Solution structure investigation of Ru(II) complex ion pairs: Quantitative NOE measurements and determination of average interionic distances

Article abstract of DOI:10.1021/ja015959g

The structure of the Ru(II) ion pairs trans-[Ru(COMe){(pz₂)CH₂}(CO)(PMe₃) ₂]X (X^- = BPh₄^-, 1a; BPh₃Me^-, 1b; BPh₃(n-Bu)^-, 1c; BPh₃

Full text of DOI:10.1021/ja015959g

11020
J. Am. Chem. Soc. 2001, 123, 11020-11028
Solution Structure Investigation of Ru(II) Complex Ion Pairs:
Quantitative NOE Measurements and Determination of Average
Interionic Distances
Cristiano Zuccaccia, Gianfranco Bellachioma, Giuseppe Cardaci, and Alceo Macchioni*
Contribution from the Department of Chemistry, UniVersity of Perugia, Via Elce di Sotto,
8 - 06123 Perugia, Italy
ReceiVed April 6, 2001
-
Abstract: The structure of the Ru(II) ion pairs trans-[Ru(COMe){(pz2)CH2}(CO)(PMe3)2]X (X- ) BPh4
,
1a; BPh3Me-, 1b; BPh3(n-Bu)-, 1c; BPh3(n-Hex)-, 1d; B(3, 5-(CF3)2(C6H3))4-, 1e; PF6-, 1f; and BF4-, 1g;
pz ) pyrazol-1-yl-ring) was investigated in solution from both a qualitative (chloroform-d, methylene chloride-
d2, nithromethane-d3) and quantitative (methylene chloride-d2) point of view by performing 1D- and 2D-NOE
NMR experiments. In particular, the relative anion-cation localization (interionic structure) was qualitatively
determined by 1H-NOESY and 19F, 1H-HOESY (heteronuclear Overhauser effect spectroscopy) NMR
experiments. The counteranion locates close to the peripheral protons of the bispyrazolyl ligand independent
of its nature and that of the solvent. In complexes 1c and 1d bearing unsymmetrical counteranions, the aliphatic
chain points away from the metal center as indicated by the absence of NOE between the terminal Me group
and any cationic protons. An estimation of the average interionic distances in solution was obtained by the
quantification of the NOE build-up versus the mixing time under the assumption that the interionic and
intramolecular correlation times (τc) are the same. Such an assumption was checked by the experimental
measurements of τc from both the dipolar contribution to the carbon-13 longitudinal relaxation time (TDD) and
1
the comparison of the intramolecular and interionic cross relaxation rate constant (σ) dependence on the
temperature. Both the methodologies indicate that anion and cation have comparable τc values. The determined
correlation time values were compared with those obtained for the neutral trans-[Ru(COMe){(pz2)BH2}(CO)-
(PMe3)2] complex (2), isosteric with the cation of 1. They were significantly shorter (∼3.8 times), indicating
that the main contribution to dipolar relaxation processes comes from the overall ion pair rotation. As a
consequence, the determined average interionic distances appear to be accurate. By using such interionic
distances, it was possible to verify that the counteranion in complex 1b also orients the BMe group far away
from the metal center.
Introduction
the behavior of transition metal ion pairs in different reactions
as a function of solvent, temperature, size of substituents on
catalysts and substrates, very little is known about their structure
in solution. Most of the structural features of transition metal
complex ion pairs in solution have been deduced either from
X-ray solid-state investigations or theoretical considerations.
Owing to the small amount of energy involved in ion pair
formation, dissociation, and eventual reorganization, the deduc-
tions may not always be true.
The importance of the ion pairing phenomenon in chemistry
is long-standing. Its effects on the reactivity of organic substrates
have been realized for several decades.1 The chemistry of
coordination and organometallic compounds2 is also strongly
affected by ion-counterion interactions, and most of the
reactions catalyzed by transition metal complexes involve
cationic compounds where the counteranion has a significant
role.2 Olefin polymerizations or copolymerizations catalyzed by
both first transition metallocene3 and late transition4 metal
complexes, for instance, always involve cationic complexes
working in low dielectric constant solvents where they are
mainly present as intimate ion pairs. Although many experi-
mental5 and theoretical6 studies have been done to understand
In the mid 1980s, it was found that NOE (nuclear Overhauser
effect) NMR experiments can be successfully applied to
investigate the structure of ion pairs in solution when intermo-
(3) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391 and
references therein. Zhou, J.; Lancaster, S. J.; Walker, D. A.; Beck, S.;
Thornton-Pett, M.; Bochman, M. J. Am. Chem. Soc. 2001, 123, 223. Beck,
S.; Lieber, S.; Schaper, F.; Geyer, A.; Brintzinger, H.-H. J. Am. Chem.
Soc. 2001, 123, 1483.
(4) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem ReV. 2000, 100,
1169 and references therein.
(5) For recent examples see: Song, X.; Thornton-Pett, M.; Bochmann,
M. Organometallics 1998, 17, 1004. Beswick, C. L.; Marks, T. J
Organometallics 1999, 18, 2410. Beswick, C. L.; Marks, T. J. J. Am. Chem.
Soc. 2000, 122, 10358.
(6) For recent examples see: Chan, M. S. W.; Vanka, K.; Pye, C. C.;
Ziegler, T. Organometallics 1999, 18, 4624. Vanka, K.; Chan, M. S. W.;
Pye, C. C.; Ziegler, T. Organometallics 2000, 19, 1841. Lanza, G.; Fragala`,
I. L.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 12764. Chan, M. S. W.;
Ziegler, T. Organometallics 2000, 19, 5182.
* E-mail for corresponding author: alceo@unipg.it.
(1) Szwarc, M. Ions and Ion Pairs in Organic Reactions; Wiley-
Interscience, New York, 1972; Vols. 1 and 2.
(2) See for examples: (a) Evans, D.; Murry, J. A.; von Matt, P.; Norcross,
R. D.; Miller, S. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 798. (b) Romeo,
R.; Arena, G.; Scolaro, L. M.; Plutino, M. R. Inorganica Chimica Acta
1995, 241, 81. (c) Carmona, D.; Cativiela, C.; Garcia`-Correas, R.; Lahoz,
F. J.; Lamata, M. P.; Lopez, J. A.; Lopez-Ram de Viu, M. P.; Oro, L. A.;
San Jose´, E.; Viguri, F. J. Chem. Soc., Chem. Commun. 1996, 1247. (d)
Lightfoot, A.; Schnider, P.; Pfaltz, A. Angew. Chem., Int. Ed. Engl. 1998,
37, 71. (e) Ku¨ndig, E. P.; Saudan, C. M.; Bernardinelli, G. Angew. Chem.,
Int. Ed. Engl. 1999, 38, 71. (f) Bellachioma, G.; Cardaci, G.; Macchioni,
A.; Zuccaccia, C. J. Organomet. Chem. 2000, 594, 119.
10.1021/ja015959g CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/10/2001

InVestigation of Ru(II) Complex Ion Pairs
J. Am. Chem. Soc., Vol. 123, No. 44, 2001 11021
lecular interactions are strong enough to stabilize a particular
anion-cation orientation so that other conformations, or dis-
sociated ions, are negligibly populated.7,8 Detailed investigations
were carried out on organic closed shell ion pairs that are
important for phase-transfer catalysis7 and organolithium ion
pairs.8 We decided to apply this methodology for investigating
transition metal complex ion pairs.9 Several Fe(II),9b,c Ru(II),9a-d,9h
Os(II),10 Pd(II),9f Pt(II),9e,g,i Ir(III),11 and Ag(I)12 complex ion
Chart 1
1
pairs were qualitatively investigated by H-NOESY and 19F,
1H-HOESY (heteronuclear Overhauser effect spectroscopy)
NMR experiments in solvents with low dielectric constants, and
the interionic structure was always well-defined. The reason
for the specificity in the cation-anion interactions, which was
generally higher than that found in other types of ion pairs, was
attributed to the noncentrosymmetric electron density distri-
bution7d,11 and/or formation of a dipolar moment in the cationic
moiety facilitated by the high polarizability of the d-metal
orbitals.
Encouraged by the results from the qualitative NOE studies,
we recently started the quantification of the NOEs in order to
estimate the average interionic distances in solution.13 Despite
the fact that the subject needs many precautions, especially
because interionic distances refer, by definition, to distances
between two noncovalently bonded nuclei which are, reasonably,
in motion relative to each other, we decided to attempt such
measurements because (1) the main presence of only one type
of intimate ion pair in solution should limit the types of motions
and (2) the estimations of average interionic distances could be
very precious to investigate transition metal catalysts in the
environment where they really “work”.
In this paper, we report the synthesis and the qualitative
interionic investigation in solution of trans-[Ru(COMe){(pz2)-
CH2}(CO)(PMe3)2]X (X- ) BPh4-, 1a; BPh3Me-, 1b; BPh3-
(n-Bu)-, 1c; BPh3(n-Hex)-, 1d; B(3,5-(CF3)2(C6H3))4-, 1e;
PF6-, 1f; and BF4-, 1g; pz ) pyrazol-1-yl-ring), complexes
bearing both symmetric and unsymmetric counteranions. Pre-
liminary results were already communicated.13 The results of
our efforts to determine the average interionic distances in
solution for complexes 1a,b by using the measurements of the
complete kinetic of NOE build-up are described. Furthermore,
two methodologies were used to determine the rotational
correlation times for complex 1a and for the neutral isosteric
trans-[Ru(COMe){(pz2)BH2}(CO)(PMe3)2] (2) complex. The
results are presented to validate average interionic distance
values obtained.
The twofold aim of this study was (1) to test both the
qualitative and quantitative NOE NMR methodologies for ion
pair investigations in solution by (2) determining the relative
cation-anion position for complexes 1a-g as a function of
anion nature, solvent, and temperature. Even if the chemistry
of such complexes is not affected by ion pairing, they have been
chosen as “model” compounds for developing and checking the
NMR investigation methodologies, because they are relatively
easy to synthesize and present all the suitable NMR properties.
The methodologies validated in this study have already been
used9f,g and will be further applied to chemical systems whose
reactivity or catalysis is affected by cation-anion interactions.
(7) See for examples: (a) Pochapsky, T. C.; Stone, P. M. J. Am. Chem.
Soc. 1990, 112, 6714. (b) Pochapsky, T. C.; Stone, P. M. J. Am. Chem.
Soc. 1991, 113, 1460. (c) Pochapsky, T. C.; Wang, A.; Stone, P. M. J. Am.
Chem. Soc. 1993, 115, 11084. (d) Mo, H.; Wang, A.; Stone-Wilkinson, P.
M.; Pochapsky, T. C. J. Am. Chem. Soc. 1997, 119, 11666. (e) Hofstetter,
C.; Stone-Wilkinson, P. M.; Pochapsky, T. C. J. Org. Chem. 1999, 64,
8794. (f) Hofstetter, C.; Pochapsky, T. C. Magn. Reson. Chem. 2000, 38,
90. For a review on intermolecular NOE investigations see: Mo, H.;
Pochapsky, T. C. Prog. Nucl. Magn. Reson. Spectrosc. 1997, 30, 1.
(8) See for examples: (a) Bauer, W.; Mu¨ller, G.; Schleyer, P. v. R.
Angew. Chem., Int. Ed. Engl. 1986, 25, 1103. (b) Avent, A. G.; Eaborn,
C.; El-Kheli, M. N. A.; Molla, M. E.; Smith, J. D.; Sullivan, A. C. J. Am.
Chem. Soc. 1986, 108, 3854. (c) Bauer, W.; Clark, T.; Schleyer, P. v. R. J.
Am. Chem. Soc. 1987, 109, 970. (d) Gu¨nther, H.; Moskau, D.; Bast, P.;
Schmalz, D. Angew. Chem., Int. Ed. Engl. 1987, 26, 1112. (e) Bauer, W.;
Schleyer, P. v. R. Magn. Reson. Chem. 1988, 26, 827. (f) Bauer, W.;
Klusener, P. A. A.; Harde, S.; Kanters, J. A.; Duisenburg, A. J. M.;
Brandsma, L.; Schleyer, P. v. R. Organometallics 1988, 7, 552. (g) Hoffman,
D.; Bauer, W.; Schleyer, P. v. R. J. Chem. Soc., Chem. Commun. 1990,
208. (h) Bauer, W.; Clark, T.; Schleyer, P. v. R. AdV. Carbanion Chem.
1992, 1, 89. (i) Hilmersson, G.; Arvidsson, P. I.; Davidson, O¨ .; Hakansson,
M. J. Am. Chem. Soc. 1998, 120, 8143. (l) Gschwind, R. M.; Rajamohanan,
P. R.; John, M.; Boche, G. Organometallics 2000, 19, 2868.
(9) (a) Bellachioma, G.; Cardaci, G.; Macchioni, A.; Reichenbach, G.;
Terenzi, S. Organometallics 1996, 15, 4349. (b) Macchioni, A.; Bellachioma,
G.; Cardaci, G.; Gramlich, V.; Ru¨egger, H.; Terenzi, S.; Venanzi, L. M.
Organometallics 1997, 16, 2139. (c) Bellachioma, G.; Cardaci, G.; Gramlich,
V.; Macchioni, A.; Valentini, M.; Zuccaccia, C. Organometallics, 1998,
17, 5025. (d) Macchioni, A.; Bellachioma, G.; Cardaci, G.; Cruciani, G.;
Foresti, E.; Sabatino, P.; Zuccaccia, C. Organometallics 1998, 17, 5549.
(e) Romeo, R.; Nastasi, N.; Monsu` Scolaro, L.; Plutino, M. R.; Albinati,
A.; Macchioni, A. Inorg. Chem. 1998, 37, 5460. (f) Macchioni, A.;
Bellachioma, G.; Cardaci, G.; Travaglia, M.; Zuccaccia, C.; Milani, B.;
Corso, G.; Zangrando, E.; Mestroni, G.; Carfagna, C.; Formica, M.
Organometallics 1999, 18, 3061. (g) Zuccaccia, C.; Macchioni, A.; Orabona,
I.; Ruffo, F. Organometallics 1999, 18, 4367. (h) Bellachioma, G.; Cardaci,
G.; D’Onofrio, F.; Macchioni, A.; Sabatini, S.; Zuccaccia, C. Eur. J. Inorg.
Chem. 2001, 1605. (i) Romeo, R.; Fenech, L.; Monsu` Scolaro, L.; Albinati,
A.; Macchioni, A.; Zuccaccia, C. Inorg. Chem. 2001, 40, 3293.
Results and Discussion
Synthesis. Complexes 1b,c (Chart 1) were synthesized by
the reaction of cis,trans-RuI(Me)(CO)2(PMe3)2 with pz2(CH2)
in methanol in the presence of a large excess of K[BPh3R]
(R ) Me (1b), n-Bu (1c)). Because of (a) the migration of the
methyl onto a cis carbonyl group, (b) the ionization of the Ru-I
bond, and (c) the coordination of the bidentate N,N-ligand
(pz2(CH2)), the acetyl ionic complex trans-[Ru(COMe){(pz2)-
CH2}(CO)(PMe3)2]I forms that exchanges the I- counteranion
with BPh3R-, affording the precipitation of the desired product.
The preparation of complex 1d (Chart 1) was similar, but owing
(10) Bellachioma, G.; Cardaci, G.; Macchioni, A.; Valentini, F.; Zuc-
caccia, C.; Foresti, E.; Sabatino, P. Organometallics 2000, 19, 4320.
(11) Macchioni, A.; Zuccaccia, C.; Clot, E.; Gruet, K.; Crabtree, R. H.
Organometallics 2001, 20, 2367.
(12) Bachechi, F.; Burini, A.; Galassi, R.; Macchioni, A.; Pietroni, B.
R.; Ziarelli, F.; Zuccaccia, C. J. Organomet. Chem. 2000, 593-594, 392.
(13) Zuccaccia, C.; Bellachioma, G.; Cardaci, G.; Macchioni, A. Orga-
nometallics 1999, 18, 1.

11022 J. Am. Chem. Soc., Vol. 123, No. 44, 2001
Zuccaccia et al.
Figure 2. Section of the 1H-NOESY spectrum of the complex 1b
showing the interionic contacts between the BMe group of the anion
and H5 and H5′ protons of the cation (400.13 MHz, 298 K, methylene
chloride-d2).
calculations, in the noncentrosymmetric distribution of the
electron density around the metal.7d The main energetic
contribution for stabilizing the observed interionic structure is
therefore of an electrostatic nature. This is also demonstrated
by the fact that relative anion-cation position is independent
of the counteranion nature. Both organic (in 1a-d complexes)
and inorganic (in 1f,g complexes) counteranions essentially
locate in the same position.
Complex 1e behaves in a particular way, because the B(3,
5-(CF3)2(C6H3))4- counterion is so weakly coordinating that it
affords nonintimate ion pairs even in methylene chloride solution
at room temperature and at NMR concentrations (10-4-10-1
M solutions). Consequently, no interionic contacts were ob-
1
served in methylene chloride-d2 in either the 19F, H-HOESY
Figure 1. 19F, 1H-HOESY spectrum of complex 1f showing the
“heteronuclear interionic contacts”. The F1 trace (indirect dimension)
relative to one component of the fluorine doublet is reported on the
right (376.65 MHz, 298 K, methylene chloride-d2).
or 1H-NOESY NMR spectra. When these last experiments were
performed for a ∼10-2 M solution of 1e in chloroform-d, weak
1
interionic contacts appeared in the 19F, H-HOESY spectrum
between the anion fluorine nuclei and 5/5′ and CH2 protons,
again. As in the other cases, no NOE was observed between
the fluorine nuclei and the COMe protons. This means that the
interionic structure of complex 1e is the same as those of the
other complexes, once the experimental parameters (solvent,
concentration, and temperature) allow the prevalence of intimate
ion pairs in solution.
to the insolubility of K[BPh3(n-Hex)] in methanol, the reaction
was carried out in nitromethane and complex 1d was then
precipitated from a methanol/water mixture. Complex 1e (Chart
1) was obtained in methanol and precipitated by the addition
of water. The synthesis of complex 1f was carried out in
methylene chloride similarly to those of complexes 1a,b with
the only difference being that the ionization of the Ru-I bond
was induced by TlPF6, with the consequent precipitation of TlI
that was then filtered off. The product was precipitated by adding
n-hexane to the solution.
Qualitative NOE Measurements. The relative anion-cation
position in solution (methylene chloride-d2) was previously
investigated for complexes 1a and 1g; only preliminary results
concerning complexes 1b-d were communicated.13 In all cases,
the counteranion showed very specific interionic interactions
and preferred to locate close to the 5/5′ and CH2 protons (Chart
1) as illustrated for complex 1f in Figure 1. A weak interionic
contact between the fluorine nuclei of the counterion and PMe3
protons can be observed in Figure 1, but its intensity is rather
small, especially considering that the intensity of a cross-peak
due to NI equivalent spins I and NS equivalent spins S is
proportional to the ratio NINS/(NI + NS).14 Such a ratio equals
4.5 for PMe3, 1.5 for CH2, and 1.5 for H5 and H5′. The intensity
of the observed PMe3-PF6- peak in Figure 1 must be divided
by 3.0 (4.5/1.5) to be quantitatively compared with other
interionic peaks, and consequently, its intensity is very small.
An explanation for the observed specific localization of the
counteranion was found, with the help of quantomechanical
Complexes 1b-d were synthesized, and their interionic
structures were investigated in order to understand whether the
presence of unsymmetrical counteranions could further enhance
the specificity of the interionic contacts. The analysis of the
results for complex 1b did not answer our question. In fact, the
1H-NOESY NMR spectrum recorded in methylene chloride-d2
(at room temperature) showed interionic contacts between both
the oH and BMe anion protons with 5/5′ and CH2 cation protons
(Figure 2). The qualitative approach did not solve the dilemma
of either (a) a preferential orientation of the counteranion with
the Me group that was still close enough to interact or (b) an
equilibrium between different anion orientations. By increasing
the aliphatic chain length of the counteranion, clearer results
were obtained. The terminal methyl groups of complexes 1c,d
did not show NOEs with any cation protons, while oH anion
protons continued to interact with 5/5′ and CH2 cation protons.
This clearly indicates that, at least in the cases of 1c,d, the
counteranion prefers to orient its aliphatic chain far from the
cationic organometallic moiety. Interestingly, the R-CH2 protons
of the counterions showed interionic contacts with the cationic
protons, which suggests that, for complex 1b, there could also
be a preferential orientation of the counterion while the BMe
group still remains close enough to be able to give NOEs with
(14) Macura, S.; Ernst, R. R. Mol. Phys. 1980, 41, 95.

InVestigation of Ru(II) Complex Ion Pairs
J. Am. Chem. Soc., Vol. 123, No. 44, 2001 11023
some cationic protons. In any case, the results relative to
complexes 1c,d indicate that the favorable van der Waals
interactions between the phenyl groups of the counterion and
the pyrazolyl rings of the cationic organometallic moiety affect
the relative anion-cation orientation. The highly specific
interionic solution structure of complexes bearing unsymmetrical
counteranions can be considered to be guided mainly by
electrostatic energy gain and then finely modulated by “stack-
inglike” π-π interactions.
The effect of solvent on the interionic structure was inves-
tigated for complex 1a. The choice of solvents was based on
their dielectric constants, keeping their coordination capability
low. The studies were performed in methylene chloride-d2
(ꢀ ) 8.93 at 298 K), acetone-d6 (ꢀ ) 20.56 at 298 K), and
nitromethane-d3 (ꢀ ) 35.94 at 298 K). The solution concentra-
tions were between 10-2-10-1 M. The 1H NMR spectra already
indicated that the intimate ion pairs were destroyed in both
acetone and nitromethane. In fact, only the resonances of the
cationic moiety that are close to the counterion in methylene
chloride (CH2, H5, and H5′) and are shielded by the π-electron
density of phenyl groups resonate at substantially higher
frequencies.15 Several 1H-NOESY spectra were recorded in
acetone-d6 and nitromethane-d3 for complex 1a with changes
in the concentration and the mixing times. In every case, very
weak interionic NOEs were detected (maximum 1.2%) but with
the “usual” cationic protons. This leads to the conclusion that
the few ion pairs that are present in such solvents have the same
interionic structure as those in methylene chloride.
Figure 3. Experimental data relative to the % NOE as a function of
τ for the irradiation of oH protons of the BPh4 (T ) 302 K) for
complex 1a.
-
Table 1. Cross Relaxation Rate Constant σIS (s-1)a and
Internuclear Distance ( rIS (Å)b) Values for Complex 1a
σIS (s-1
linear fitting from linear
)
rIS (Å)
σIS
rIS
(Å)
rIS
(s-1
)
up to 0.4 s
fitting
(Å)c
mH{oH}
H5{oH}
CH2{oH} 0.0035
H4{H5} 0.0358
CH2{H5} 0.0345
oH{H5}
H5{H4}
H3{H4}
H5{CH2} 0.0350
oH{CH2} 0.0036
0.0743d 2.47e
0.0678
0.0055
0.0027
0.0294
0.0256
0.0046
0.0293
0.0310
0.0254
0.0024
2.47e
4.13
4.65
2.84
3.20
4.26
2.84
2.81
3.20
4.74
2.47
0.0058
4.16
4.52
2.79
3.08
4.30
2.80
2.79
3.08
4.50
2.66
0.0047
0.0349
0.0356
2.66
2.65
2.94
Quantitative NOE Measurements. The quantitative NOE
experiments were performed for complexes 1a (2 × 10-1 M)16
and 1b (4 × 10-2 M) in methylene chloride-d2 at 302 K by
using the selno17 and selnogp18 sequences. The target resonances
were chosen on the basis of the qualitative interionic structure:
oH, H5, H4, and CH2 for complex 1a (H5′ was not selectively
inverted because it was buried under the pH protons of the
anion), and oH, CH2, BMe, PMe3 for complex 1b. The NOEs
between the inverted and all the dipolarly coupled nuclei were
measured as a function of the mixing time (τm). The data relative
to the NOE percentages as a function of τ are presented in the
Supporting Information (see also Figure 3).
a All the fits have R2 values higher than 0.97, and the estimated error
for a single σIS value is less than 5%. b The distances relative to the
protons that undergo dynamic processes were increased by 10% and
are reported in bold. c Values derived from the X-ray structure by
averaging all the possible distances in the case of equivalent protons.
d The spin oH-mH subsystem must be treated carefully. In fact, while
the 8 ortho protons and the 8 meta protons are magnetically equivalent,
it is not correct to divide the rate of NOE build-up by 8 (the number
of irradiated spins), because each oH relaxes mainly with “its own”
mH (i.e., the adjacent mH). e The oH-mH distance was calculated by
averaging the data derived from neutron scattering investigations.20
In the case of complex 1a, τ was varied from 10-4 to 10 s in
each experiment, and consequently, the complete time course
of the NOE enhancements was recorded. In Figure 3, the
experimental time course of the NOE build-up for the dipolarly
coupled nuclei after inversion of oH protons is illustrated. The
experimental data were treated in two different ways: (a) they
were fitted by eq 1,19 valid for two isolated spins, using R (total
longitudinal relaxation rate constant) and σIS as fittable pa-
rameters, and (b) the fittings were limited to the first points
where the kinetic of NOE build-up is almost linear
(τ e 0.4 s).
1
ISτ
NOEI{S}(τ) ) e-(R-σ )τ(1 - e-2σ
)
(1)
IS
2
(15) Bellachioma, G.; Cardaci, G.; Macchioni, A.; Zuccaccia, C. In
preparation.
In both cases, the time-average values of the cross relaxation
rate ( σIS ), reported in Table 1, were determined by dividing
the σIS values derived from fittings by the number of inverted
protons. For the “symmetric” experiments, that is, I spin
inversion and NOE detection on S and vice versa, the “sym-
metry” rule is satisfied; for example, the rate of NOE build-up
on oH protons after the inversion of CH2 protons is equal, within
experimental error, to the rate of NOE build-up on CH2 protons
after the inversion of oH protons (Table 1).
(16) It must be said that at this high concentration value ion quadrupoles
are also present (Zuccaccia, C.; Bellachioma, G.; Cardaci, G.; Macchioni,
A. Organometallics 2000, 19, 4663.). Despite this, the qualitative interionic
structure of ion quadrupoles was found to be the same as that of ion pairs,
and the quantification of NOE should not be affected. Furthermore, unlike
the results reported in ref 7f, no negative NOE was observed indicating for
our system a lower level of aggregation.
(17) Kessler, H.; Oschinat, H.; Griesinger, G.; Bermel, W. J. Magn. Res.
1986, 70, 106. Bauer, C. J.; Freeman, R.; Frenkiel, T.; Keeler, J.; Shaka, J.
J. Magn. Res. 1984, 58, 442.
(18) Stonehouse, J.; Adel, P.; Keeler, J.; Shaka, J. J. Am. Chem. Soc.
1994, 116, 6037. Stott, K.; Stonehouse, J.; Keeler, J.; Hwang, T.-L.; Shaka,
J. J. Am. Chem. Soc. 1995, 117, 4199. An interesting application of the
selnogp sequence to the investigation of the solution conformation of a
tetraanion-tetracation ion pair has been recently reported: McCord, D. J.;
Small, J. H.; Greaves, J.; Van, Q. N.; Shaka, A. J.; Fleischer, E. B.; Shea,
K. J. J. Am. Chem. Soc. 1998, 120, 9763.
With the assumption that ωI ) ωS ) ω, σIS depends on the
correlation time (τc) and the internuclear distance according to
the following equation:
µ0 2p2γ2I γ2
6
-6
IS
σIS )
S‚τc
- 1 r
(2)
(19) Neuhaus, D.; Williamson, M. The Nuclear OVerhauser Effect in
Structural and Conformational Analysis; VCH Publishers: New York, 1989.
2
4π
10
( )
(
)
1 + (2ωτc)

11024 J. Am. Chem. Soc., Vol. 123, No. 44, 2001
Zuccaccia et al.
Table 2. Cross Relaxation Rate Constant ( σIS (s-1)a) and
Internuclear Distance ( rIS (Å)b) (oH-mH Distance Is Used as
Reference Distance) for Complex 1b
σIS (s-1
)
rIS (Å)
mH{oH}
0.0440
0.0070
0.0021
0.0021
0.0220
0.0009
0.0024
0.0071
0.0026
0.0002
0.0025
2.47c
3.69
4.51
4.51
3.05
5.19
4.41
3.68
4.35
6.67
4.38
BMe{oH}
CH2{oH}
oH{CH2}
H5{CH2}
BMe{CH2}
PMe3{CH2}
oH{BMe}
H5{BMe}
mH{PMe3}
CH2{PMe3}
a All the fits have R2 values higher than 0.9 and the estimated error
for a single σIS value is about 10%. b All the rIS values, except the
reference distance, were increased by 10% in order to correct for the
overestimation of the short distances. c The oH-mH distance is
calculated by averaging the data derived from neutron scattering
investigations.20
Figure 4. Three-dimensional structure of 1a-d Ru(II) ion pairs derived
from qualitative and quantitative interionic NOE measurements showing
as the aliphatic chain (in 1b-d) points away from the metal center.
toward the metal, the average CH2-oH distance should be
longer than that in the other ion pair. In conclusion, the
quantitative NOE investigation of complex 1b allows us to
unambiguously understand that the BMe group points away from
the cationic fragment, as in complexes 1c,d as schematized in
Figure 4. This is probably due to a maximization of lipophilic
interactions.
The average distances, rIS , were then calculated from the
obtained σ values by using eq 3 and the intramolecular reference
distance (rAB) oH-mH (Table 1) with the assumption that the
tumbling of the system can be described by a single rotational
correlation time.
Measurements of the Correlation Time. As mentioned
above, the average internuclear distances were determined from
kinetic NOE experiments using an intramolecular reference
distance and by assuming that the tumbling of the system can
be described by a single rotational correlation time. This
assumption is valid if the molecular system can be reasonably
approximated to a rigid spherical body tumbling in solution
because of Brownian motion. On the other hand, our systems
consist of two separated fragments, linked together only by
noncovalent forces, and this assumption has to be verified.
6
σAB
rIS ) rAB
(3)
σ
x
IS
The oH-mH distance (2.47 Å) was derived from experi-
mental data available by neutron diffraction investigations.20 The
distances involving the protons of the counteranion that
undergoes dynamic motions (demonstrated by the equivalence
of the four phenyl groups) and those of CH2 and H5 protons
(interested in the chair-boat inversion of the six-member Ru-
N-N-CH2-N-N ring) were increased by 10% in order to
correct, at least partially, for the overestimation of the short
distances and are indicated in bold in Table 1.19
For this reason, we decided to estimate the rotational
correlation time (τc) for complex 1a in methylene chloride-d2
by using two different approaches: (a) 13C relaxation measure-
1
ments and (b) dependence of the H-1H cross relaxation on
As mentioned above, the interionic structure of complex 1b
bearing an unsymmetrical counteranion is not completely
defined by the qualitative NOE investigation. In particular, the
orientation of the BMe group with respect to the organometallic
moiety is not clear. For such a complex, kinetic experiments
were performed to determine the rate of the NOE build-up by
using only the first linear part of the curve, (τm e 0.4 s); all the
experiments were performed using the PFG version of the selno
sequence (selnogp).18 The estimations of the average cross
temperature.
(a) Carbon T1 Determinations. It is well-known that the
most important pathway of relaxation for nonquaternary carbon
nuclei is dipole-dipole relaxation by the attached protons.21 In
an isotropic system, at the extreme narrowing limit (i.e., when
the product ωτc , 1), the dipolar contribution to the total
relaxation is directly proportional to the rotational correlation
time:22
relaxation rate constants ( σIS ) and internuclear distances ( rIS
are reported in Table 2.
)
2
µ0
γ2H γ2Cp
r6CH
1
TD1 D
) RD1 D
)
N
‚τC ) NKτc
(4)
( )
4π
The 0.7 Å difference between the average distances CH2-
MeB (5.19 Å) and CH2-oH (4.51 Å) indicate two possible
conclusions: (1) the situation for complex 1b is the same as
that of complexes 1c,d, with the methyl group pointing away
from the cation; (2) two different populated ion pairs are present,
one with the aliphatic tail pointing away from the cation, the
other with the aliphatic tail pointing toward the cation. The latter
interpretation can be excluded because the CH2-oH distance
for complex 1b (4.51 Å) is very similar to that of 1a (4.51 and
4.62 Å with total and linear fittings, respectively), and this would
not be the case if 1b were present in solution as two different
ion pairs. In fact, in the hypothetical ion pair where BMe points
where the longitudinal relaxation time and relaxation rate
constant refer to 13C, N is the number of protons attached to a
given carbon, and K ) 2.0325 × 1010 s-2, if the C-H bond
lengths, rCH, are assumed to be constant (1.1 Å).
(21) (a) Abraham, R. J.; Loftus, P. Proton and Carbon-13 NMR
Spectroscopy; Wiley Heyden Ltd.: Chichester, U.K., 1985. (b) Kowalewski,
J.; Ma¨ler, L. In Methods for Structure Elucidation by High-Resolution NMR;
Batta, G., Ko¨ver K. E., Szantay, C., Jr., Eds.; Elsevier: Amsterdam, 1997,
Chapter 16.
(22) (a) Abragam, A. The Principle of Nuclear Magnetism; Claredon
Press: Oxford, 1961. (b) Bu¨hl, M.; Hopp, G.; vin Philipsborn, W.; Beck,
S.; Prosenc, M.-H.; Rief, U.; Brintzinger, H.-H. Organometallics 1996, 15,
778. (c) Gaemers, S.; van Slageren, J.; O’Connor, C. M.; Vos, J. G.; Hage,
R.; Elsevier, C. J. Organometallics 1999, 18, 5238.
(20) Albinati, A.; Bracher, G.; Carmona, D.; Jans, J. H. P.; Klooster,
W. T.; Koetzle, T. F.; Macchioni, A.; Ricci, J. S.; Thouvenot, R.; Venanzi,
L. M. Inorg. Chim. Acta 1997, 265, 255.

InVestigation of Ru(II) Complex Ion Pairs
J. Am. Chem. Soc., Vol. 123, No. 44, 2001 11025
Table 3. Relaxation Times (T1 and TDD), NOE Factors and
Rotational Correlation Times (τc) for Complex 1a in Methylene
Chloride-d2
Table 4. Relaxation Times (T1 and TDD), NOE Factors and
Rotational Correlation Times (τc) for Complex 2 in Methylene
Chloride-d2
1
1
signal
NOE(13C{1H})
T1 (s)
TD1 D (s)
τc (ps)
signal
NOE(13C{1H})
T1 (s)
TD1 D (s)
τc (ps)
C3
1.68
1.88
1.67
1.72
1.78
1.70
1.70
1.70
1.87
1.69
1.52
1.94
0.484
0.513
0.458
0.858
0.476
0.858
0.62
0.575
0.543
0.545
0.993
0.533
1.003
0.727
0.776
0.704
0.384
2.510
1.408
86
90
90
49
92
49
67
63
70
64
7
C3
C3′
C5′
C5
C4
C4′
COMe
PMe3
1.65
1.56
1.58
1.84
1.85
1.58
1.06
1.84
1.883
2.027
1.962
1.988
1.848
1.923
5.472
2.153
2.270
2.592
2.479
2.151
1.989
2.417
10.251
2.322
22
19
20
23
25
20
2
C3′
C5′
oC
C5
mC
pC
C4
0.66
7
C4′
CH2
COMe
PMe3
0.661
0.326
1.917
1.376
all the assumptions involved, this value is in good agreement
with that from previous PGSE measurements15 (∼3) and
indicates, for sure, that the τc values of complex 1 are governed
by the overall ion pair rotation that determines similar τc values
for anion and cation. Furthermore, the invariance of τc values
for complex 2 indicates that it tumbles in solution like a rigid
spherical body. Crystallographic inspection of the cationic
moiety of complex 1 indicates that it is reasonably correct to
consider 2 as spherical. The three molecular axes defined (1)
from the carbon of the acetyl group to C5′ (7.41 Å), (2) from
the largest distance between two carbons belonging to the
different phosphine groups (7.06 Å), and (3) from C4 to C4′
(6.47 Å) are very similar. In contrast, in the case of complex
1a, the formation of the ion pair or, eventually, higher
aggregates, destroys the pseudospherical symmetry at the cation
and anion. This is reflected in the different correlation time
values for the internuclear vectors that form different angles
with the principal axes of rotation.
11
The dipolar contribution to the total relaxation can be
estimated from the NOE data; in fact, if the CHn systems relax
entirely by dipole-dipole interactions, at the extreme narrowing
limit the expected value for the NOE(13C{1H}) is 1.987, and
we can write
1.987‚T1
NOE(13C{1H}) )
(5)
TD1 D
where T1 is the total longitudinal relaxation time.
The NOE(13C{1H}) can be obtained by comparing the
standard gated decoupling 13C{1H} spectrum, with the corre-
sponding inVerse gated decoupling spectrum, in which the
decoupler is only switched on during the acquisition time; the
T1 value can be obtained by the standard inVersion recoVery
sequence. The results concerning the measured values of NOE-
(b) Dependence of the Cross Relaxation Rate Constant
on the Temperature. Another methodology to obtain informa-
tion about the correlation time of the various internuclear vectors
is to measure the cross relaxation rate as a function of
temperature. A number of studies have shown that the rotational
correlation time varies with temperature according to the
following equation:23
(13C{1H}), T1, TDD, and τc for complex 1a are reported in
1
Table 3. As expected, the correlation times of COMe and PMe3
groups are shorter than the others because of the contribution
of the fast internal rotations of the CH3 groups around the C-C
or P-C bonds, respectively. Apart from these two groups, the
correlation times of the cation (64-92 ps) and those of the anion
(49-67 ps) are comparable. Slightly different correlation times
for the pz-cationic CH vectors were observed. Whereas the CH
vectors in positions 3, 3′, 5, and 5′ reorient with the same
correlation time, those in position 4 and 4′ reorient faster. On
the other hand, the correlation times for the anion are also
different; in particular, the oC-oH and mC-mH vectors reorient
with the same correlation times (49 ps), but the pC-pH vector
reorients more slowly (67 ps), with the same correlation time
as those of the 4 and 4′ cation vectors.
The similar correlation time values for both anion and cation
could be due to their similar size. To ascertain if the two
fragments influence the motion of each other, we decided to
investigate neutral complex trans-[Ru(COMe){(pz2)BH2}(CO)-
(PMe3)2] (2), which only differs from the cationic part of 1a
with the substitution of one carbon atom with a boron, and it is
isostructural, isosteric, and isomass with the cation itself. The
experimental results for a 2 × 10-1 M solution of complex 2 in
methylene chloride-d2 are reported in Table 4. All the C-H
vectors in the bispyrazolylborate ligand have similar correlation
times (∼23 ps), whereas, as expected, the correlation times for
the COMe and PMe3 groups are lower. By excluding COMe
and PMe3 groups, the average ratio between the correlation times
relative to CH values of complex 1 and those of complex 2 is
∼3.8. From the Debye equation19 that relates τc with the volume
of the rotating particle, assumed to be spherical, we can consider
3.8 as a first approximation of the ratio of volumes. Considering
τc ) τ0‚eE /k T
(6)
R
B
where ER is the activation energy for rotational reorientation,
τ0 is a constant, and kb is the Boltzmann constant.
By substituting the τc expression (eq 6) into classical eq 2,
that relates σIS with τc and r, the expression for the temperature
dependence of σIS is obtained. Its analysis indicates that σIS
reaches a maximum at the same τc or T value independent of
the internuclear distance and it has a zero cross point for
ωτc ) 1.12.
We performed 1D-transient experiments by inverting oH or
H5 in the 217-302 K temperature range for complex 1a in
methylene chloride-d2 (the data are reported in Supporting
Information). Because the experiments were rather time-
consuming, the σIS values were estimated by using only one τ
value that was set at 0.4 s as a compromise between the initial
rate approximation condition and a good signal-to-noise ratio
(especially for the temperature near the zero cross point, i.e.,
when ωτc ) 1.12). The experimental data points were fitted
setting ER and τ0 as fittable parameters and introducing the
scaling factor P1 (0 < P1 < 1). The results of this procedure
are summarized in Table 5. It is important to outline that the
position of the maximum (∼285 K) and that of the zero cross
(23) (a) Doddrell, D. M.; Bendall, M. R.; O’Connor, A. J.; Pegg, D. T.
Aust. J. Chem. 1977, 30, 943. (b) Farrar, T. C.; Becker, E. D. Pulse and
Fourier Transform N. M. R.; Academic Press: New York, 1971.

11026 J. Am. Chem. Soc., Vol. 123, No. 44, 2001
Zuccaccia et al.
Table 5. Values of rIS (Å), ER (kJ/mol), τ0 (ps), and P1 for
Complex 1a Estimated from the Best Nonlinear Least-Squares Fit of
Data Points
that averages them does exist. There can be two possibilities:
(a) relative anion-cation rotation inside the ion pair, and (b)
ion pair dissociation and reassociation with different anion-
cation orientation. Both possibilities have to occur in less than
∼10-2 s in order to average the differences in chemical shift,
but it is not immediately clear which is the lowest limit. In his
elegant papers,7 Pochapsky and co-workers showed that for
organic closed-shell ion pairs, the “contact” time is ∼10-5 s.7d
Despite the fact that, in such ion pairs, the anion is compen-
etrated with the cation, and consequently, the dissociation could
be slowed, we consider it to be reasonable to estimate our
“contact” time between 10-6 and 10-7 s, because it (point (b))
must not contribute significantly to the dipolar relaxation
processes. Concerning point (a), in the case of Pochapsky’s ion
pairs, the anion is so small (BH4-) that it can “freely” rotate
inside the ion pair as indicated by its very short correlation time
(4 ps).7c In our compounds, the (a) motion could be partially
inhibited by steric interactions. This explains why, whatever
the (a) or (b) process is that averages the phenyl groups, its
rate is smaller than 1/τc and contributes little to the dipolar
relaxation processes.
rIS (Å) ER (kJ/mol) τ0 (ps)
P1
τc (ps) (302 K)
mH{oH}a
H4{H5}
CH2{H5}
H5{oH}
oH{H5}
oH{CH2}
2.47
2.81
2.91
3.92
3.80
4.43
10.4
9.5
7.8
10.5
11.1
9.5
1.9
3.0
7.0
2.0
1.4
3.2
0.614
0.614
0.614
0.614
0.614
0.614
120
132
156
131
117
141
a In the fit of mH{oH} data points, the rIS is maintained as fixed
and the P1 is varied between 0 and 1, while in the other fits the P1 was
maintained at a fixed value of 0.614 and rIS was varied.
Table 6. Summarizing Table of Main Average Internuclear
Distances ( rIS (Å)) for Complex 1a
rIS (Å)a
rIS (Å)b
rIS (Å)c
H4/H5
H5/CH2
H5/oH
CH2/oH
2.8
3.1
4.2
4.5
2.8
3.2
4.2
4.7
2.8
2.9
3.9
4.4
a Derived by fitting the complete NOE built up with eq 2. b Derived
from the linear fits of NOE built up with τm e 0.4 s. c Derived from
the dependence of the cross relaxation rate constant on temperature.
Conclusions
We have shown that the interionic structure of Ru(II) complex
ion pairs 1a-g can be directly and deeply investigated in
solution by NOE NMR measurements. Qualitative 1H-NOESY
point (around 230 K) are very similar for all the fittings. This
strongly indicates that both the intramolecular and interionic
vector reorientations have the same temperature dependence.
As a general consideration, the information from the tem-
perature variation of σIS also seems to indicate that the
assumption of a single correlation time in the interionic distance
estimation is acceptable. Regarding the numerical values of τc
(Table 5), they are higher than those obtained from the 13C
relaxation investigations, but considering the assumptions
underlying both methodologies and the experimental errors, they
can be considered in good agreement. The activation energy
values for rotational reorientation (ER) amount to ∼10 kJ/mol,
in agreement with data previously reported in the literature.22a
The average distances extracted from the fits are consistent with
those coming from the kinetics of NOE build-up. To facilitate
the comparison, the main distances are summarized in Table 6.
It can be noted that the agreement is very good.
General Discussion. After having reported on the measure-
ments of the rotational correlation times, we can now discuss
the accuracy of the average interionic distance determinations,
a subject strictly related to the rates of the motions present in
the ion pairs that can contribute to dipolar relaxation processes.
Inside the Ru(II) ion pairs here investigated, there are three types
of motions: (1) overall rotation of ion pairs and/or ions
constituting them, (2) ion pair dissociation and formation, and
(3) internal motions (Me group rotations around single bonds
and chair-boat inversion of the six-member Ru-N-N-CH2-
N-N cycle). As stated above, the use of eq 3 for the
determination of both intramolecular and interionic distances
assumes that the correlation times of the unknown and reference
distances are the same. The τc measurements (with both the
methodologies) clearly indicate that the cation and anion have
substantially the same rotational correlation times, higher than
those of complex 2 which has the same size and mass as the
cation of 1. It is reasonable to conclude that the oVerall ion
pair rotation makes the greatest contribution to the dipolar
relaxation processes, and consequently, the “contact” time is
long enough to allow rather accurate aVerage interionic
distances to be estimated. On the other hand, the four or three
phenyl groups of the 1a,b anions are always equivalent, even
at low temperatures (down to ∼200 K), indicating that a motion
1
and 19F, H-HOESY indicate that interionic interactions are
strong enough to determine a preferential anion-cation orienta-
tion that is not affected by changing the solvent: the counter-
anion prefers to locate close to the peripheral protons of
bispyrazolyl ligand maximizing the electrostatic and van der
Waals (in the case of “organic” anions) interaction energy as
indicated in Figure 4. Of course, by increasing the dielectric
constant of the solvent, the percentage of intimate ion pairs
decreases, and NOEs become smaller. The quantification of
NOEs allowed the average interionic distances to be estimated;
these were checked and found to be both precise and accurate.
In fact, by using two different methodologies, it was found that
the rotational correlation times of cation and anion are similar
and, very importantly, higher than that of neutral complex 2
that is isosteric with the cation of 1. Consequently, the dipolar
relaxation processes are determined mainly by the overall ion
pair rotation. The determination of average interionic distances
may afford valuable structural information. For example, only
by comparing the interionic distances between BMe/CH2 and
oH/CH2 was it possible to determine that in complex 1b the
anion orients the BMe group far away from the metal center.
Generally speaking, the results of this study indicate that NOE
NMR spectroscopy is a promising technique for investigating
transition metal complex ion pairs in solution. We are currently
applying the technique to transition metal complexes whose
reactivity or catalytic properties are affected by ion pairing
phenomena.
Experimental Section
General Considerations. Reactions were generally carried out in a
dried apparatus under a dry inert atmosphere of nitrogen using standard
Schlenk techniques. Solvents were purified by conventional methods
prior to use.24 Pz2(CH2),25 cis,trans-RuI(Me)(CO)2(PMe3)2,26 trans-[Ru-
(24) Weissberger, A.; Proskauer, E. S. Techniques of Organic Chemistry;
Interscience: New York, 1955; Vol. VII.
(25) Julia, S.; Sala, P.; del Mazo, J.; Sancho, M.; Ochao, C.; Elguero,
J.; Fayet, J. -F.; Vertut, M.-C. J. Heterocycl. Chem. 1982, 19, 1141. Julia,
S.; del Mazo, J.; Avila, L.; Elgero, J. Org. Prep. Proced. Int. 1984, 16,
299.

InVestigation of Ru(II) Complex Ion Pairs
J. Am. Chem. Soc., Vol. 123, No. 44, 2001 11027
(COMe){(pz2)CH2}(CO)(PMe3)2]BPh4 (1a),7b trans-[Ru(COMe){(pz2)-
CH2}(CO)(PMe3)2]BF4 (1g),7b and trans-Ru(COMe){(pz2)BH2}(CO)-
(PMe3)2 (2)27 were prepared according to the literature. BPh3 was
prepared by the reaction of the adduct (BPh3‚NaOH)aq with HClaq. MeLi,
n-BuLi, and n-HexLi reagents were purchased and used without further
purifications.
strength was set accordingly to the value suggested in the Bruker
microprograms.
NMR Data. All the values in Tables 1 and 2 are presented with
two digits, assuming a precision of 0.01 Å; of course, this is an
underestimation of the error. First, the experimental σIS data are
affected by an error of about 5% due to the goodness of nonlinear
regression; second the reference distance, in the case of oH-mH, is
again affected by a 4% experimental error. In light of this, distances,
as a rough estimation, must be considered precise within (0.1-0.2 Å.
Finally, the 10% correction due to the overestimation of short distances
is rather empirical, even if it is often used in the literature.19 In Table
1, the intramolecular solid-state distances from the X-ray investigations9b
are also reported; they are in very good agreement with the NOE
Characterization of Complexes. All the complexes were investi-
gated in solution by IR spectroscopy using an FT-IR Perkin-Elmer
1725X. All the carbonyl bands are in the range 1500-2200 cm-1 and
are diagnostic for the relative positions of different CdO groups
belonging to the same complexes or for the formation of the COMe
group. Elemental analyses were made on a Carlo Erba 1106 elemental
microanalyzer.
measurements. Despite
σ values determined by fitting complete
All the NMR measurements were performed on a Bruker Avance
DRX 400 spectrometer operating at 400.13 MHz (1H), 376.65 MHz
(19F), 161.98 MHz (31P), and 100.61 MHz (13C). Gradient pulses, only
along the z direction, were generated by using the Great 1/10 gradient
unit. Shaped pulses were generated by the standard controls available
in a Bruker Avance DRX 400 spectrometer; the shape was Gaussian
with a 1% truncation. Sample temperature was controlled using the
BVT 3000 variable temperature unit equipped with digital control; the
value was checked using a 4% CH3OH in CD3OD standard tube. Proton,
carbon, and phosphorus spectra were acquired using the 5 mm QNP
probe or the 5 mm TBI (triple resonance broadband inverse) probe.
Fluorine spectra were acquired using the QNP probe. Referencing is
experimental data with eq 1 and those coming from linear fittings up
to a 0.4 s mixing time which are slightly different, the derived
internuclear distances are in excellent agreement within experimental
error. From the comparison of both intramolecular and interionic
distances of complexes 1a,b reported in Tables 1 and 2, it appears that
they are excellently reproduced.
Synthesis of K[BPh3R] (R ) Me, n-Bu, n-Hex). RLi (11 mmol,
Et2O solution) was added to a solution of BPh3 (3 g, 10.6 mmol) in
Et2O. The resulting solution was stirred for 1 h at room temperature.
The solvent was evaporated under reduced pressure. A volume of 50
mL of H2O was added to the residual solid, obtaining a suspension
that was then filtered; the final solution was saturated with KCl and
K[BPh3R] precipitated. It was washed with H2O and dried.
1
relative to external TMS for H and 13C, external H3PO4 for 31P, and
external CCl3F for 19F. All FIDs (free induction decays) for proton,
fluorine, and phosphorus were acquired using 32K or 64K points; the
FIDs for carbon spectra were acquired using 64K or 128K data points.
R ) Me:28 yield ) 78%. Anal. calcd (found) for C19H18BK: H,
6.12 (6.07); C, 77.03 (77.45). 1H NMR (methanol-d4, 298 K): δ 7.22
3
3
(m, oH), 6.91 (t, JHH ) 7.2, mH), 6.75 (t, JHH ) 7.1, pH), 0.21 (q,
2JBH ) 3.8, BMe). R ) n-Bu: yield ) 73%. Anal. calcd (found) for
1
Carbon and phosphorus spectra were acquired by decoupling the H
nucleus. In the 2D NMR experiments, the number of digital points
dedicated to direct and indirect dimension was fixed according to the
desired resolution and to the total experimental time. Each transient
(direct dimension) was acquired using 1K, 2K, or 4K points; the number
of transients (indirect dimension) was from 512 to 2K, and the number
of scans was set at 16, 24, or 32, depending on solute concentration.
All bidimensional spectra were transformed using the zero filling
procedure in both dimensions. The longitudinal relaxation times for
the 13C nuclei were measured by the standard inversion recovery method
1
C22H24BK: H, 7.15 (7.07); C, 78.10 (78.54). H NMR (methanol-d4,
298 K): δ 7.35 (m, oH), 6.97 (t, 3JHH ) 7.2, mH), 6.80 (t, 3JHH ) 7.1,
3
pH), 1.24 (q, JHH ) 7.2 CH2(3)), 0.99 (m, CH2(1) and CH2(2)), 0.81
(t, 3JHH ) 7.2, Me(4)). R ) n-Hex: yield ) 54%. Anal. calcd (found)
for C24H28BK: H, 7.70 (7.58); C, 78.68 (78.46). 1H NMR (nitromethane-
3
3
d3, 298 K): δ 7.23 (m, oH), 6.97 (t, JHH ) 7.4, mH), 6.69 (t, JHH
7.2, pH), 1.11 (m, CH2(2), CH2(4), CH2(5)), 0.87 (m, CH2(1) and CH2-
(3)), 0.81 (t, JHH ) 7.3, Me(6)).
)
3
Synthesis of Na[B(3,5-(CF3)2C6H3)4].29 A solution of 3,5-(CF3)2C6H3-
Br (10.26 g, 35 mmol) in 50 mL of Et2O was slowly added to 10 mL
of Et2O containing 1 g of Mg and some I2 crystals. The mixture was
refluxed for 1 h, and then NaBF4 (0.68 g, 6 mmol) was added. The
solution was stirred for 48 h, and Na2CO3 (15 g in 200 mL of H2O)
was added. The solution was stirred for an additional 20 min. A solid
separated from the solution and was filtered off. The resulting solution
was extracted with Et2O (4 × 40 mL). The ether solution was dried
under anhydrous Na2SO4 and treated with decolorant carbon; the solvent
was then removed, and the residual solid washed with small aliquots
of CH2Cl2. The final product Na[B(3,5-(CF3)2C6H3)4] was recrystallized
from CH2Cl2/n-hexane. Yield ) 70%. Anal. calcd (found) for C32H12-
F24BNa: H, 1.35 (1.42); C, 43.34 (43.51). 1H NMR (acetone-d6,
298K): δ 7.78 (m, oH), 7.67 (s, pH). 19F NMR (acetone-d6, 298 K):
δ -58.5 (s).
1
with H decoupling. A total of 32 experiments, having the relaxation
delay ranging from 0.001 to 28 s, were acquired, each of them consisting
of 512 scans. The total relaxation delay between two consecutive scans
was 15 s. All single experiments were Fourier transformed by using
an exponential function with line broadening of 3.0 Hz. Frequency
domain spectra were processed with a standard T1/T2 software package
available on Bruker spectrometers to extract the relaxation parameters.
The 1H,1H monodimensional NOE experiments for the quantitative
investigation were carried out using the “selno” 17 or the “selnogp” 18
sequences; both are available in the standard pulse microprogram library
of the Bruker spectrometers. All kinetic experiments using the selno
sequence were performed by setting the central frequency (O1) on
resonance with the “target” multiplet (i.e., the multiplet that has to be
excited) and increasing the spectral width until all peaks were observed.
A total of 32 experiments, having the mixing time period ranging from
0.0001 to 10 s, were acquired, each of them consisting of 64 scans.
The total relaxation delay between two consecutive scans was 15 s.
All single FIDs were Fourier transformed by using an exponential
function with line broadening of 1.5 Hz. The spectrum with the shortest
mixing time value was used as a reference spectrum; the integral value
of the excited resonance was set at -nS‚100, and the integral values of
all the other spectra were in reference to the previous one. The observed
integral value on a given enhanced signal, divided by the number of
protons corresponding to the signal itself (nI), directly yields the
percentage of NOE. The experiments using the selnogp sequence were
acquired and processed in the same way; the total scan number
depended on the concentration of the sample. The pulsed field gradient
Synthesis of trans-[Ru(COMe){(pz2)CH2}(CO)(PMe3)2]BPh3R
(R ) Me (1b), n-Bu (1c)). cis,trans-RuI(Me)(CO)2(PMe3)2 (100 mg,
0.22 mmol) and pz2(CH2) (53 mg, 0.30 mmol) were dissolved in 5 mL
of MeOH. K[BPh3R] (large excess) was added, and a white solid
precipitated. The solid was filtered, washed with cold Et2O, and dried.
Characterization of 1b: yield ) 63%. Anal. calcd (found) for C35H47-
BN4O2P2Ru: H, 6.48 (6.52); C, 57.61 (57.16); N, 7.68 (7.75). IR (CH2-
Cl2): νCO ) 1950.0 cm-1; νCOMe ) 1602.6 cm-1. 1H NMR (methylene
3
3
chloride-d2, 298 K): δ 8.29 (d, JHH ) 2.4, H3), 7.68 (d, JHH ) 2.0,
H3′), 7.41 (m, oH), 7.07 (d, 3JHH ) 2.6, H5′), 7.02 (d, 3JHH ) 2.1, H5),
3
3
7.01 (t, JHH ) 7.2, mH), 6.84 (t, JHH ) 7.1, pH), 6.34 (m, H4 and
H4′), 4.64 (s, CH2), 2.45 (s, COMe), 0.96 (Harris t,30 | JPH + JPH| )
2
4
(28) Siegmann, K.; Pregosin, P. S.; Venanzi, L. M. Organometallics
1989, 8, 2659.
(29) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992,
11, 3920.
(26) Bellachioma, G.; Cardaci, G.; Macchioni, A.; Madami, A. Inorg.
Chem. 1993, 32, 554.
(27) Bellachioma, G.; Cardaci, G.; Gramlich, V.; Macchioni, A.; Pieroni,
F.; Venanzi, L. M. J. Chem. Soc., Dalton Trans. 1998, 947.
(30) Harris, R. K. Can. J. Chem. 1964, 42, 2275.

11028 J. Am. Chem. Soc., Vol. 123, No. 44, 2001
Zuccaccia et al.
6.4, PMe3), 0.34 (q, 2JBH ) 3.8, BMe). 13C NMR (methylene chloride-
Synthesis of trans-[Ru(COMe){(pz2)CH2}(CO)(PMe3)2]PF6 (1f).
cis,trans-RuI(Me)(CO)2(PMe3)2 (100 mg, 0.22 mmol) was dissolved
in 5 mL of methylene chloride. Solid TlPF6 (78 mg, 0.22 mmol) was
added to the solution, and TlI precipitated immediately. The solid was
filtered off, and pz2(CH2) (53 mg, 0.30 mmol) was added to the solution.
After 30 min, n-hexane was added until incipient precipitation of
complex 1f occurred. The solid was washed with cold n-hexane and
dried under vacuum. Yield ) 69%. Anal. calcd (found) for C16H30N4-
F6O2P3Ru: H, 4.89 (4.94); C, 31.08 (31.53); N, 9.06 (9.17). IR (CH2-
Cl2): νCO ) 1953.9 cm-1; νCOMe ) 1606.9 cm-1. 1H NMR (methylene
1
d2, 298 K): δ 258.5 (br, COMe), 205.2 (br, CO), 168.1 (q, JBC
)
49.1, C-ipso), 149.3 (s, C3), 146.1 (s, C3′), 137.1 (s, C5), 135.7 (s,
C5′), 135.3 (s, oC), 126.9 (s, mC), 122.9 (s, pC), 109.1 (s, C4), 108.3
1
(s, C4′), 61.6 (s, CH2), 50.7 (s, COMe), 15.6 (Harris t, | JPC + 3JPC| )
29.4, PMe3), 13.9 (q, 1JBC ) 42.2, BMe). 31P NMR (methylene chloride-
d2, 298 K): δ -3.6 (s). Characterization of 1c: yield ) 35%. Anal.
calcd (found) for C38H53BN4O2P2Ru: H, 6.87 (6.97); C, 59.14 (59.35);
N, 7.26 (7.31). IR (CH2Cl2): νCO ) 1950.0 cm-1; νCOMe ) 1601.4 cm-1
.
1H NMR (methylene chloride-d2, 298 K): δ 8.32 (d, 3JHH ) 2.3, H3),
7.71 (d, 3JHH ) 2.3, H3′), 7.49 (m, oH), 7.03 (d, 3JHH ) 2.6, H5′), 7.03
(t, 3JHH ) 7.2, mH), 6.94 (d, 3JHH ) 2.6, H5), 6.84 (t, 3JHH ) 7.1, pH),
6.39 (m, H4 and H4′), 4.49 (s, CH2), 2.49 (s, COMe), 1.31 (m, CH2-
3
3
chloride-d2, 298 K): δ 8.49 (d, JHH ) 1.6, H3), 8.20 (d, JHH ) 2.2,
H3′), 8.18 (d, 3JHH ) 2.8, H5), 7.88 (d, 3JHH ) 2.7, H5′), 6.59 (m, H4
2
and H4′), 6.21 (s, CH2), 2.55 (s, COMe), 1.09 (Harris t, | JPH
+
2
(3)), 1.03 (m, CH2(1) and CH2(2)), 0.99 (Harris t, | JPH + 4JPH| ) 6.5,
4JPH| ) 6.6, PMe3). 19F NMR (methylene chloride-d2, 298 K): δ -72.46
3
1
PMe3), 0.85 (t, JHH ) 7.3, Me(4)).
(d, JFP ) 711).
Synthesis of trans-[Ru(COMe){(pz2)CH2}(CO)(PMe3)2]BPh3(n-
Hex) (1d). cis,trans-RuI(Me)(CO)2(PMe3)2 (141 mg, 0.31 mmol) and
pz2(CH2) (62.5 mg, 0.42 mmol) were dissolved in 10 mL of CH3NO2.
After a few hours, the carbonyl stretching bands of the starting material
disappeared, and K[BPh3(n-Hex)] (115 mg, 0.31 mmol) was added to
the solution. The solution was filtered, the solvent was removed, and
the residual was dissolved in MeOH. After the addition of H2O, the
final product precipitated; it was washed with cold Et2O and dried.
Yield ) 37%. Anal. calcd (found) for C40H58BN4O2P2Ru: H, 7.30
Acknowledgment. The authors thank Dr. Marco Gobbino
for the synthesis of complexes 1b-e and Professors Tiziana
Beringhelli (University of Milano) and Roberto Gobetto (Uni-
versity of Torino) for helpful discussions and suggestions. This
work was supported by grants from the Ministero dell’ Uni-
versita` e della Ricerca Scientifica e Tecnologica (MURST,
Rome, Italy), Programma di Rilevante Interesse Nazionale,
Cofinanziamento 2000-2001.
(7.24); C, 60.00 (59.78); N, 7.00 (7.12). IR (CH2Cl2, 298 K): νCO
)
1950.0 cm-1; νCOMe ) 1602.6 cm-1. 1H NMR (methylene chloride-d2,
Supporting Information Available: NOE percentages as
a function of mixing time (τm) relative to trans-[Ru(COMe)-
{(pz2)CH2}(CO)(PMe3)2]BPh4 (1a) (2 × 10-1 M) measured in
methylene chloride-d2 at 302 K (Table 1S); NOE percentages
as a function of mixing time (τm) relative to trans-[Ru(COMe)-
{(pz2)CH2}(CO)(PMe3)2]BPh3Me (1b) (4 × 10-2 M) measured
in methylene chloride-d2 at 302 K (Table 2S); dipolar cross
relaxation rate constants as a function of temperature for trans-
[Ru(COMe){(pz2)CH2}(CO)(PMe3)2]BPh3Me (1a) (2 × 10-1
M) measured in methylene chloride-d2 with a mixing time of
0.4 s (Table 3S); trend of % NOE as a function of τ for the
irradiation of H5 proton of the cation (T ) 302 K) for complex
1a (Figure 1S); theoric trend of σIS as a function of rotational
correlation time τc for three pairs of protons 2.5, 3.0, and 3.5 Å
apart (Figure 2S); plot of the best nonlinear least-squares fit of
intramolecular (Figure 3S) and interionic (Figure 4S) σIS as a
function of temperature for complex 1a (pdf). This material is
available free of charge via the Internet at http://pubs.acs.org.
298 K): δ 8.33 (d, 3JHH ) 2.3, H3), 7.71 (d, 3JHH ) 1.5, H3′), 7.48 (m,
3
3
oH), 7.02 (t, JHH ) 7.2, mH), 7.01 (d, JHH ) 2.3, H5′), 6.93 (d,
3JHH ) 2.2, H5), 6.83 (t, 3JHH)7.2, pH), 6.39 (m, H4 and H4′), 4.52 (s,
CH2), 2.49 (s, COMe), 1.28 (m, CH2(2), CH2(4), CH2(5)), 1.04 (m,
2
4
CH2(1) and CH2(3)), 1.00 (Harris t, | JPH + JPH| ) 6.5, PMe3), 0.88
3
(t, JHH ) 6.6, Me(6)).
Synthesis of trans-[Ru(COMe){(pz2)CH2}(CO)(PMe3)2]B(3,5-
(CF3)2(C6H3))4 (1e). cis,trans-RuI(Me)(CO)2(PMe3)2 (150 mg, 0.33
mmol) and pz2(CH2) (63 mg, 0.42 mmol) were dissolved in 10 mL of
MeOH and stirred until the two carbonyl stretching bands of the starting
material disappeared from the IR spectrum. Na[B(3,5-(CF3)2C6H3)4]
(432 mg, 0.48 mmol) was added, and after the addition of H2O, the
final product precipitated. Yield ) 70%. Anal. calcd (found) for
C48H42N4BF24O2P2Ru: H, 3.09 (3.21); C, 43.16 (43.38); N, 4.19 (4.17).
1
IR (CH2Cl2): νCO ) 1953.9 cm-1; νCOMe ) 1606.9 cm-1. H NMR
3
(methylene chloride-d2, 298 K): δ 8.61 (d, JHH ) 2.4, H3), 7.96 (d,
3
3
3JHH ) 2.2, H3′), 7.91 (d, JHH ) 2.8, H5), 7.88 (d, JHH ) 2.7, H5′),
3
7.78 (m, oH), 7.62 (s, pH), 6.67 (dd, JHH ) 2.4, H4), 6.64 (dd,
3JHH ) 2.4, H4′), 5.97 (s, CH2), 2.58 (s, COMe), 1.11 (Harris t,
| JPH + 4JPH| ) 6.4, PMe3). 19F NMR (methylene chloride-d2, 298 K):
2
δ -63.2 (s).
JA015959G