Preparation and solution behavior of diphosphinepalladium nitrite complexes

Article abstract of DOI:10.1016/S0277-5387(00)00407-1

Diphosphinepalladium nitrite complexes of the type [Pd(NO₂)₂L₂] (L₂ = dppm, dppe, dppp) have been prepared by treatment of [PdCl₂(CH₃CN)₂] with silver nitrite, followed by addition of the appropriate diphosphine. The nitrite complexes are fluxional in solution at 298 K, but on cooling to 220 K signals due to three species could be detected. These have been assigned to the N-bonded, the O-bonded, and the mixed coordination compounds. Treatment of [PdCl₂(dppe)] with two equivalents of TlPF₆, followed by addition of Na¹⁵NO₂, produced [Pd(¹⁵NO₂)₂(dppe)], whose ³¹P NMR spectrum allowed identification of the three isomers. The analogous platinum complex [Pt(NO₂)₂(dppe)] exists as a single isomer, which is static at 298 K, and a ¹⁵NO₂-labeling experiment indicated that it contains N-bonded nitrite ligands. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(00)00407-1

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 1329–1332
Preparation and solution behavior of diphosphinepalladium nitrite
complexes
Jennifer A. Moser, Robert A. Stockland Jr., Gordon K. Anderson *
Department of Chemistry, Uni6ersity of Missouri - St. Louis, 8001 Natural Bridge Road, St. Louis, MI 63121, USA
Received 29 November 1999; accepted 3 April 2000
Abstract
Diphosphinepalladium nitrite complexes of the type [Pd(NO2)2L2] (L2=dppm, dppe, dppp) have been prepared by treatment
of [PdCl2(CH3CN)2] with silver nitrite, followed by addition of the appropriate diphosphine. The nitrite complexes are fluxional
in solution at 298 K, but on cooling to 220 K signals due to three species could be detected. These have been assigned to the
N-bonded, the O-bonded, and the mixed coordination compounds. Treatment of [PdCl2(dppe)] with two equivalents of TlPF6,
followed by addition of Na15NO2, produced [Pd(15NO2)2(dppe)], whose 31P NMR spectrum allowed identification of the three
isomers. The analogous platinum complex [Pt(NO2)2(dppe)] exists as a single isomer, which is static at 298 K, and a
15NO2-labeling experiment indicated that it contains N-bonded nitrite ligands. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Palladium; Platinum; Nitrite; Diphosphines; Isomerism; Fluxionality
1. Introduction
Reversible nitro–nitrito linkage isomerization at a
ruthenium center may be redox-induced [7–9]. For the
chromium complex [Cr(h5-C5H5)(NO)2(NO2)], a mix-
ture of linkage isomers was found, the nitrito form
predominating, but exchange between the two forms
was slow on the NMR timescale [10].
Palladium nitrite complexes, in particular, exhibit
reactivity in a number of areas, including the oxidation
of alkenes to ketones [11], in carboxylation reactions
[12], and in oxygen transfer reactions [13]. Oxygen
transfer reactions catalyzed by nickel nitrite complexes
have been suggested to involve intermediate O-bonded
nitrite ligands [14]. Interconversion between N- and
O-bonded nitrites may be important in many of these
reactions, but such rearrangements are typically slow,
at least under ambient conditions.
We have prepared a series of palladium nitrite com-
plexes of the type [Pd(NO2)2L2] (L2=dppm, dppe,
dppp), which exhibit broad 31P NMR resonances at
ambient temperature. This behavior suggests that they
are fluxional at this temperature and that the energy
barrier between the different forms is quite low. We
have employed variable temperature NMR spec-
troscopy, as well as 15N-labeling experiments, in order
to establish the nature of this behavior. The results of
these studies will be presented here.
The phenomenon of linkage isomerism, which is
exhibited by ambidentate ligands such as thiocyanate,
nitrite, and dimethylsulfoxide, has been known for
many years, and it is discussed in most inorganic text-
books. The thiocyanate ligand, for example, usually
acts as an S-donor towards soft metal centers, but as a
N-donor towards smaller, hard metals. Steric effects
may also be important, particularly for the bulkier
dimethylsulfoxide. Although one form is often consid-
erably more thermodynamically stable than the other,
rearrangement of the kinetic product into the thermo-
dynamic one may be slow. Such is the case for the
classic example of [Co(NH3)5(NO2)]Cl2 [1], where con-
version of the nitrito to the corresponding nitro com-
plex takes place over several days at ambient
temperature. Nitrito to nitro isomerization of octahe-
dral cobalt(III) complexes may be base-catalyzed [2],
whereas the reverse process has been shown to be
catalyzed by acid [3]. Isomerization may proceed in
liquid ammonia [4] or even in the solid state [5,6].
* Corresponding author. Tel.: +1-314-516-5437; fax: +1-314-516-
5342.
E-mail address: ganderson@umsl.edu (G.K. Anderson).
0277-5387/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(00)00407-1

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J.A. Moser et al. / Polyhedron 19 (2000) 1329–1332
Table 1
31P{1H} NMR parameters for [Pd(NO2)2L2], [Pd(NO2)(ONO)L2] and [Pd(ONO)2L2] a
L2
[Pd(NO2)2L2]
lP
[Pd(NO2)(ONO)L2]
[Pd(ONO)2L2]
lP
Isomer ratio
lP
2J(P,P)
90
dppm
dppe
dppp
−43.5s
53.6s
−45.6d
−44.4d
54.4d
57.4d
−0.3d
9.0d
−46.2 s
59.3 s
1.00:0.49:0.06
1.00:0.60:0.05
1.00:0.31:B0.05
19
1.5s
44
not observed
a Recorded for CDCl3 solutions at 220 K. Coupling constants are in hertz.
2. Results and discussion
(NO2)(dppe)] has been reported, but no 31P NMR data
were provided [11].
Treatment of [PdCl2(CH3CN)2] with excess silver ni-
trite in CH3CN solution produced [Pd(NO2)2-
(CH3CN)2], along with a precipitate of AgCl [15–18].
Treatment of the platinum complex [PtCl2(dppe)]
with AgNO2 resulted in the formation of the expected
[Pt(NO2)2(dppe)], which is static in solution. The reac-
Following filtration, addition of one molar equivalent
of the appropriate diphosphine gave a complex of the
type [Pd(NO2)2L2] (L2=dppm, dppe, dppp). Each of
these complexes contains two monodentate nitrite lig-
ands, which are forced to adopt a cis configuration due
to the presence of the chelate ring. In each case, when
tion takes place in
a
stepwise fashion, with
1
[PtCl(NO2)(dppe)] (lP 28.2 br d, J(Pt,P)=2923 Hz,
2J(P,P)=8 Hz; lP 37.8 d, J(Pt,P)=3707 Hz) being
1
formed when one molar equivalent of AgNO2 was
added. Addition of a second equivalent of AgNO2
resulted in complete conversion to [Pt(NO2)2(dppe)] (lP
1
1
the H NMR spectrum was recorded in CDCl3 solu-
27.9, J(Pt,P)=3052 Hz).
tion, the resonances expected for the diphosphine lig-
and were observed, but no resonances could be detected
in the 31P{1H} NMR spectrum at ambient temperature.
Upon cooling to 220 K, however, 31P signals were
observed in each case (Table 1). The 31P{1H} NMR
spectrum of [Pd(NO2)2(dppm)] at 220 K is shown in
Fig. 1. It contains signals due to three species; two of
these must contain equivalent P atoms and give rise to
singlet resonances, whereas the third has non-equivalent
P atoms and produces a pair of doublets (AB quartet).
When the 31P{1H} NMR spectrum was recorded in
CD2Cl2 solution at temperatures between 180 and 250
K, there was no discernible change in the relative
concentrations of the three components. We propose
that the three sets of resonances are due to the linkage
isomers shown in Fig. 2.
In order to assign the linkage isomers, we carried out
15N-labeling experiments. When up to ten equivalents
of Na15NO2 was added to a CH3CN solution of
[PtCl2(dppe)], mixtures of [PtCl(15NO2)(dppe)] and
[PtCl2(dppe)] only were obtained. In order to ensure
complete conversion to the bis(nitrite)platinum species,
[PtCl2(dppe)] was first treated with TlPF6 in acetone,
followed by addition of two equivalents of Na15NO2.
Several palladium nitrite complexes containing
monodentate phosphines have been prepared [19,20].
Using 15N-labeling experiments, the nitrite ligand was
shown to be coordinated through nitrogen in each case,
but no fluxional behavior was reported. The complex
trans-[Pd(NO2)2(PMePh2)2] was prepared by treatment
of K2[Pd(NO2)4] with PMePh2 [21], and we have pre-
pared this complex by an alternative route, namely,
treatment of [PdCl2(PMePh2)2] with silver nitrite. We
have found that trans-[Pd(NO2)2(PMePh2)2] is not flux-
ional in solution, as shown by sharp resonances in its
31P NMR spectrum over the temperature range 220–
298 K. The diphosphinepalladium complex [PdPh-
Fig. 1. 31P{1H} NMR spectrum of [Pd(NO2)2(dppm)] (a),
[Pd(NO2)(ONO)(dppm)] (b) and [Pd(ONO)2(dppm)] (c) recorded in
CDCl3 solution at 220 K.
Fig. 2. Equilibria involving N- and O-bonded nitrite complexes.

J.A. Moser et al. / Polyhedron 19 (2000) 1329–1332
1331
This generated [Pt(15NO2)2(dppe)] quantitatively. Its 31P
NMR spectrum contained a central doublet at 27.9
ppm (2J(P,N)=65 Hz) and, consistently, the 15N NMR
spectrum exhibited a doublet at 464 ppm with a separa-
tion of 65 Hz. These data confirmed that the two nitrite
ligands were coordinated to platinum through nitrogen.
(The 31P and 15N NMR spectra should both exhibit
AA%XX% coupling patterns [22], but the magnitudes of
most of the coupling constants were small and only the
separation of the main doublet could be ascertained.)
The corresponding palladium complex [Pd(15NO2)2-
(dppe)] was prepared by the same method. Its 31P
NMR spectrum was again very broad at ambient tem-
perature, but cooling to 220 K allowed identification of
three species. The major species exhibited a broad
doublet (actually part of an AA%XX% system) centered
at 54.5 ppm (2J(P,N)=56 Hz), indicating that the
mode of coordination of the two nitrites was through
nitrogen, namely, [Pd(15NO2)2(dppe)]. A second species
exhibited a doublet of doublets at 58.5 ppm (2J(P,P)=
bonds in [Pt(NO2)2(dppe)] may preclude ligand rear-
rangement even though the nitrites lie trans to phos-
phino groups.
3. Summary
The results described above provide evidence that
palladium nitrite complexes may exchange readily be-
tween N- and O-bonded forms when the nitrite group
lies opposite a ligand with a high trans-influence [24].
This is in contrast to a number of platinum and palla-
dium complexes that have been reported previously to
be coordinated through nitrogen exclusively. Forma-
tion of oxygen-bound palladium nitrite complexes may
be the first step in many palladium-catalyzed oxygen
transfer reactions, and this study provides evidence that
such species do exist and, indeed, may make up a
significant proportion of the metal complex in solution,
even at low temperatures.
2
16 Hz, J(P,N)=64 Hz) and a doublet at 55.1 ppm
(2J(P,P)=16 Hz). This was consistent with
[Pd(15NO2)(O15NO)(dppe)], the signal at lP 58.5 being
due to the P atom lying trans to N. Only a trace of
[Pd(O15NO)2(dppe)] was observed, which exhibited a
single resonance at 59.3 ppm. Thus, the three linkage
isomers could be identified at low temperature, but
interconversion between the three was rapid at 298 K.
In each case, [Pd(NO2)2L2] is the major species, with
the dinitritopalladium complex being least favored. The
dinitropalladium derivative is present in the greatest
concentration for the bulkiest ligand dppp, perhaps
unexpectedly because the ꢀNO2 group is more sterically
demanding than the ꢀONO fragment, but the other two
cases do not follow this trend. In contrast, the
analogous thiocyanate complexes were isolated as
4. Experimental
[PdCl2(CH3CN)2] and [Pd(NO2)2(CH3CN)2] were
prepared as described previously [11]. All reactions
were carried out under an argon atmosphere and in the
absence of light. AgNO2 and Na15NO2 were purchased
from Aldrich and Cambridge Isotope Laboratories,
respectively, and used as received. NMR spectra were
recorded on a Bruker ARX-500 spectrometer. 1H chem-
ical shifts were measured in ppm relative to the residual
solvent signal, and 31P and 15N shifts were determined
relative to external H3PO4 and MeNO2, respectively,
positive shifts representing deshielding. Elemental
analyses were performed by Atlantic Microlab Inc.,
Norcross, GA. The ratios of isomers of [Pd(NO2)2L2]
were obtained by integration of the 31P{1H} NMR
spectra (gated decoupled).
[Pd(SCN)2(dppm)],
[Pd(SCN)(NCS)(dppe)]
and
[Pd(NCS)2(dppp)], respectively [23]. Here it appears
that the more sterically demanding ꢀSCN ligands are
favored with the less bulky diphosphine ligands. It
should be noted, however, that this study was of solid
state structures, and it does not necessarily follow that
these are the more stable species in solution.
4.1. Preparation of [Pd(NO2)2(dppm)]
[PdCl2(CH3CN)2] (0.10 g, 0.39 mmol) was dissolved
in CH3CN (30 ml) and AgNO2 (0.18 g, 1.2 mmol) was
added. The reaction mixture was allowed to stir for 12
h. The solution was filtered and dppm (0.15 g, 0.39
mmol) was added to the filtrate. The mixture was
stirred for a further 5 min, then the solvents were
removed. The residue was extracted with CH2Cl2 and
filtered. The solvent was removed and the resulting
solid was dried in vacuo to leave the product as a pale
yellow solid (0.22 g, 99%). Anal. Calc. for
C25H22N2O4P2Pd: C, 51.52; H, 3.81. Found: C, 51.10;
The difference in solution behavior between the
monodentate phosphine complexes trans-[Pd(NO2)2L2]
(L=PPh3, PMePh2, PMe2Ph, PEt3) and those where
L2=dppm, dppe or dppp may be attributed to the high
trans-influence [24] of the phosphine ligand. When
monodentate phosphines are employed, they adopt a
mutually trans arrangement, causing the nitrite ligands
to be trans to each other. In the diphosphine com-
plexes, however, the nitrite ligands are forced to lie
trans to tertiary phosphino groups. The greater trans-
influence of the latter weakens the palladium–nitrite
bond, thereby allowing isomerization between N- and
O-bonded ligands more readily. The stronger PtꢀN
1
2
H, 3.92%. H NMR (CDCl3): l 4.05 (t, J(P,H)=10.3
Hz, 2H, PCH2); 7.4–7.75 (m, 20H, P(C6H5)2).

1332
J.A. Moser et al. / Polyhedron 19 (2000) 1329–1332
[Pd(NO2)2(dppp)] was prepared analogously, and iso-
Acknowledgements
lated as a colorless powder in 43% yield. Anal. Calc. for
C27H26N2O4P2Pd: C, 53.08; H, 4.30. Found: C, 52.99; H,
4.23%. 1H NMR (CDCl3): l 2.1 (br, 2H, PCH2CH2); 2.4
(br, 4H, PCH2); 7.4–7.7 (m, 20H, P(C6H5)2).
Thanks are expressed to the National Science Founda-
tion (grant No. CHE-9508228) for support of this work,
to Mallinckrodt Inc. for a graduate student fellowship
(R.A.S.J.), to the donors of the Brunngraber Fellowship
(J.A.M.), and to Dr Janet Braddock-Wilking for help
with some of the NMR experiments. Thanks are also
expressed to the US Department of Energy (grant No.
DE-FG02-92CH10499) for funds to purchase the NMR
spectrometer.
4.2. Preparation of [Pd(NO2)2(dppe)]
[PdCl2(CH3CN)2] (0.10 g, 0.39 mmol) was dissolved in
CH3CN (30 ml) and AgNO2 (0.18 g, 1.2 mmol) was added.
The reaction mixture was allowed to stir for 12 h. The
solution was filtered, then dppe (0.15 g, 0.39 mmol) was
added. A precipitate formed, which was collected by
filtration. It was washed with CH3CN and ether, then
dried in vacuo to leave the product as a colorless powder
(0.16 g, 71%). Anal. Calc. for C26H24N2O4P2Pd: C, 52.31;
H, 4.03. Found: C, 51.80; H, 4.06%. 1H NMR (CDCl3):
l 2.21–2.35 (m, 4H, PCH2); 7.4–7.5 (m, 20H, P(C6H5)2).
References
[1] W.M. Philips, S. Choi, J.A. Larabee, J. Chem. Educ. 67 (1990)
267.
[2] F. Bozoglian, M. Queirolo, B. Sienra, Polyhedron 17 (1998)
3551.
[3] W.G. Jackson, Inorg. Chem. 26 (1987) 3857.
[4] S. Balt, H.J.A.M. Kuipers, W.E. Renkema, J. Chem. Soc.,
Dalton Trans. (1983) 1739.
4.3. Preparation of [Pt(NO2)2(dppe)]
[5] E.V. Boldyreva, A.A. Sidel’nikov, N.Z. Lyakhov, Thermochim.
Acta 92 (1985) 109.
[6] M. Kubota, S. Ohba, Acta Crystallogr., Sect. B: Struct. Sci. B48
(1992) 627.
[7] H. Nishimura, H. Nagao, F.S. Howell, M. Mukaida, H. Kaki-
hana, Chem. Lett. (1990) 133.
[8] D. Ooyama, N. Nagao, H. Nagao, Y. Miura, A. Hasegawa, K.
Ando, F.S. Howell, M. Mukaida, K. Tanaka, Inorg. Chem. 34
(1995) 6024.
[PtCl2(dppe)] (0.15 g, 0.23 mmol) was dissolved in
CH3CN (30 ml). AgNO2 (0.14 g, 0.90 mmol) was added
and the mixture was allowed to stir for 48 h. The solution
was filtered and the solvent was removed. The resulting
solid was extracted with CH2Cl2 and filtered. The solvent
was removed again to leave the product as a colorless solid
(0.12 g, 77%). Anal. Calc. for C26H24N2O4P2Pd: C, 45.54;
H, 3.50. Found: C, 45.47; H, 3.60%. 1H NMR (CDCl3):
l 2.20–2.50 (m, 4H, PCH2); 7.5–7.8 (m, 20H, P(C6H5)2).
[9] D. Ooyama, H. Nagao, N. Nagao, F.S. Howell, M. Mukaida,
Chem. Lett. (1996) 759.
[10] J.L. Hubbard, C.R. Zoch, W.L. Elcesser, Inorg. Chem. 32 (1993)
3333.
4.4. Preparation of [Pd(NO2)2(PMePh2)2]
[11] M.A. Andrews, K.P. Kelly, J. Am. Chem. Soc. 103 (1981) 2894.
[12] R. Ugo, A. Chiesa, J. Chem. Soc., Perkin Trans. (1987) 2625.
[13] C.J. Jones, J.A. McCleverty, A.S. Rothin, H. Adams, N.A.
Bailey, J. Chem. Soc., Dalton Trans. (1986) 2055.
[14] J. Kriege-Simondsen, G. Elbaze, M. Dartiguenave, R.D.
Feltham, Y. Dartiguenave, Inorg. Chem. 21 (1982) 230.
[15] R.J. Puddephatt, P.J. Thompson, J. Chem. Soc., Dalton Trans.
(1976) 2091.
[16] J.A. Kaduk, J.A. Ibers, Inorg. Chem. 16 (1977) 3278.
[17] S.A. Bhaduri, I. Bratt, B.F.G. Johnson, A. Khair, J.A. Segal, R.
Walters, C. Zurraco, J. Chem. Soc., Dalton Trans. (1981) 234.
[18] R.A. Middleton, G. Wilkinson, M.B. Hursthouse, N.P. Walker,
J. Chem. Soc., Dalton Trans. (1982) 663.
[19] R.D. Feltham, J. Kriege, J. Am. Chem. Soc. 101 (1979) 5064.
[20] J. Kriege-Simondsen, R.D. Feltham, Inorg. Chim. Acta 71
(1983) 185.
[21] R.D. Feltham, G. Elbaze, R. Ortega, C. Eck, J. Dubrawski,
Inorg. Chem. 24 (1985) 1503.
[22] F.A. Bovey, Nuclear Magnetic Resonance Spectroscopy, Aca-
demic, New York, 1969, pp. 87–127.
[23] G.J. Palenik, M. Mathew, W.L. Steffen, G. Beran, J. Am. Chem.
Soc. 97 (1975) 1059.
[PdCl2(PMePh2)2] (0.10 g, 0.17 mmol) was dissolved in
CH2Cl2 (30 ml), and AgNO2 (0.080 g, 0.52 mmol) and
methanol (2 ml) were added. The reaction mixture was
allowed to stir for 24 h, the solvents were removed and
the residue was extracted with CH2Cl2 and filtered. The
solvent was removed again and the sample was dried in
vacuo to leave the product as a colorless solid (0.086 g,
83%).
4.5. Preparation of 15NO2-labeled complexes
In a typical experiment, [MCl2L2] (ca. 0.02 g) was
dissolved in acetone. TlPF6 (two molar equivalents) was
added and the reaction was allowed to stir for 2 h. After
this time, Na15NO2 (two molar equivalents) was added
and the mixture was allowed to stir for a further 2 h. The
acetone was removed, and the solid was extracted with
CH2Cl2. The solvent was removed again and the product
was dried in vacuo.
[24] T.G. Appleton, H.C. Clark, L.E. Manzer, Coord. Chem. Rev. 10
(1973) 335.
.
.