experimental and theoretical analysis (vide infra) of the ECD (see Fig. S7 and Table S19‡). The ECD spectra of complexes
spectra of Ru complexes 6–10 with the aim of obtaining infor- (Rp,S,XM)-7a and (Rp,S,XM)-7b, coming from ligand (Rp,S)-2 but
−
−
mation regarding the absolute stereochemistry of the Ru(II) differing in the nature of the anion, PF6 for 7a and BPh4 for
stereogenic center. ECD spectra of 6–10 and of ligand (Rp,S)-2 7b, fully coincide up to 450 nm, showing that the anion nature
were then recorded in THF in the 200–500 nm range (Table 1, affects neither the main spectral features nor the stereo-
Fig. 3 and Fig. S5 in ESI‡).
chemistry at the metal. ECD spectra of complexes 6–10, all
The ECD spectrum of ligand (Rp,S)-2 shows a weak negative having (Rp) configured quinolinophane moiety but different
Cotton effect at 338 nm, a strong positive band at 208 nm and chirality at C-23 of the oxazolinic ring, appear almost super-
three bands of moderate intensity in the 220–320 nm range: a imposable, showing that all these complexes share the same
positive one at 303 nm and two negative ones at 280 and metal AC. As expected, a mirror image ECD spectrum is shown
248 nm (Table 1 and Fig. S5 in ESI‡). The strong positive band by ent-7b having (Sp,R,XM) AC (Fig. 3). Therefore, it is held that
can be ascribed to the quinolinophane chromophoric moiety the p-quinolinophane moiety exerts full control of stereo-
by comparison with the ECD spectrum of (Rp)-2,4-dimethyl-[2]- chemistry at the metal, while central chirality at C-23 of the
paracyclo[2](5,8)quinolinophane [(Rp)-14] (see Fig. S6 in ESI‡). oxazoline moiety does not affect the AC at the metal. The only
In the same spectral range, the ECD spectrum of the corres- spectral difference is to be found in the 220–230 nm range
ponding Ru complex (Rp,S,XM)-7a appears much more intense, where (Rp,S,XM)-7a, (Rp,S,XM)-9 and (Rp,XM)-10 show a negative
displaying a series of strong and oppositely signed Cotton shoulder, while (Rp,R,XM)-6a and (Rp,R,XM)-8 show a positive
effects, typical of chiral Ru(II) arene complexes.27 Comparing Cotton effect. Such a difference could be ascribed to the oppo-
the ECD spectrum of the complex with that of (Rp)-2,4- site AC at the stereogenic carbon in the oxazoline ring which
dimethyl[2]paracyclo[2](5,8)quinolinophane [(Rp)-14] it appears induces a different conformational arrangement of the arene
evident that in the long wavelength range above 250 nm, the ligand around the metal–η6-p-cymene centroid while preser-
complex spectrum is dominated by Cotton effects mainly due ving the AC at the metal. The ECD spectrum of (Rp,S,XM)-9
to the stereogenic ruthenium atom which fully overlaps the remained unchanged after one week of storage in THF, con-
spectral features of the chiral ligand moiety. Such intense firming the remarkable configurational stability of these com-
Cotton effects are attributable to metal-to-metal (MM) d–d plexes even in solution.
transitions, metal-to-ligand and ligand-to-metal charge trans-
fer, and π–π* transitions,29 as indicated by a detailed analysis
Theoretical determination of the AC of [(η6-p-cymene)-
of some calculated transitions above 250 nm given in the ESI
Ru(QUIPHANOX)Cl]+Y−
The AC of ruthenium complexes 6–10 was determined by com-
paring the experimental and computed chiroptical properties;
in particular, the ECD as well as the optical rotation (OR) at
the sodium D-line.14a As described above, compounds 6–10
differ in nature and orientation of the substituents on the ox-
azoline ring. In each case, the chirality of both the substituted
carbon on the oxazoline ring and the quinolinophane group is
known. Therefore, calculations were carried out for all
diastereomers resulting from fixing the quinolinophane and
substituted oxazoline to known chiralities, taking into account
both the RM and the SM configurations at the metal atom.
Owing to the large molecular size, the counter-anion
was omitted in all the calculations and only the most stable
conformation of each cation was searched for and used in
the subsequent chiroptical property determination. The
conformational flexibility of the various complexes depends on
Fig. 3 Experimental ECD spectra (THF) of complexes 6–10.
1640 | Dalton Trans., 2014, 43, 1636–1650
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orientation of either the phenyl or isopropyl groups attached
A complication in the calculation of the OR in the UV-vis
to the oxazoline ring in 6–9. Consequently, for each of the ten range of many metal complexes is that it requires the treat-
complexes examined here, a preliminary search for the most ment of anomalous dispersion around electronic transitions.
stable conformation was performed using the molecular mecha- First-principle computations of damped OR, which allows for
nics MMFF94 approach, as implemented in Spartan02,30 and the covering of the resonant region, were developed36a,b and
by profiling the strain energy for full rotations of both the successfully adopted in the calculation of the ORD in a
p-cymene isopropyl group and p-cymene itself in 6–10, as well number of cases (see for example ref. 34a for a recent appli-
as the full rotation of both the phenyl and isopropyl group in cation to some bis-tridentate complexes). So far, the procedure
the oxazoline ring of 6–9.
implemented within the Gaussian 09 package provides the OR
Subsequently, molecular geometries of the most stable MM as a trace of the mixed electric-dipole–magnetic-dipole polariz-
conformations were optimized at the quantum mechanical ability tensor, the latter obtained by means of a linear-
(QM) density functional theory (DFT) level. Long range inter- response calculation without damping, i.e., the OR diverges as
actions, such as those expected to occur in 6–10, require the frequency of the incident light approaches an excitation
neither a trivial nor an occasional choice of the density func- energy of the molecules. In order to treat the anomalous dis-
tional which must include dispersion corrections if for no persion, we exploited the possibility of computing the OR as a
other reason than to obtain plausible equilibrium geometries. sum over states (SOS) of damped elements as described in
Thus, we adopted the B97-D density functional31 in combi- ref. 36c,d.
nation with the SV(P) basis set,32 as implemented within
Gaussian 09.33
Specifically, for each complex the first 100 electronic tran-
sitions were determined at the B3LYP/SV(P) level using the
Chiroptical properties were calculated for the QM equili- B97-D/SV(P) optimized geometry of the most stable confor-
brium geometries at the time-dependent density functional mation. These electronic transitions were then used to work
theory (TDDFT) level by combining the B3LYP density func- out the ECD spectra (see below) and to obtain the ORs
tional with the SV(P) basis set. The B3LYP functional used at 589.3 nm as SOS, assuming a global damping constant of
here for the OR and ECD calculations is an old functional of 0.2 eV.36d Linear-response and SOS calculations without
the global-hybrid class. Actually, as shown in a recent set of damping were also accomplished.
papers by Rudolph et al.34a,c and Srebro et al.,34b more modern
Table 2 reports MM and QM energy differences between the
functionals with range-separated exchange do not necessarily SM and the RM configurations of the ruthenium atom for com-
outperform standard global hybrids such as B3LYP or PBE0 for plexes 6–10 and calculated optical rotations at the sodium
compact chiral metal complexes. However, the results may be D-line. Experimental ORs at 589.3 nm are collected in the
improved using system-specific long-range corrected func- last column of Table 2.
tional, by tuning with respect to some energetic criteria.34d,e
As can be observed, in all cases the RM configured cationic
The double-zeta SV(P) basis set, which includes polarization complexes are definitely more stable (from 8.5 to 12 kcal
functions on all of the atoms except hydrogens, was retained mol−1) than those having the SM configuration; in particular,
here in order to keep the computational effort at a feasible both levels of theory (MM and QM) provide quite concordant
level, relying on the finding that this basis set provides suc- estimates.
cessful predictions of chiroptical properties, as shown for
Optical rotations at 589.3 nm deserve further consideration.
several prepared metallahelicenes35 also containing Ru.35c Firstly, the issue concerning the error associated with the SOS
Solvent effects were not included as large quantitative agree- truncation must be addressed. This can be appreciated com-
ments are not necessary to accomplish our goal.
paring the linear-response results provided by the Gaussian 09
Table 2 MMFF94 and B97-D/SV(P) energy differences between SM and RM configurations for complexes 6–10 and B3LYP/SV(P)//B97-D/SV(P)
optical rotationsa
SOS calculation with damping of
Linear-response
calculation
without damping
SOS calculation without damping
0.2 eV
Length
SM
Length
Velocity
Velocity
Complex
ΔMM
ΔQM
9.19
8.51
11.87
9.85
SM
RM
SM
RM
SM
RM
809
1408
546
1602
978
RM
852
1208
733
SM
RM
622
1010
399
Expt.
(Rp,R,XM)-6a
(Rp,S,XM)-7a
(Rp,R,XM)-8
(Rp,S,XM)-9
(Rp,XM)-10
6.75
7.23
11.09
4.68
−299
1191
1728
966
1744
1431
−268
1291
−596
−368
−1260
1115
1636
983
1791
1435
570
1819
328
550
305
611
1294
428
805
1537
641
298
514
221
575
357
1381
−497
−71
577
408
1428
1015
700
578
1273
683
11.99
8.54
−1041
a Δ is the energy difference between SM and RM complexes in kcal mol−1. Optical rotations in degrees [dm(g cm−3)]−1 at 589.3 nm; 100 states were
considered in the SOS calculations.
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package, which are without damping, with the SOS values substantial fraction of the summations. On the other hand, the
obtained for a vanishing damping constant. As can be observed, large deviations between length and velocity results obtained
the comparison reveals rather good agreement for the length without damping underline some drawbacks of the adopted
formalism (compare columns 4 and 6 for the SM configured basis set. The deviation between length and velocity formalisms
complexes and columns 5 and 7 for the RM ones), indicating becomes much less pronounced using a damping constant of
that the number of excited states is large enough to get a 0.2 eV. Nevertheless, the comparison between damped ORs and
Fig. 4 B3LYP/SV(P) ECD spectra of 6–10 for the various combinations of chiral ligands considered in the paper. The shapes of the component CD
Cotton effects have been approximated by the Gaussian distribution curve method, assuming half of the bandwidth at 1/e of peak height of 0.2 eV.
For each complex, spectra for the two ruthenium configurations are superimposed. As indicated, red/green lines correspond to the length/velocity
formalism of calculation.
1642 | Dalton Trans., 2014, 43, 1636–1650
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experimental results for a single wavelength does not provide
any firm conclusion about the Ru configuration.
Table 3 Pearson correlation coefficients between experimental and
theoretical spectra computed over the 260–370 nm rangea
Let us now turn our attention to ECD. Since it is practically
unfeasible to work out rotational strengths for single electronic
transitions from one experimental ECD spectrum, it is custom-
ary to do the task the other way around, i.e., by obtaining a
theoretical spectrum approximating ECD Cotton effects by
Gaussian (or Lorentzian) distribution curves, one for each
computed rotational strength.37 Theoretical ECD spectra,
obtained as the sum of Gaussian distribution curves, are
shown in Fig. 4. They are formulated in terms of computed
electronic rotational strengths and transition energies by
assuming half of the bandwidth at 1/e peak height of 0.2 eV.
Owing to the limited basis set employed in the calculations
and without considering a number of effects as listed above,
the best agreement between experimental and theoretical
RM/SM spectra cannot be easily appreciated. However, accord-
ing to our previous experience,38 our goal can be achieved pro-
vided that the theoretical spectra of RM and SM epimers differ
substantially in a relatively large range of wavelengths. Whilst
this is always true for an enantiomeric pair, in the case of
diastereomers the above condition must be verified in advance.
This can be easily inferred by inspecting Fig. 4 showing the
B3LYP/SV(P) ECD spectra of the five diastereomeric pairs.
Complex
SM
RM
Red shift (nm)
(Rp,S,XM)-7a
(Rp,R,XM)-6a
(Rp,R,XM)-8
(Rp,S,XM)-9
(Rp,XM)-10
−0.17
−0.47
−0.21
−0.29
−0.18
0.48
0.93
0.87
0.48
0.80
23
23
25
30
23
a Data within each set are 0.1 nm apart; theoretical data in the velocity
formalism.
from 0.5 to 0.9 for the (RM) configuration whilst they are always
negative for the (SM) configuration. The shift (20–30 nm) is an
educated one and fairly constant towards red. This conclusion
has been confirmed by an extensive analysis of the PCCs, for-
mulated as a function of the wavelength shift, for the whole
range of the spectra and for both the length and the velocity
formalisms, reported in the ESI.‡
Superposition of experimental and theoretical spectra in
the 260–370 nm wavelength range is shown in Fig. 5. Theore-
tical spectra have been plotted for the velocity formalism
applying the wavelength shifts reported in Table 3. The better
agreement between the theoretical prediction obtained for the
(RM) configuration and experimental spectra can be readily
appreciated. Therefore, theoretical calculations assign the R
configuration at the ruthenium atom to all the complexes
6–10, supporting the results obtained by the NMR techniques.
Rather encouragingly, in all cases below 370 nm the ECD
spectra for (RM) are quite different from those for (SM) con-
figurations and display opposite signs within a large range of
wavelengths. This occurs for 6a, 7a in the 210–370 nm range
while for 8–9 this behaviour is restricted to the 260–370 nm
range. Moreover, the ECD shape for a given ruthenium con-
X-ray crystallography section
figuration is the same over the 260–370 nm range for all the The complex 7b can be identified as a three-legged piano stool
complexes, irrespective of the ligand configurations, indicating complex with a pseudo-octahedral geometry in which the
that the ECD feature in this wavelength range is characteristic arene ligand (p-cymene) occupies three adjacent sites of the
of the ruthenium configuration. Another interesting ECD octahedron as in other similar complexes reported in the lit-
feature, most probably due to aromatic π–π* transitions, is the erature.40 The cymene plane, defined by the C1–C3–C5 atoms
different sign of the band around 220 nm for 6a and 7a result- (Fig. 6), is staggered with respect to the other three ligands
ing from the inversion of the ruthenium configuration.
and is bent 53.5° with respect to the N1–Ru–N2 plane, with
The comparison of experimental and theoretical spectra the cymene moiety that aims to stay as far as possible with the
has been quantified by calculating the Pearson correlation Cl ligand, and with its isopropyl substituent on the same side
coefficient (PCC)39 over the 260–370 nm range. For our pur- of the Cl ligand. The angle between the cymene centroid, Ru
poses, PCC has two appealing features. The first is that it can and the centroid of C11 and C12 atoms is 138.9°.
assume negative values, indicating that the correlation is
The paracyclophane moiety shows a pseudo-perpendicular
reversed in some sense, that is, a positive correlation for one configuration with respect to the pyridyl moiety. In fact the
diastereomer is supported by a negative correlation for the plane described by C17–C20–C23–C26 (the four atoms
other. Secondly, the PCC does not change for an arbitrary bounded to the two ethylenic groups) is 85.32° with respect to
scaling of the data sets. This is rather useful as the predicted the N1–Ru–N2 plane and is 97.55° with respect to the pyridyl
molar ECD could be significantly underestimated with respect moiety. The angle between the p-cymene plane and the phenyl
to the experiment.38 Since the predicted theoretical spectrum plane is about 131°. Furthermore, the average of the two
could also be shifted in wavelengths with respect to the experi- planes that described the paracyclophane moiety, which are
mental one, PCCs were determined applying a wavelength C15–C16–C18–C19 and C24–C25–C27–C28, is bent about 150°
shift which maximizes the positive coefficient. Table 3 reports from the p-cymene plane, defined by the C1–C3–C5 atoms (the
PCC values for the wavelength shift that provides the methyl substituent of the cymene ligand is on the same side of
maximum positive correlation.
the paracyclophane moiety), and it is bent about 28° from the
As can be observed, in all the examined cases we found a N1–Ru–N2 plane.
positive correlation when the R configuration at Ru was chosen
Noteworthy, contrary to what occurs in the related cationic
in the calculation. Rather satisfactorily, the PCC values range Ru(L)(p-cymene)(bpy)+ complex, where the two heteroaromatic
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Fig. 5 Comparison of experimental spectra (blue) of 6–10 with those computed (velocity formalism) for both the R (green) and the S (red)
configurations at Ru in the 260–370 nm wavelength range. A wavelength shift has been applied, see text. For further details, see Fig. 4 caption.
rings of the bpy ligand are perfectly coplanar,41 the quinoline (Sp,R,SRu)-7b is perfectly specular to that of (Rp,S,RRu)-7b
and oxazoline moieties of the N,N-bidentate ligand are not (Fig. 3), giving a definitive confirmation of both the correct-
coplanar, exhibiting a torsion angle of −18.1(8)°. Moreover, a ness and the validity of the spectroscopic methods in the
T-shaped arrangement of the p-cymene ligand with respect to assignment of the AC (XM) at the metal.
the phenyl group of the oxazoline ring, that should contribute
A selection of some characterizing bond lengths, angles,
to stabilizing the complex by 1 kcal mol−1 at least,42 is clearly and torsions of (Sp,R,SRu)-7b from the X-ray measurement is
visible. From the ORTEP structure, the AC at the metal can be shown in Table 4, along with the corresponding estimates
easily inferred as XRu = SRu. Coherently, the ECD spectrum of from the geometry optimization of its enantiomer (Rp,S,RM)-7a
1644 | Dalton Trans., 2014, 43, 1636–1650
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evaluating the performance of these organometallic species as
potential catalysts in stereoselective reactions.
Experimental section
Generalities
Starting materials were purchased from Aldrich-Fluka
(CH-9479 Buchs). All commercial reagents were used without
further purification. Air and moisture sensitive compounds
were stored in Schlenk tubes or Schlenk burettes. They were
protected by and handled under an atmosphere of 99.995%
pure nitrogen, using appropriate glassware. Tetrahydrofuran
and diethyl ether were stored over potassium hydroxide pellets
in the presence of cuprous chloride, from which they were dis-
tilled, before being redistilled from sodium wire in the pres-
ence of benzophenone.
Fig. 6 Thermal ellipsoid plot (probability 50%) of (Sp,R,SRu)-7b.
Melting ranges (mp) given were found to be reproducible
after resolidification unless decomposition (“dec.”) is speci-
fied. The temperature readings were corrected using a cali-
bration curve established with authentic standards.
Table 4 Selected bond lengths, angles, and torsions of (Sp,R,XRu)-7b
from X-ray measurement vs. B97-D/SV(P) optimization of (Rp,S,RM)-7a.
Bond lengths are in Å, angles and torsions in degree
1H, and 13C spectra, as well as COSY and NOESY experi-
ments, were carried out at 400 and 100.6 MHz, respectively,
the samples having been dissolved in deuterochloroform.
Chemical shifts (δ) are given in ppm relative to the internal
standards tetramethylsilane, coupling constants (J) are given
in Hz. IR spectra were taken of chloroform solutions in the
4000–625 cm−1 frequency range. The purity of all final pro-
ducts was testified by elemental analyses performed on Carlo
Erba Elemental Analyzer Mod. 1106.
Bonds
X-ray
DFT
Angles
X-ray
76.86
DFT
76.2
Ru–Cl
2.414
2.082
2.217
2.159
2.212
1.275
1.348
1.458
1.505
1.509
2.425
2.072
2.228
2.234
2.256
1.296
1.350
1.458
1.601
1.599
N1–Ru–N2
Ru–N1
Ru–N2
Cl–Ru–N1
Cl–Ru–N2
81.37
92.71
81.0
91.1
Ru–C1
Ru–C4
N1–C11
N2–C12
C11–C12
C21–C22
C29–C30
Ru–N1–C11
Ru–N2–C12
N1–C11–C12
N2–C12–C11
N1–C11–C12–N2
Cl–Ru–N1–C31
Cl–Ru–N2–C16
115.0
116.7
111.6
119.2
113.7
16.4
66.1
−107.4
110.20
120.60
113.60
−18.1
−67.0
104.6
Absorption and ECD spectra were recorded at 25 °C on a
JASCO J815 spectropolarimeter, using 0.1 mm cells and con-
centrations of about 1 × 10−3 M in tetrahydrofuran (THF).
During the measurement, the instrument was thoroughly
purged with nitrogen.
at the B97-D/SV(P) level. As can be observed the agreement is
rather good despite the small basis set employed. It should be
noticed that as the two complexes 7a and 7b form an enantio-
meric pair, the signs of X-ray and DFT torsions should be sys-
tematically reversed, as they are.
Intermediates and final products
(R)-(−)-2-Cyano-4-methyl[2]paracyclo[2](5,8)quinolinophane
[(Rp)-13].14 Hydroxylamine hydrochloride (0.24 g, 3.5 mmol)
was added to (R)-(−)-4-methyl[2]paracyclo[2](5,8)quinolino-
phane-2-carbaldehyde (5.0 g, 16.6 mmol) in dry chloroform
Conclusions
To summarize, 1H NMR and ECD proved to be, once again, (100 mL) and the mixture was refluxed for 5 min. Pyridine
two effective complementary spectroscopic methods for the (1.3 g, 16.5 mmol) was added and the reflux was continued for
structural determination of chiral organometallic compounds 7 h. Selenium dioxide (1.83 g, 16.5 mmol) was added in por-
in solution. Whilst the ECD technique assisted by theoretical tions during a 15 min period and the mixture was allowed to
calculations allows the assignment of the AC at the metallic react until the oxime intermediate disappeared (2 h by tlc
stereogenic center, the NMR provides valuable indications at a monitoring). After cooling, the mixture was filtered on celite,
conformational level as well. Thus, we found that optically the solid was washed with chloroform (30 × 10 mL) and the fil-
pure quinolinophane-oxazoline ligands exhibiting both planar trate was evaporated at reduced pressure. Chromatography of
and central chirality, combined with ruthenium(II)-η6-arene the residue on silica gel (eluent, diethyl ether–petroleum ether
reagents, give only one of the two possible diastereoisomers 3 : 1) allowed for the recovery of an orange rhombic crystals
where the η6-arene ligand and the phane moiety are arranged (3.45 g, 70%): mp 127–128 °C; [α]D20 = −64.4 (c = 0.68, CHCl3);
on the opposite sides of the quinoline–oxazoline plane. There- elemental analysis, calc. for C21H18N2 (298.38), C 84.53,
fore, planar chirality is by far the most important, if not the H 6.08; N 9.39; found, C 84.57, H 6.03, N 9.44. IR: νmax/cm−1
unique, factor determining the structural characteristic of the 2235 (CuN); MS: m/z (%) 298 (M+, 26), 194 (11), 104 (100).
complex. All these elements are of primary importance in 1H NMR: δ 7.40 (s, 1 H), 7.00–6.91 (four peaks, AB system,
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JAB = 7.3, 2 H), 6.54–6.48 (split AB system, JAB = 7.9, 4J = 1.6, 2 H), (m, 2 H), 5.79 (dd, J = 6.5 and 1.5, 1 H), 5.52 (dd, J = 6.5 and
5.74 (dd, J = 7.8 and 1.5, 1 H), 5.43 (dd, J = 7.8 and 1.8, 1 H), 1.5, 1 H), 5.50 (dd, J = 10 and 8.5, 1 H), 4.97 (dd, J = 10 and
4.33–4.25 (m, 1 H), 3.85 (dd, J = 14 and 9.1, 1 H), 3.22–2.91 8.5, 1 H), 4.50 (t, J = 8.5, 1 H), 4.4 (m, 1 H), 3.89 (dd, J = 14
(m, 5 H), 2.75 (s, 3 H), 2.58 (dt, J = 13 and 9.1, 1 H); and 9.0, 1 H), 3.2 (m, 2 H), 3.0 (m, 3 H), 2.76 (s, 3 H), 2.61 (dt,
13C-NMR: δ 150.8, 144.5, 140.7, 139.7, 137.8, 137.1, 136.5, J = 13 and 9.0, 1 H). 13C NMR: δ 164.9, 150.2, 143.6, 143.3,
133.6, 132.7, 131.3, 130.9, 130.3, 128.8, 128.7, 125.4, 123.0, 142.1, 140.7, 139.9, 137.9, 136.8, 135.3, 132.8, 132.6, 131.1,
37.7, 35.2, 34.4, 31.5, 22.6.
130.9, 129.0, 128.9 (2 C), 128.5, 127.8, 126.9 (2 C), 123.0, 75.3,
(S)-(+)-2-Cyano-4-methyl[2]paracyclo[2](5,8)quinolinophane 70.4, 37.8, 35.4, 34.6, 31.9, 22.8.
[(Sp)-13] was prepared analogously from (S)-(+)-4-methyl[2]-
Similarly, the enantiomer (Sp,R)-4,5-dihydro-2-{4′-methyl[2]-
paracyclo[2](5,8)quinolinophane-2-carbaldehyde (1.0 g, 3.3 mmol) paracyclo[2](5′,8′)quinolinophane-2′-yl}-4-phenyloxazole, [(Sp,R)-2]
(yield, 0.64 g, 65%): mp 126–128 °C; [α]2D0 = +63.9 (c = 0.57, was obtained from nitrile (Sp)-13 (0.50 g, 1.67 mmol) and (R)-2-
CHCl3); elemental analysis, calc. for C21H18N2 (298.38), C 84.53, amino-2-phenylethanol (yield, 0.47 g, 68%): mp 66–67 °C;
H 6.08, N 9.39; found, C 84.61, H 6.11, N 9.42. IR, 1H [α]D20 +105 (c = 0.83, CHCl3); elemental analysis, calc. for
and 13C NMR spectra were identical to those of its enantiomer C29H26N2O (418.20), C 83.22, H 6.26, N 6.69; found, C 83.42,
1
(Rp,S)-13.
H 6.20, N 6.75. IR, H and 13C NMR spectra were identical to
those of its enantiomer (Rp,S)-2.
General procedure to prepare (Rp)-4′-aryl/alkyl-4′,5′-dihydro-2′-
{4-methyl[2]paracyclo[2](5,8)quinolinophane-2-yl}oxazoles
1–515
(Rp,R)-4,5-Dihydro-4-isopropyl-2-(4′-methyl[2]paracyclo[2]-
(5′,8′)quinolinophane-2′-yl)oxazole [(Rp,R)-3] was obtained ana-
logously from nitrile (Rp)-13 (0.50 g, 1.67 mmol) and (R)-2-
A solution of nitrile 13 (0.5 g, 1.7 mmol) and the suitable amino-3-methyl-1-butanol (0.19 g, 1.8 mmol) as white flat crys-
2-aryl/alkyl-2-aminoethanol (1.8 mmol) in chlorobenzene tals (0.49 g, 76%): mp 179–182 °C; [α]2D3 +33 (c = 0.56, CHCl3);
(20 mL) was concentrated up to half the initial volume in a elemental analysis, calc. for C26H28N2O (384.51), C 81.21,
1
Dean–Stark apparatus. Zinc chloride (0.5 mmol), freshly fused H 7.34, N 7.29; found, C 81.32, H 7.29, N 7.35. H NMR δ 7.93
in vacuum, was added and the mixture was refluxed under an (s, 1 H), 6.92–6.81 (four peaks, AB system, JAB = 7.2, 2 H), 6.48
argon atmosphere until the complete disappearance of the (tight, AB system, JAB = 10, 2 H), 5.74 (d, J = 7.9, 1 H), 5.46 (d,
starting nitrile (76 h, by tlc monitoring). After solvent evapor- J = 7.9, 1 H), 4.6 (m, 1 H), 4.37 (quint, J = 9.4, 1 H), 4.29–4.20
ation at reduced pressure, chromatography of the residue on (m, 2 H), 3.85 (dd, J = 14 and 9.1, 1 H), 3.19–3.12 (m, 2 H),
silica gel (eluent, 4 : 6 diethyl ether–petroleum ether mixture) 3.04–2.96 (m, 3 H), 2.74 (s, 3 H), 2.56 (dt, J = 13 and 9.0,
afforded the expected products which were characterized as 1 H), 1.92 (octet, J = 6.7, 1 H), 1.10 (d, J = 6.7, 3 H), 0.99 (d, J =
follows:
6.7, 3 H); 13C NMR δ 163.5, 150.0, 143.5, 143.4, 140.6, 139.7,
(Rp,R)-4,5-Dihydro-2-{4′-methyl[2]paracyclo[2](5′,8′)quinoli- 137.8, 136.7, 135.1, 132.6, 132.4, 131.0, 130.7, 128.7, 128.3,
nophane-2′-yl}-4-phenyloxazole (Rp,R)-1 was obtained as pale 122.7, 72.8, 70.8, 37.7, 35.2, 34.5, 32.9, 31.9, 22.6, 19.0, 18.3.
yellow rhombic crystals from nitrile (Rp)-13 (0.50 g, 1.67 mmol)
(Rp,S)-4,5-Dihydro-4-isopropyl-2-{4′-methyl[2]paracyclo[2]-
and (R)-2-amino-2-phenylethanol (0.25 g, 1.8 mmol) (yield (5′,8′)quinolinophane-2′-yl}oxazole [(Rp,S)-4] was obtained ana-
0.50 g, 71%): mp 65–67 °C; [α]2D3 +29 (c = 0.39, CHCl3); elemen- logously from nitrile (Rp)-13 (0.50 g, 1.67 mmol) and (S)-2-
tal analysis, calc. for C29H26N2O (418.20), C 83.22, H 6.26, amino-3-methyl-1-butanol (0.19 g, 1.8 mmol) as white flat crys-
N 6.69; found, C 83.31, H 6.13, N 6.74. IR: νmax/cm−1 3023, tals (0.46 g, 72%), mp 129–131 °C; [α]D23 −90 (c = 0.64, CHCl3);
2930–2857. 1H NMR: δ 8.03 (s, 1 H), 7.4 (m, 5 H), 6.96–6.85 elemental analysis: calc. for C26H28N2O (384.51), C 81.21,
(four peaks, AB system, JAB = 7.2, 2 H), 6.54–6.50 (four doub- H 7.34, N 7.29; found, C 80.95, H 7.30, N 7.38. IR (CDCl3)
4
lets, split AB system, JAB = 7.9, J = 1.5, 2 H), 5.80 (dd, J = 7.7 νmax/cm−1 3031, 3009–2875, 1702, 1641, 1418, 1360, 1237,
1
and 1.2, 1 H), 5.53 (dd, J = 10 and 8.8, 1 H), 5.50 (dd, J = 8.3 1091, 902. H NMR δ 7.92 (s, 1 H), 6.91–6.80 (AB system, JAB
=
and 1.3, 1 H), 5.01 (dd, J = 10 and 8.4, 1 H), 4.45 (t, J = 8.6, 7.2, 2 H), 6.52–6.47 (split AB system, JAB = 7.9 and 1.5, 2 H),
1 H), 4.4 (m, 1 H), 3.88 (dd, J = 14 and 8.9, 1 H), 3.1 (m, 5 H), 5.74 (dd, J = 7.7 and 1.4, 1 H), 5.48 (dd, J = 7.7 and 1.5, 1 H),
2.76 (s, 3 H), 2.60 (dt, J = 13 and 9.0, 1 H). 13C NMR: δ 165.0, 4.57 (dd, J = 9.8 and 8.4, 1 H), 4.3 (m, 2 H), 4.2 (m, 1 H), 3.85
150.1, 143.7, 143.3, 142.1, 140.7, 139.8, 137.9, 136.9, 135.4, (dd, J = 14 and 9.0, 1 H), 3.2 (m, 2 H), 3.0 (m, 3 H), 2.74 (s,
132.8, 132.6, 131.1, 130.9, 128.8 (3 C), 128.5, 127.7, 126.9 (2 C), 3 H), 2.57 (dt, J = 13 and 8.9, 1 H), 1.98 (octet, J = 6.7, 1 H),
122.9, 75.5, 70.4, 37.8, 35.3, 34.6, 32.0, 22.7.
1.14 (d, J = 6.7, 3 H), 1.01 (d, J = 6.7, 3 H). 13C NMR: δ 163.6,
(Rp,S)-4,5-Dihydro-2-{4′-methyl[2]paracyclo[2](5′,8′)quinoli- 150.0, 143.6, 143.4, 140.6, 139.7, 137.8, 136.7, 135.1, 132.6,
nophane-2′-yl}-4-phenyloxazole [(Rp,S)-2] was obtained from 132.4, 131.6, 131.0, 130.6, 128.8, 128.3, 122.7, 72.8, 70.6, 37.7,
(Rp)-13 (0.50 g, 1.67 mmol) and (S)-2-amino-2-phenylethanol 35.2, 34.5, 32.8, 31.8, 22.6, 19.2, 18.1.
(0.25 g, 1.8 mmol) as pale yellow rhombic crystals (0.47 g,
(Rp)-4,5-Dihydro-4,4-dimethyl-2-{4′-methyl[2]paracyclo[2]-
68%): mp 65–67.5 °C; [α]D23 −106 (c = 0.92, CHCl3); elemental (5′,8′)quinolinophane-2′-yl}oxazole [(Rp)-5]. Likewise, starting
analysis, calc. for C29H26N2O (418.20): C 83.22, H 6.26, N 6.69; from nitrile (Rp)-13 (0.50 g, 1.76 mmol) and 2-amino-2-methyl-
found, C 83.51, H 6.24, N 6.72. IR νmax/cm−1 3023, 2930–2857, propanol (0.16 g, 1.8 mmol), the expected product was
1
1711, 1636, 1364, 1214, 704. H NMR δ 8.01 (s, 1 H), 7.46–7.33 obtained as white flat crystals (0.44 g, 67%), mp 173–175 °C;
(m, 5 H), 6.95–6.85 (four peaks, AB system, J = 7.3, 2 H), 6.5 [α]2D3 −37 (c = 0.88, CHCl3); elemental analysis, calc. for
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Paper
C25H26N2O (370.49), C 81.05, H 7.07, N 7.56; found, C 81.29,
(Rp,S,RRu)-{(η6-p-cymene)Ru[4,5-dihydro-2-(4′-methyl[2]-
H 7.12, N 7.41. IR (CDCl3) νmax/cm−1 3031–3009, 2930, 1712, paracyclo[2](5′,8′)quinolinophane-2′-yl)-4-phenyloxazole]Cl}+BPh4
−
1
1418, 1364, 1234, 903. H NMR δ 7.95 (s, 1 H), 6.94–6.83 (four [(Rp,S,RRu)-7a] (79%): [α]2D0 = +514 (c = 0.081, CHCl3); elemental
peaks, AB system, JAB = 7.2, 2 H), 6.52 (split tight AB system, analysis: calc. for C63H60BClN2ORu (1008.50), C 75.03, H 6.00,
JAB = 7.9, 4J = 1.5, 2 H), 5.77 (dd, J = 7.7 and 1.3, 1 H), 5.51 (dd, N 2.78; found, C 74.92, H 6.05, N 2.85. 1H NMR (CD2Cl2) δ 7.90
J = 7.7 and 1.3, 1 H), 4.4 (m, 1 H), 4.34–4.29 (four peaks, AB (s, 1 H), 7. 7 (m, 3 H), 7.6 (m, 2 H), 7.3 (broad m, 8 H),
system, JAB = 8.1, 2 H), 3.87 (dd, J = 14 and 9.0, 1 H), 3.26–3.15 7.29–7.16 (four peaks, AB system, JAB = 7.5, 2 H), 7.05 (t, J = 7.4,
(m, 2 H), 3.08–2.98 (m, 3 H), 2.76 (s, 3 H), 2.59 (dt, J = 13 and 8 H), 6.90 (t, J = 7.2, 4 H), 6.79–6.63 (split AB system, JAB = 8.0,
9.0, 1 H), 1.53 (s, 3 H), 1.46 (s, 3 H); 13C NMR: δ 162.2, 150.0, 4J = 1.8, 2 H), 5.62 (dd, J = 7.8 and 1.8, 1 H), 5.59 (dd, J = 7.2
143.6, 143.4, 140.5, 139.7, 137.8, 136.7, 135.1, 132.6, 132.4, and 1.8, 1 H), 5.46 (dd, J = 7.8 and 1.8, 1 H), 5.42 (dd, J = 10
130.9, 130.6, 128.9, 128.4, 122.6, 79.5, 67.7, 37.7, 35.2, 34.5, and 9.3, 1 H), 5.05 (t, J = 8.5, 1 H), 4.06 (dd, J = 14 and 9.0,
31.8, 28.4, 28.2, 22.3.
1 H), 3.40 (dd, J = 12 and 9.7, 1 H), 3.30–3.08 (m, 3 H), 3.07
(s, 3 H), 2.96 (ddd, J = 14, 11 and 3.8, 1 H), 2.87 (dt, J = 13
and 8.9, 1 H), 2.25 (very broad s, 1 H), 1.56 (s, 3 H), 0.9–0.12
General procedure to prepare [(η6-p-cymene)Ru(QUIPHANOX)Cl]+-
B(C6H5)4− (6a, 7a, 8, 9, 10) and [(η6-p-cymene)-
(very broad
6 H), the aromatic proton absorptions of
Ru(QUIPHANOX)Cl]+PF6− complexes (6b, 7b)11
η6-p-cymene, δ 4.00 and 6.00, are very broad and hardly
The suitable quinolinophaneoxazoline ligand (QUIPHANOX) perceptible.
was stirred into dichloro(p-cymene)ruthenium(II) dimer in
(Rp,S,RRu)-{(η6-p-cymene)Ru[4,5-dihydro-2-(4′-methyl[2]-
methanol at 25 °C and the mixture was allowed to react for paracyclo[2](5′,8′)quinolinophane-2′-yl)-4-phenyloxazole]Cl}+PF6
−
2.5 h before either sodium tetraphenylborate or ammonium [(Rp,S,RRu)-7b]
(83%):
elemental
analysis,
calc.
for
hexafluorophosphate was added. After 15 min, the resulting C39H40ClF6N2OPRu (834.23), C 56.15, H 4.83, N 3.36; found,
red precipitate was filtered and washed with hexane (3 × C 56.35, H 4.61, N 3.42. 1H NMR δ 8.12 (s, 1 H), 8.00–7.50
10 mL). Crystallization from dichloromethane–diethyl ether (broad, 5 H), 7.87 (very broad s, 2 H), 7.64 (broad m, 2 H), 7.58
mixture and then from dichloromethane afforded the pure (q, J = 7.3, 1 H), 7.25–7.09 (four peaks, AB system, JAB = 7.3, 2 H),
ruthenium complex salt in 78–89% yield. Earlier, the complex 6.72–6.59 (split AB system, JAB = 7.8, 4J = 1.6, 2 H), 5.59–5.47 (m,
salts had been kept at 40 °C/0.2 mmHg for 4 h and then 4 H), 5.36 (dd, J = 10 and 9.0, 1 H), 5.16 (t, J = 9.9, 1 H), 4.09 (dd,
characterized as follows:
J = 15 and 9.4, 1 H), 3.35 (dd, J = 13 and 9.8, 1 H), 3.24–3.12
(Rp,R,RRu)-{(η6-p-cymene)Ru[4,5-dihydro-2-(4′-methyl[2]-para- (m, 3 H), 3.09 (s, 3 H), 2.97–2.90 (m, 1 H), 2.84 (dt, J = 13
−
cyclo[2](5′,8′)quinolinophane-2′-yl)-4-phenyloxazole]Cl}+BPh4
and 8.9, 1 H), 1.87 (very broad s, 3 H), the aromatic proton
[(Rp,R,RRu)-6a], (80%): [α]2D0 = +298 (c = 0.098, CHCl3); elemen- absorptions of η6-p-cymene, δ 4.00 and 6.00, are very broad and
tal analysis, calc. for C63H60BClN2ORu (1008.50), C 75.03, hardly perceptible.
1
H 6.00, N 2.78; found, C 74.96, H 6.11, N 2.82. H NMR δ 7.84
(Rp,R,RRu)-{(η6-p-cymene)Ru[4,5-dihydro-2-(4′-methyl[2]-
(s, 1 H), 7.47–7.39 (m, 13 H), 7.17–7.06 (four peaks, AB system, paracyclo[2](5′,8′)quinolinophane-2′-yl)-4-isopropyloxazole]Cl}+BPh4
JAB = 7.4, 2 H), 7.03–7.00 (m, 8 H), 6.91–6.88 (m, 4 H), [(Rp,R,RRu)-8] (89%): [α]2D0 = +221 (c = 0.069, CHCl3); elemental
6.75–6.60 (four peaks, AB system, JAB = 7.8, 2 H), 5.59 (ddd, J = analysis, calc. for C60H62BClN2ORu (974.48), C 73.95, H 6.41,
14, 10 and 3.5, 1 H), 5.55 (s, 2 H), 5.22 (t, J = 11, 1 H), 5.05 N 2.87; found, C 73.88, H 6.50, N 2.95. 1H NMR δ 7.73 (s, 1 H),
(broad s, 2 H), 4.92 (t, J = 9.5, 1 H), 4.82 (broad d, J = 5.5, 1 H), 7.38 (broad s, 8 H), 7.11 (d, JAB = 7.4, 1 H), 7.01 (t, J = 7.2, 9 H),
4.77 (broad d, J = 5.3, 1 H), 4.47 (dd, J = 11 and 9.3, 1 H), 6.91 (t, J = 7.2, 4 H), 6.72–6.58 (split AB system, J = 7.9, 4J = 1.6, 2 H),
−
4.00 (dd, J = 15 and 9.0, 1 H), 3.34 (dd, J = 12 and 9.7, 1 H), 5.56 (ddd, J = 14, 11 and 3.4, 1 H), 5.51–5.44 (split AB system, JAB
3.1 (m, 2 H), 3.12–3.05 (m, 1 H), 3.02 (s, 3 H), 2.8 (m, 2 H), 1.5 7.9, 4J = 1.8, 2 H), 5.12 (d, J = 5.9, 1 H), 4.90–4.85 (AB system, JAB
=
=
(bs, J = 6.7, 1 H), 1.00 (very broad s, 3 H), 0.45 (very broad s, 5.9, 2 H), 4.79 (d, J = 6.0, 1 H), 4.65 (dd, J = 9.2 and 5.9, 1 H), 4.50
3 H), 0.43 (d, J = 6.9, 3 H).
(t, J = 9.8, 1 H), 4.23 (ddd, J = 9.7, 5.8 and 2.7, 1 H), 4.01 (dd, J =
(Rp,R,RRu)-{(η6-p-cymene)Ru[4,5-dihydro-2-(4′-methyl[2]- 15 and 8.9, 1 H), 3.35 (dd, J = 13 and 9.8, 1 H), 3.22–3.04 (m, 3 H),
−
paracyclo[2](5′,8′)quinolinophane-2′-yl)-4-phenyloxazole]Cl}+PF6
3.01 (s, 3 H), 2.89 (ddd, J = 14, 11 and 3.4, 1 H), 2.79 (dt, J = 13
[(Rp,R,RRu)-6b],
84%:
elemental
analysis,
calc.
for and 9.1, 1 H), 2.37 (sept d, J = 7.0 and 2.4, 1 H), 2.02 (broad sept,
C39H40ClF6N2OPRu (834.23), C 56.15, H 4.83, N 3.36; found, J = 6.7, 1 H), 1.18 (s, 3 H), 1.07 (d, J = 7.2, 3 H), 0.90 (d, J = 6.5,
C 56.19, H 4.94, N 3.42. 1H NMR: δ 7.90 (s, 1 H), 7.73 (d, J = 6.9, 3 H), 0.88 (d, J = 6.9, 3 H), 0.69 (d, J = 6.9, 3 H).
2 H), 7.5 (m, 3 H), 7.23–7.08 (four peaks, AB system, JAB = 7.4, 2
(Rp,S,RRu)-{(η6-p-cymene)Ru[4,5-dihydro-2-(4′-methyl[2]-para-
4
−
H), 6.76–6.62 (split AB system, JAB = 8.0, and J = 1.2, 2 H), 6.30 cyclo[2](5′,8′)quinolinophane-2′-yl)-4-isopropyloxazole]Cl}+BPh4
(t, J = 11, 1 H), 5.79 (d, J = 6.0, 1 H), 5.73 (d, J = 6.0, 1 H), [(Rp,S,RRu)-9] (85%): [α]D20
= +575 (c = 0.105, CHCl3);
5.66–5.57 (m, 3 H), 5.49 (d, J = 6.1, 1 H), 5.25 (d, J = 5.9, 1 H), elemental analysis, calc. for C60H62BClN2ORu (974.48),
5.13 (d, J = 5.9, 1 H), 4.73 (dd, J = 11 and 8.5, 1 H), 4.09 (dd, J = C 73.95, H 6.41, N 2.87; found, C 74.18, H 6.48, N 2.95. 1H
14 and 9.1, 1 H), 3.38 (dd, J = 13 and 10, 1 H), 3.33–3.14 (m, 3 NMR: δ 7.59 (s, 1 H), 7.4 (broad m, 8 H), 7.13–7.04 (four peaks,
H), 3.09 (s, 3 H), 2.95–2.89 (m, 1 H) 2.87 (dt, J = 13 and 9.6, 1 H), AB system, JAB, 7.4, 2 H), 7.00 (t, J = 7.2, 8 H), 6.89 (t, J = 7.1,
1.73, (very broad s, 1 H), 1.25 (very broad s, 3 H), 0.56 (very 4 H), 6.60–6.58 (split AB system, JAB = 7.9, 4J = 1.7, 2 H),
broad s, 3 H), 0.51 (d, J = 6.8, 3.42).
5.49–5.38 (split AB system, JAB = 7.9, 4J = 1.8, 2 H), 5.28–5.22
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Dalton Transactions
(m, 1 H), 5.07 (broad s, 1 H), 4.95 (d, J = 5.6, 1 H), 4.8 (m, AB
portion of an ABX system, 2 H), 4.50 (X portion of a ABX
system, 1 H), 3.96 (dd, J = 15 and 9.2, 1 H), 3.34 (dd, J = 13 and
9.8, 1 H), 3.17 (dt, J = 15 and 9.1, 1 H), 3.0 (m, 2 H), 2.9
(m, 1 H), 2.91 (s, 3 H), 2.79 (dt, J = 13 and 8.5, 1 H), 2.31
(broad s, 1 H), 1.60 (very broad s, 3 H), 1.12 (d, J = 7.0, 3 H),
0.99 (d, J = 6.8, 3 H), 0.6–0.0 (very broad system).
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(Rp,RRu)-{(η6-p-cymene)Ru[4,5-dihydro-2-(4′-methyl[2]-para-
cyclo[2](5′,8′)quinolinophane-2′-yl)-4,4-dimethyloxazole]Cl}+BPh4
−
[(Rp,RRu)-10] (79%): [α]D20 = +357 (c = 0.086, CHCl3); elemental
analysis, calc. for C59H60BClN2ORu (960.45), C 73.78, H 6.30,
1
N 2.92; found C 73.61, H 6.24, N 3.01. H NMR δ 7.66 (broad s,
1 H), 7.39–7.36 (m, 8 H), 7.17–7.06 (four peaks, AB system, JAB
=
7.4, 2 H), 7.01 (t, J = 7.3, 8 H), 6.89 (t, J = 7.1, 4 H), 6.73–6.59 (split
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4
AB system, JAB = 7.8, J = 1.7, 2 H), 5.50–5.42 (split AB system,
4
JAB = 7.8, J = 1.8, 2 H), 5.42 (broad s, 1 H), 5.13 (d, J = 5.8, 1 H),
5.03 (d, J = 5.7, 1 H), 4.57–4.46 (four peaks, AB system, JAB = 8.7,
2 H), 4.01 (dd, J = 14 and 8.9, 1 H), 3.36 (dd, J = 13 and 9.8, 1 H),
3.19 (dt, J = 14 and 9.0, 1 H), 3.15–3.08 (m, 2 H), 2.99 (s 3 H),
2.94–2.87 (m, 1 H), 2.81 (dt, J = 13 and 9.1, 1 H), 2.3 (very broad
s, 1 H), 1.58 (s, 3 H), 1.56 (s, 3 H), 1.4 (s, 3 H), 0.8–00 (broad m,
p-cymene protons not better identified).
6 (a) C. Bruneau, J.-L. Renaud and B. Demerseman,
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Acknowledgements
We would like to thank the Italian “Ministero dell’Istruzione,
dell’Università
e della Ricerca Scientifica” for financial
support, under the auspices of the PRIN 2008LYSESR
program.
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