New products in an old reaction: Isomeric products from H2 addition to Vaska's complex and its analogues

Article abstract of DOI:10.1039/a905590h

para-Hydrogen enhanced NMR signals aid detection of minor isomers of complexes IrH₂(L)₂(CO)Cl (L = PPh₃, PMe₃, PPh₂Cl and AsPh₃) containing magnetically inequivalent hydride ligands that ar

Full text of DOI:10.1039/a905590h

New products in an old reaction: isomeric products from H2 addition to
Vaska’s complex and its analogues
Sarah K. Hasnip,a Simon B. Duckett,*a Christopher J. Sleigh,a Diana R. Taylor,b Graham K. Barlowb and
Mike J. Taylorb
a Department of Chemistry, University of York, Heslington, York, UK YO10 5DD. E-mail: sbd3@york.ac.uk
b BP Amoco Chemicals, BP Chemicals Limited, Salt End, Hull, UK HU12 8DS
Received (in Cambridge, UK) 12th July 1999, Accepted 29th July 1999
para-Hydrogen enhanced NMR signals aid detection of
minor isomers of complexes IrH2(L)2(CO)Cl (L = PPh3,
PMe3, PPh2Cl and AsPh3) containing magnetically in-
equivalent hydride ligands that are produced via addition
across the L–Ir–L axis of Ir(L)2(CO)Cl: in the case of L =
PPh3, reaction with CO and H2 is shown to yield the
substitution product IrH2(CO)2(PPh3)Cl which reacts fur-
ther via HCl transfer to form IrH(CO)(PPh3)2Cl2 and
thereby enables the detection of IrH3(CO)2(PPh3).
pre-formed materials and suggests the ratio of 2a to 3a is ≈ 100
if similar enhancements are assumed for each species. There is
no evidence to suggest that these two species interconvert on the
NMR time scale.
In order to test the generality of this reaction pathway we
repeated this procedure with Ir(CO)(L)2Cl [L = PMe3 1b, Fig.
1(b) and L = AsPh3 1c]. In both these cases dihydride products
corresponding to addition over the Cl–Ir–CO and L–Ir–L axes
of trans-IrCl(L)2(CO) are detected. The spectral features of the
PMe3 products 2b and 3b are similar to those of their PPh3
analogues.† However, hydride resonances for both these
species are visible at 333 K for extended periods. The failure to
observe 3b with normal hydrogen suggests that while both 2b
and 3b are accessible by H2 addition to 1b, both are able to
reductively eliminate H2 at 333 K, otherwise 3b would become
a significant reaction product.
When the reaction with L = AsPh3 is monitored with normal
hydrogen, two isomers, cis,trans-IrH2(CO)(AsPh3)2Cl 2c (with
hydride resonances at d 27.11 and 218.91) and cis,cis-
IrH2(CO)(AsPh3)2Cl 3c (hydride d 29.62) are detected in the
ratio 1+2.85, respectively, at 295 K.† After five days the ratio
became 1+0.1, this suggests that addition over the As–Ir–As
It has been shown that the addition of H2 enriched in the para
spin state to a transition metal centre leads to greatly enhanced
hydride signals in associated 1H NMR spectra.1 This phenome-
non is a powerful tool available to characterise minor reaction
2
products such as all-cis Ru(H)2(CO)2(PMe3)2 and investigate
hydrogenation kinetics.3 The addition of H2 to Vaska’s
complex,4 trans-Ir(CO)(PPh3)2Cl 1a, and its analogues has
been the subject of much investigation and it is currently
accepted that, in general, addition proceeds exclusively over the
OC–Ir–Cl axis. However, calculations by Sargent and Hall5
revealed that H2 addition over the OC–Ir–Cl axis rather than the
P–Ir–P axis is favoured by 9.5 kJ mol21 in the case of
Ir(CO)(PMe3)2Cl. Furthermore, calculations indicate that in-
creasing the p-accepting nature of the phosphine will favour H2
addition across the P–Ir–P axis. Here, we describe studies that
test this prediction by monitoring reactions of Ir(L)2(CO)Cl
(L = PPh3, PMe3, PPh2Cl and AsPh3) with para-hydrogen
(p-H2).
It has already been shown that when a solution containing
cis,trans-IrH2(CO)(PPh3)2Cl 2a is examined by 1H NMR
spectroscopy the corresponding hydride resonances are po-
larised (2a, 0.1 mmol dm23, toluene-d8, 343 K and 3 atm p-
H2).6 Under these conditions, H2 addition is reversible, and
exchange leads to hydride signal enhancements that are 11 fold.
At 298 K the hydride resonances of 2a are unaffected by the
presence of p-H2 because exchange is suppressed. However,
when the reaction of 1a with p-H2 is monitored by NMR
spectroscopy, in-situ, the dihydride products are detected as
they form and prior to spin population relaxation. In the
1
corresponding H spectrum polarised hydride resonances are
detected for 2a, and a second previously unreported species, as
shown in Fig. 1(a). The enhanced resonance of this minor
product is only visible when 1a is reacting with p-H2 and the
associated signals decay away rapidly. Furthermore, the extra
hydride resonance arises from a second order spin system with
magnetically distinct hydrides.6,7 The 31P and 13C chemical
shifts and the multiplicities of the hydride, phosphine and
carbonyl resonances of this product, located via a series of 2D
NMR experiments, are consistent with their origin in cis,cis-
IrH2(CO)(PPh3)2Cl 3a. This product is formed by H2 addition
over the P–Ir–P axis of 1a, as shown in Scheme 1.† At 295 K,
the difference in hydride signal intensities of 2a and 3a was
determined by the application of a pulse sequence involving a
90° 1H pulse, a pulsed field gradient, a 100 ms delay, and a 45°
1H read pulse. This procedure enables the instantaneous
observation of the ratio of 2a+3a by suppressing signals from
Fig. 1(a) 1H NMR spectrum (400 MHz, 295 K) of a 0.1 mM solution of 1a
in benzene-d6 under 3 atm of p-H2. The weak second order resonance arises
from cis,cis-IrH2(CO)(PPh3)2Cl 3a. (b) 1H NMR spectrum (400 MHz, 333
K) of a 0.1 mM solution of 1b in benzene-d6 under 3 atm of p-H2 with
1
resonances due to 2b and 3b indicated. (c) H NMR spectrum (400 MHz,
333 K) of a 0.1 mM solution of 2a in benzene-d6 in the presence of a four-
fold excess of PPh2Cl under 3 atm of p-H2 showing resonances due to 2a,
2d, 2e and 3d.
Chem. Commun., 1999, 1717–1718
1717

and 5c and the fac-trans isomer of IrH3(CO)2(PPh3) 7a are
detected as p-H2 enhanced products (Scheme 1).9 These
observations are consistent with HCl transfer from IrH2-
(CO)2Cl(PPh3) to 1a, followed by H2 addition to yield the
dicarbonyl trihydride product.
Here we have demonstrated that H2 addition to a series of
iridium(i) carbonyl complexes based on Ir(CO)(PPh3)2Cl
involves a minor reaction pathway where addition proceeds
across the P–Ir–P axis. We also show that in the presence of
mixtures of CO and H2, IrH2(CO)(PPh3)2Cl and the phosphine
substitution product IrH2(CO)2Cl(L) are detected in addition to
HCl transfer products and a series of trihydride complexes.
Financial support from the EPSRC (Spectrometer, C. J. S.
and S. K. H.), BP Chemicals (CASE award S. K. H.), the Royal
Society, NATO and Bruker UK, and discussions with Professor
R. N. Perutz, and Dr R. J. Mawby are gratefully acknowl-
edged.
Notes and references
† Selected spectroscopic data at 400.13 MHz (1H) and 161.45 MHz (31P)
1
and 100.2 MHz (13C) in benzene-d6 (couplings Hz): 2a: H, d 26.64 {H,
J(PH) 18.8, J(HH) 25.7}, 217.48 {H, J(PH) 13.7, J(HH) 25.7}; 31P, d 8.4.
1
2b: H, d 27.80 {H, J(PH) 20.8, J(HH) 25.2}, 218.90 {H, J(PH) 14.7,
J(HH) 25.2}, 31P, d 241.1; 13C, d 183.0 {CO}. 2c (296 K): 1H, d 27.11
{H, J(COH) 44.5, J(HH) 25.0}, 218.91 {H, J(CH) 3.7, J(HH) 25.0}; 13C,
d 177.2 {CO, s}. 2d 1H, d 26.81 {H, J(PH) 14.4, J(PH) 14.7, J(HH) 24.7},
216.99 {H, J(PH) 17.3, J(PH) 17.3, J(HH) 24.7}, 31P, d 9.0 {PPh3, J(PP)
398.6}, 66.2 {PPh2Cl, J(PP) 398.6}, 13C, d 176.3 {CO, J(PC) 7.9}. 2e 1H,
d 27.24 {H, J(PH) 20.3, J(COH) 44.0, J(HH) 25.0}, 216.73 {H, J(PH)
15.0, J (COH) 3.5, J(HH) 25.0}; 31P, d 65.1; 13C, d 174.8 {CO, J(PC) 7.9}.
3a (295 K): 1H, d 28.10 {m, second order}; 31P, d 25.9; 13C, d 167.1 {CO}.
Scheme 1 Species observed in the reactions of a series of chlorocarbonyl-
bis(phosphine)iridium(i) and dihydridochlorocarbonylbis(phosphine)-
iridium-(iii) complexes under p-H2.
axis is kinetically preferred while addition over the OC–Ir–Cl
axis leads to the thermodynamic product. Product 2c was
characterised by 2D NMR methods at 333 K where the rate of
H2 exchange is fast and the p-H2 signal enhancements are long-
lived.† The structure of the cis,cis product 3c was confirmed by
1
3b (333 K): H d 28.15 {m, second order}; 31P d 27.98, 13C, d 173.37
{CO}. 3c (296 K): 1H, d 29.62 {H, J(COH) 6.4}; 13C d 165.7 {CO, s}. 3d:
1H, d 28.13 {H, J(PH) 222.4, J(PH) 15.0, J(HH) 21.8}, 28.34 {H, J(PH)
160.8, J(PH) 15.9, J(HH) 21.8}; 31P, d 24.0 {PPh3, J(PP) 28.5}, 65.2
1
13C labelling experiments, H integral measurements, and the
presence of two n(IrH) modes at 2083 and 2115 cm21 in the
corresponding IR spectrum.
1
{PPh2Cl, J(PP) 28.5}; 13C, d 165.3 {CO, J(PC) 6}. 3e: H, d 28.37 {m,
second order}; 31P, d 62.9. 4a (338 K): 1H, d 211.70 {H, J(COH) 3.4,
J(HH) 25.9}, 218.50 {H, J(CH) 4.7, J(HH) 25.9}; 13C, d 170.3 {CO, s}.
4b: 1H, d 27.52 {H, J(PH) 16, J(PH) 23, J(CH) 48, J(HH) 22.9}, 29.50
{H, J(PH) 202.2, J(PH) 23.0, J(CH) 4.4, J(HH) 22.9}; 31P, d 65.3 {PPh2Cl,
br}; 13C, d 174 {CO}. 5a: 1H, d 27.42 {H, J(PH) 14.5, J(COH) 56.9 and 7,
J(HH) 27}, 28.37 {H, J(PH) 162.0, J(COH) 5, J(HH) 27}; 31P, d 25.7;
13C, d 169.8 {CO, J(PC) 8}, 161.7 {CO, J(PC) 123}. 5b: 1H, d 27.97 {H,
J(PH) 17.4, J(COH) 45.5 and 6, J(HH) 25}, 216.51 {H, J(PH) 16.0,
J(COH) 2.8, J(HH) 25}; 31P, d 5.2 {J(PC) 123.4}; 13C, d 170.8 {CO, J(PC)
8}, 166.1 {CO, J(PC) 123}. 5c: 1H, d 28.09 {H, J(PH) 18.8, J(COH) 57.2
and 6}. 6a (333 K): 1H, d 214.60 {H, J(PH) 11.6, J(COH) 5.2}; 31P, d 22.9
{J(PC) 7.9}; 13C, d 163.2 {CO}. 6b: 1H, d 214.22 {H, td, J(PH) 12.3,
J(COH) 5.0}, 31P, d 23.0 {P, PPh3, J(PP) 456}, 55.7 {P, PPh2Cl, J(PP)
456}, 13C, d 162.3 {CO}. 7a: 1H, d 28.8 {H, J(PH) 136.9, J(HH) 2.7}, 29.7
{H, J(PH) 121.6, J(HH) 22.4}: dp 43.2.
Interestingly, a third p-H2 enhanced isomer, 4a, was detected
at 338 K in the H NMR spectrum that was not previously
1
visible [Fig. 1(c)]. The chemical shifts of the hydride reso-
nances of this product, d 211.70 and 218.50, suggest hydride
locations trans to CO or arsine, and trans to chloride,
respectively. NOE measurements revealed that the hydride
ligand of 4a which resonates at d 211.70 is close in space to a
single set of ortho-phenyl protons (d 7.65) while that which is
trans to chloride is adjacent to two different sets (d 7.65 and
7.60). 4a therefore contains two inequivalent arsine ligands.
When a 13CO labelled sample was examined, the d 211.70
1
signal showed a small H–13CO coupling of 3.4 Hz indicating
that the hydride is trans to arsine rather than CO. In the NOE
experiment, no interconversion between 2c, 3c and 4a was
observed which suggests that the most probable route to
formation of the minor isomer 4a is H2 addition over the OC–Ir–
As axis of the cis isomer of Ir(CO)(AsPh3)2Cl (Scheme 1).
A sample containing a four-fold excess of PPh2Cl relative to
2a was examined to test the p-accepting role of the phosphine
[Fig. 1(c)]. While this spectrum contains no resonances that can
be assigned to trisphosphine species, signals corresponding to
the hydride resonances of mono- and bis-phosphine exchange
products (2d, 2e, Scheme 1) containing cis hydrides and trans
phosphines were readily assigned. Furthermore, resonances for
the cis,cis isomers 3d and 3e were present in significantly higher
proportions than those seen in the reaction with 1a described
above. Significantly, by virtue of the mixed phosphines, 3d
contains inequivalent hydride ligands which resonate at d
28.13 and 28.34 with the resonances having doublet of doublet
of doublet multiplicities consistent with trans and cis phosphine
connections. This product is formed by H2 addition across the
P–Ir–P axis of Ir(CO)(PPh3)(PPh2Cl)Cl. Weak signals due to 4b
were also present.† Ultimately the signals of all these species
disappear, and the major hydride resonances are associated with
the bisphosphine carbonyl dichloride monohydride complexes
6a and 6b.8 Complex 6a is also observed in the reaction
chemistry of 1a when both CO and p-H2 are present. Under
these conditions, three isomers of IrH2(CO)2(PPh3)Cl, 5a, 5b
1 C. R. Bowers and D. P. Weitekamp, J. Am. Chem. Soc., 1987, 109, 5541;
R. Eisenberg, Acc. Chem. Res., 1991, 24, 110; J. Natterer and J. Bargon,
Prog. Nucl. Magn. Reson. Spectrosc., 1997, 31, 293; C. J. Sleigh and
S. B. Duckett. Prog. Nucl. Magn. Reson. Spectrosc., 1999, 34, 71.
2 S. B. Duckett, C. L. Newell and R. Eisenberg, J. Am. Chem. Soc., 1994,
116, 10548; S. B. Duckett and R. Eisenberg, ibid., 1993, 115, 5292; S. B.
Duckett, R. J. Mawby and M. G. Partridge, Chem. Commun., 1996, 383;
P. D. Morran, S. A. Colebrooke, S. B. Duckett, J. A. B. Lohmann and R.
Eisenberg, J. Chem. Soc., Dalton. Trans., 1998, 3363.
3 P. Hubler, R. Giernoth, G. Kummerle and J. Bargon, J. Am. Chem. Soc.,
1999, 121, 5311.
4 L. Vaska and M. F. Wernke, Trans. N. Y. Acad. Sci., 1971, 33, 80; P. B.
Chock and J. Halpern, J. Am. Chem. Soc., 1966, 88, 3511; R. Ugo, A.
Pasini and S. Cenini, ibid., 1972, 94, 7364.
5 A. L. Sargent and M. B. Hall, Inorg. Chem., 1992, 31, 317.
6 B. A. Messerle, C. J. Sleigh, M. G. Partridge and S. B. Duckett, J. Chem.
Soc., Dalton Trans., 1999, 1429. C. J. Sleigh, S. B. Duckett, and B. A.
Messerle, Chem. Commun., 1996, 2395; S. B. Duckett, C. L. Newell and
R. Eisenberg, J. Am. Chem. Soc., 1993, 115, 1156.
7 S. B. Duckett, G. K. Barlow, M. G. Partridge and B. A. Messerle,
J. Chem. Soc., Dalton Trans., 1995, 3427.
8 R. A. Vanderpool and H. B. Abrahamson, Inorg. Chem., 1985, 24,
2985.
9 S. K. Hasnip, S. B. Duckett, D. R. Taylor and M. J. Taylor, Chem.
Commun., 1998, 923.
Communication 9/05590H
1718
Chem. Commun., 1999, 1717–1718