A remarkable temperature-dependent, accidental degeneracy of 31P NMR chemical shifts in Ru(II) diphosphine/diimine complexes

Article abstract of DOI:10.1039/b103473c

Several cis-RuX₂((R)-BINAP)(diimine) complexes have been prepared, and many of these exhibit an unusual temperature-dependent, accidental degeneracy of the ³¹P shifts in their solution NMR spectra.

Full text of DOI:10.1039/b103473c

A remarkable temperature-dependent, accidental degeneracy of 31P
NMR chemical shifts in Ru(II) diphosphine/diimine complexes
Paul W. Cyr, Brian O. Patrick and Brian R. James*
Department of Chemistry, The University of British Columbia, Vancouver, British Columbia V6T 1Z1,
Canada. E-mail: brj@chem.ubc.ca
Received (in Cambridge, UK) 18th April 2001, Accepted 28th June 2001
First published as an Advance Article on the web 2nd August 2001
Several cis-RuX2((R)-BINAP)(diimine) complexes have
been prepared, and many of these exhibit an unusual
temperature-dependent, accidental degeneracy of the 31P
shifts in their solution NMR spectra.
expected3b l-conformation of the (R)-BINAP chelate ring, and
only the R,L diastereomer is seen (where L refers to the
chirality about the Ru atom; a parallel structural refinement was
carried out for both L and D isomers, but only the L structure
refined to convergence). Further, the solution 31P spectra reveal
the presence of only one set of signals at any given temperature,
while diastereomeric Ru(II) BINAP complexes have been
differentiated by 31P NMR data.6b,9 Cis-3a is seen solely as the
R,D diastereomer; whether the halide metathesis reaction
occurs with retention or inversion of stereochemistry at Ru
remains to be established.10 The solution 31P NMR behaviour is
There is on-going research in this laboratory on Ru(II
)
complexes possessing one chelating, ditertiary phosphine (P–P)
per Ru atom, because of their proven ability as hydrogenation
catalysts,1 especially for enantioselective catalysis when P–P is
a chiral diphosphine ligand;2 of such ligands, the C2-symmetric
BINAP (2,2A-bis(diphenylphosphino)-1,1A-binaphthyl) and its
derivatives have been employed very successfully in asym-
metric catalysis.3,4 We have also studied Ru(II) complexes with
mixed P- and N-donor ligand sets, where the N-donor is either
incorporated into the phosphine ligand,5 or with separate P- and
N-donor ligands,6 and the use of Ru(II) systems with tetra-
dentate ‘P2N2’ ligands for catalytic hydrogenation7 and epox-
idation8 reactions has been explored recently. Of particular
note, spectacular success has been achieved in the use of chiral
Ru(II) complexes with phosphine (either mono- or bidentate)
and diamine ligands in catalytic enantioselective hydro-
genation.4 We report here a warning concerning interpretation
of the 31P NMR spectra of such systems: ‘apparent complica-
tions’ can result from a remarkable temperature-dependent
degeneracy of 31P NMR chemical shifts. More specifically,
some systems, where two P atoms are trans to different ligands
(an N-donor, and a halogen), generate a singlet 31P{1H} signal
resulting from an authentic, accidental degeneracy.
Complexes of formula cis-RuCl2((R)-BINAP)(L2), where L2
= a bidentate (N–N) ligand,† were prepared by reaction of L2
with RuCl2((R)-BINAP)(PPh3),‡ and halide metathesis using
NaX (X = Br, I) in acetone afforded the corresponding cis-
RuX2((R)-BINAP)(L2) complexes. The structure of cis-
RuBr2((R)-BINAP)(bipy) (1b) is shown in Fig. 1.§ The
crystallographic and solution 31P{1H} NMR data (Table 1)
indicate that these complexes are formed stereoselectively. For
example, the pseudo-octahedral structure of 1b has the
Fig. 1 ORTEP representation (50% probability ellipsoids) of cis-RuBr2((R)-
BINAP)(bipy) (1b). Solvates and H-atoms have been omitted for clarity.
Selected bond distances (Å) and angles (°): Ru–Br(1), 2.6175(6); Ru–Br(2),
2.5476(7); Ru–P(1), 2.305(1); Ru–P(2), 2.317(2); Ru–N(1), 2.128(4); Ru–
N(2), 2.091(4); Br(1)–Ru–Br(2), 89.42(2); P(1)–Ru–P(2), 93.13(5); N(1)–
Ru–N(2), 77.7(2). Only the ipso carbon atoms (C(11), C(17), C(43) and
C(49)) of the phenyl groups are shown.
Table 1 31P{1H} NMR spectroscopic data for cis-RuX2((R)-BINAP)(L2)a
dA, dB [2JAB/Hz]b
Complex
C6D6
CDCl3
CD2Cl2
cis-RuCl2((R)-BINAP)(bipy) 1a
cis-RuBr2((R)-BINAP)(bipy) 1b
cis-RuI2((R)-BINAP)(bipy) 1c
cis-RuCl2((R)-BINAP)(dmbipy) 2
cis-RuCl2((R)-BINAP)(phen) 3a
cis-RuBr2((R)-BINAP)(phen) 3b
cis-RuI2((R)-BINAP)(phen) 3c
cis-RuCl2((R)-BINAP)(batho) 4
cis-RuCl2((R)-BINAP)(bpa) 5
48.0, 46.0 [32.3]
47.8, 45.2 [32.1]
49.4, 42.5 [30.0]
47.5, 47.2 [33.4]g
48.4, 46.2 [34.0]
48.7, 45.7 [32.8]
50.8, 42.6 [29.7]
47.8, 45.5 [33.4]
50.7, 47.1 [33.3]
47.1, 46.6 [33.5]c
47.3, 46.7 [33.3]e
48.8, 44.2 [30.6]
48.3, 47.4 [34.6]
47.2h
48.0, 47.0 [33.1]j
49.6, 44.6 [31.3]
47.6, 46.9 [33.6]
50.0, 47.6 [33.5]
47.2d
47.2, 46.9 [33.7]f
48.4, 44.6 [31.7]
48.0, 47.5 [34.4]
47.9, 47.5 [34.4]i
47.5, 46.9 [34.0]k
49.3, 45.2 [31.5]
47.4, 47.1 [33.7]l
49.5, 48.0 [33.6]
a Satisfactory elemental analyses were obtained for all the complexes listed.10 b At room temperature ( ~ 20 °C) except where noted (121 MHz spectrometer
frequency). c At 50 °C: d 47.2 (s). d At 230 °C: dA = 47.8, dB = 47.4 [33.8]; at 35 °C: dA = 47.4, dB = 47.0 [34.5]. e At 260 °C: d 47.3 (s). f At 10 °C:
d 47.2 (s); at 220 °C: dA = 47.6, dB = 47.2 [32.9]. g At 25 °C: d 47.3 (s); at 40 °C: dA = 47.4, dB = 47.0 [33.6]. h At 230 °C: dA = 47.8, dB = 47.2 [34.4];
at 60 °C: dA = 47.2, dB = 46.9 [33.9]. i At 0 °C: d 47.9 (s); at 290 °C: dA = 48.4, dB = 48.1 [34.9]. j At 240 °C: d 47.7 (s). k At 210 °C: d 47.6 (s); at
220 °C: dA = 47.6, dB = 47.2 [32.9]. l At 25 °C: d 47.2 (s).
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Chem. Commun., 2001, 1570–1571
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103473c

remarkable in that the expected AB pattern is sometimes ‘lost’
(Table 1). Thus, 3a in CDCl3 shows a singlet (d 47.2) in its room
dependent degeneracy of 31P NMR shifts. These data, partic-
ularly the observance of the degeneracy at room temperature in
common NMR solvents, indicate that caution should be taken in
the analysis of 31P NMR data, especially for the widely studied
chiral P–P systems, where such spectra remain a major
characterization technique. The ‘impossible’ observation of a
singlet NMR signal must be interrogated further by variation of
temperature and variation of solvent.
temperature spectrum, while in CD2Cl2 an AB pattern (dA
=
47.9, dB = 47.5, 2JAB = 34.4 Hz) is present; 1a demonstrates
the opposite behaviour (an AB pattern in the CDCl3 spectrum
and a singlet in CD2Cl2).
Variable temperature (VT) NMR studies, conducted on all
the complexes, are exemplified by the data shown in Fig. 2.
From 0 to 10 °C, the spectrum of 1b consists of a sharp singlet,
while an AB pattern is observed either side of this range with
increasing separation of the signals; there is clearly no dynamic
exchange process between, for example, two species giving a
time-averaged singlet. A temperature-dependent, accidental
degeneracy of the two signals of the 4-line AB pattern gives rise
to the singlet. At a specific solvent and temperature combina-
tion, an A2 pattern is observed, the 31P nuclei having become
isochronous. This degeneracy occurs in at least one solvent
studied (usually chlorinated) for most of the cis-RuX2((R)-
BINAP)(L2) complexes, while cis-RuCl2((R)-BINAP)(dmbipy)
(2) is the only complex exhibiting degeneracy in C6D6 and not
in CDCl3 or CD2Cl2. The dibromo complexes 1b and 3b exhibit
degeneracy at lower temperatures than the corresponding
dichloro analogues (1a and 3a); the diiodo complexes (1c and
3c) show no degeneracy from 290 to 60 °C. Changing from the
planar bipy- or phen-based ligand systems (1–4) to that with
bis(o-pyridyl)amine (5) sufficiently separates the two 31P shifts
that degeneracy is not seen (Table 1). The related cis-
RuCl2(DPPB)(L2) complexes (DPPB = 1,4-bis(diphenylphos-
phino)butane) possess well separated (Dd > 10) signals in their
31P NMR spectra.6c
We thank Colonial Metals Inc. for a loan of RuCl3·3H2O, Dr
S. King (formerly of Merck Research) for a gift of (R)-BINAP,
and NSERC of Canada for financial support.
Notes and references
† Abbreviations used are: bipy (2,2A-bipyridine), dmbipy (4,4A-dimethyl-
2,2A-bipyridine), phen (1,10-phenanthroline), batho (4,7-diphenyl-
1,10-phenanthroline, or bathophenanthroline) and bpa (bis(o-pyridyl)-
amine).
‡ A representative synthesis is as follows: RuCl2((R)-BINAP)(PPh3)1b
(0.19 g, 0.18 mmol) and bipy (0.37 g, 0.24 mmol) were dissolved in 7 mL
of C6H6 and the solution was refluxed for 3 h. The orange product (1a, L2
= bipy), precipitated by the addition of 30 mL hexanes, was washed with
hexanes and dried in vacuo. Yield: 0.11 g (65%). Anal. Calc. for
C54H40N2Cl2P2Ru: C, 68.21; H, 4.24; N, 2.95. Found: C, 68.24; H, 4.23; N,
3.01%.
§ Crystal data for 1b: C54H40N2Br2P2Ru·3C6D6, M = 1292.09, mono-
clinic, space group P21, a = 13.3564(7), b = 14.2879(7), c = 15.4367(9)
Å, b = 98.448(4)°, V = 2913.9(2) Å3, Z = 2, Dc = 1.473 g cm23, m =
17.45 cm21, T = 2100 °C, 25153 reflections measured, 6590 unique (Rint
= 0.089), R (Rw) = 0.079 (0.092) on all data. X-ray crystal data are also
available for 3a and 5.
CCDC reference number 164469. See http://www.rsc.org/suppdata/cc/
b1/b103473c/ for crystallographic data in CIF or other electronic format.
1 For example: (a) B. R. James, R. S. MacMillan, R. H. Morris and
D. K. W. Wang, in Transition Metal Hydrides, ed. R. Bau, ACS
Symposium Series 167, Washington, DC, 1978, p. 122; (b) A. M. Joshi,
I. S. Thorburn, S. J. Rettig and B. R. James, Inorg. Chim. Acta, 1992,
198–200, 283; (c) D. E. Fogg, B. R. James and M. Kilner, Inorg. Chim.
Acta, 1994, 222, 85.
2 (a) K. S. MacFarlane, I. S. Thorburn, P. W. Cyr, D. E. K.-Y. Chau, S. J.
Rettig and B. R. James, Inorg. Chim. Acta, 1998, 270, 130; (b) K. S.
MacFarlane, S. J. Rettig, Z. Liu and B. R. James, J. Organomet. Chem.,
1998, 557, 213.
3 For example: (a) R. Noyori and H. Takaya, Acc. Chem. Res., 1990, 23,
345; (b) R. Noyori, CHEMTECH, 1992, 22, 360; (c) R. Noyori, Acta
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4 (a) R. Noyori and T. Ohkuma, Angew. Chem., Int. Ed., 2001, 40, 40; (b)
K. Abdur-Rashid, A. J. Lough and R. H. Morris, Organometallics, 2001,
20, 1047.
Fig. 2 VT 31P{1H} NMR spectra (CD2Cl2, 121 MHz) of cis-RuBr2((R)-
BINAP)(bipy) (1b) from 30 to 230 °C. Spectra are plotted in 10 °C
increments.
5 For example: (a) C. R. S. M. Hampton, I. R. Butler, W. R. Cullen, B. R.
James, J.-P. Charland and J. Simpson, Inorg. Chem., 1992, 31, 5509; (b)
E. S. F. Ma, S. J. Rettig and B. R. James, Chem. Commun., 1999, 2463;
(c) R. P. Schutte, S. J. Rettig, A. M. Joshi and B. R. James, Inorg. Chem.,
1997, 36, 5809; (d) N. D. Jones, K. S. MacFarlane, M. B. Smith, R. P.
Schutte, S. J. Rettig and B. R. James, Inorg. Chem., 1999, 38, 3956.
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10 P. W. Cyr, PhD Dissertation, University of British Columbia,
Vancouver, 2001.
Such accidental degeneracy is likely involved in some
‘anomalies’ in earlier work from this laboratory. Within the
L(DPPB)Ru(m-Cl)3RuCl(DPPB) complexes (L = nitrile), a
31P{1H} singlet, rather than the expected AB (or AX) pattern, is
seen for the two P atoms at Ru at 20 °C in CD2Cl2 (i.e. a singlet
and 2 doublets are observed), while the expected 4 doublets are
seen in C6D6 or CDCl3;6b at 240 °C in CD2Cl2 the 2 sets of AB
patterns are seen.11 When L is Me2S, the AB pattern is seen at
20 °C in C6D6, but not in CDCl3, while the reverse holds true
when L is tetrahydrothiophene, although VT NMR experiments
were not performed.12
The temperature-dependence of 31P NMR shifts is well
documented, and indeed has been used for measuring sample
temperature in VT work; e.g. the dP values for PPh3 and ONPPh3
change linearly with temperature ( ~ 1.3 Hz °C).13 Further, the
temperature-dependence of the dP values for the dimetallic,
mixed-halide ClPd(m-DPPM)2PdI species (DPPM = bis(diphe-
nylphosphino)methane) formed in situ varies with solvent, and
the A2B2 pattern observed in CDCl3 at 220 °C ‘collapses’ to a
singlet at 35 °C, and reemerges above 45 °C,14 behaviour
similar to that of our Ru complexes.
11 D. E. Fogg, PhD Dissertation, Univeristy of British Columbia,
Vancouver, 1994.
12 K. S. MacFarlane, A. M. Joshi, S. J. Rettig and B. R. James, Inorg.
Chem., 1996, 35, 7304.
To our knowledge, the cis-RuX2((R)-BINAP)(L2) complexes
are the first isolated complexes to exhibit temperature-
13 F. L. Dickert and S. W. Hellmann, Anal. Chem., 1980, 52, 966.
14 C. T. Hunt and A. L. Balch, Inorg. Chem., 1982, 21, 1641.
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