Synthesis, structure, and reactions of a nitroxyl complex of iridium(III), cis,trans-IrHCl2(NH=O)(PPh3)2

Article abstract of DOI:10.1039/b111645b

Reaction of Ir(NO)(PPh₃)₃ with anhydrous HCl results in addition of 2 equivalents of HCl with formal protonation of the nitrosyl ligand, affording the unusual six-co-ordinate nitroxyl complex cis,trans-IrHCl₂(NH=O)(PPh₃)₂.

Full text of DOI:10.1039/b111645b

Synthesis, structure, and reactions of a nitroxyl complex of iridium(III),
cis,trans-IrHCl2(NHNO)(PPh3)2
Rory Melenkivitz and Gregory L. Hillhouse*
Searle Chemistry Laboratory, Department of Chemistry, The University of Chicago, Chicago, Illinois 60637,
USA. E-mail: g-hillhouse@uchicago.edu
Received (in Purdue, IN, USA) 21st December 2001, Accepted 6th February 2002
First published as an Advance Article on the web 27th February 2002
Reaction of Ir(NO)(PPh3)3 with anhydrous HCl results in
addition of 2 equivalents of HCl with formal protonation of
the nitrosyl ligand, affording the unusual six-co-ordinate
(1)
nitroxyl complex cis,trans-IrHCl2(NHNO)(PPh3)2.
Nitroxyl (HNNO) is a highly reactive, unstable molecule
attracting current interest primarily because of its relationship to
nitric oxide. The nitrosonium cation (NO+) and the nitroside
anion (NO2), as well as the conjugate acid of NO2, nitroxyl,
have been suggested to be responsible for some of the myriad
functions attributed to nitric oxide in mammalian biochem-
istry.1 Interest in HNNO also stems from its postulated
intermediacy in photochemical and free-radical reactions and
the role its formation and decomposition may play in mecha-
nisms for the combustion of nitrogen-containing fuels and the
oxidation of atmospheric nitrogen.2 In some of these functions,
metal co-ordination is likely involved, for example in HNNO co-
ordination to heme iron,3 so knowledge of how nitroxyl
interacts with and is stabilised by metals is desirable. We and
others have been interested in elucidating the co-ordination
chemistry of this novel molecule.3–7 Two nitroxyl complexes
have been structurally characterised, OsCl2(NHNO)-
(CO)(PPh3)2 by Roper and Ibers,5 and Ru(NHNO){2,6-bis(2-
The 1H NMR spectrum of 3 exhibits the diagnostic downfield
resonance expected for the nitroxyl ligand’s proton (d 22.75)3–6
and a characteristic upfield resonance for the hydride ligand (d
218.90, triplet, 2JPH = 27.8 Hz). In the 1H NMR spectrum of
3-15N, the nitroxyl proton resonance appears as a doublet-of-
triplets in which both the 15N–H (77.6 Hz) and 31P–H (1.7 Hz)
1
couplings are resolved. The value of JNH is in the range
expected for a proton attached to an sp2-hybridized nitrogen co-
ordinated to a transition metal, and is similar to those reported
for other nitroxyl and related diazene complexes.3,4,5b,9
The molecular structure of 3 is shown in Fig. 1, and Table 1
summarises pertinent bond lengths and angles for 3 and the
other structurally characterised nitroxyl complexes. The d6
iridium adopts a pseudo-octahedral co-ordination geometry
with trans phosphines and cis chlorides. The hydride ligand was
not crystallographically located, but it exerts a typical trans-
lengthening influence on Cl(2) relative to Cl(1). The nitroxyl
ligand lies in the molecule’s (non-crystallographic) mirror
plane, which allows for backbonding with an orthogonal filled
d-orbital of p-symmetry. Although there is no evidence for
mercapto-3,5-di-tert-butylphenylthio)dimethylpyridine}
by
Sellmann.6
Roper and co-workers have previously observed that reaction
of the four-co-ordinate nitrosyl complex Ir(NO)(PPh3)3 (1) with
an excess of HCl results in reduction of the nitrosyl ligand of 1
to give a six-co-ordinate iridium(III) hydroxylamine derivative,
IrCl3(NH2OH)(PPh3)2 (2).5a,8 We were interested in reinvest-
igating this reaction in an attempt to intercept the intermediates
in what clearly is a multi-step reaction. Possible intermediates,
such as IrCl(NHNO)(PPh3)2 (a Vaska’s complex analogue),
seemed reasonable since, like carbon monoxide, ligated nitroxyl
is a strong p-acid.4b A comparison of nCO in Re(NHNO)-
(CO)3(PPh3)2 (2082, 2007, 1977 cm21) with nCO in Re-
+
(NH2OH)(CO)3(PPh3)2 (2061, 1966, 1926 cm21) shows a
+
large increase in nCO (D ~ 21–51 cm21) when the purely s-
donating NH2OH ligand is replaced by the HNNO ligand in an
otherwise identical co-ordination environment. This reflects
substantial competition for metal p-electron density by a p*
orbital of the nitroxyl ligand (backbonding), and this has been
further supported by DFT calculations.4b
Reaction of 18 with a controlled excess of anhydrous HCl
(CH2Cl2, 278 °C) gives cis,trans-IrHCl2(NHNO)(PPh3)2 (3) as
pink crystals in 60% isolated yield (eqn. 1).† The labelled
isotopomer cis,trans-IrHCl2(15NHNO)(PPh3)2 (3-15N) was
analogously prepared from Ir(15NO)(PPh3)3. Use of three
equivalents of HCl seems to afford optimal yields of 3, whereas
addition of a single-equivalent of acid results in reduced yields
of 3 and recovered starting material. Complex 3 was charac-
terised by NMR (1H and 31P) and infrared spectroscopy,
elemental analysis, and a single crystal X-ray diffraction study.‡
The infrared spectrum of 3 shows vibrations associated with the
nitroxyl ligand (nNO = 1493(s), nNH = 2810(w) cm21) which
Fig. 1 A view of the molecular structure of 3. Selected metrical parameters:
Ir–N = 1.879(7), N–O(1) = 1.235(11), Ir–Cl(1) = 2.403(2), Ir–Cl(2) =
2.476(2), Ir–P(1)
= 2.351(3), Ir–P(2) = 2.345(3) Å; Ir–N–O(1) =
129.8(7), N–Ir–Cl(2) = 83.6(3), N–Ir–Cl(1) = 177.0(3), N–Ir–P(1) =
94.2(3), N–Ir–P(2) = 93.0(3), P(1)–Ir–P(2) = 165.19(8)°.
Table 1 Comparison of bond lengths and angles for HNNO ligands
Compound
M–N/Å
N–O/Å
M–N–O/°
IrHCl2(NHNO)(PPh3)2 (3)a
OsCl2(NHNO)(CO)(PPh3)2
Ru(S4py)(NHNO)c
1.879(7)
1.915(6)
1.875(7)
1.235(11)
1.193(7)
1.242(9)
129.8(7)
136.9(6)
130.0(6)
b
are isotopically shifted in the infrared spectrum of 3-15N (nNO
1465(s), nNH = 2801(w) cm21).
=
a This work. b Ref. 5b. c Ref. 6.
660
CHEM. COMMUN., 2002, 660–661
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intramolecular hydrogen bonding, the nitroxyl proton is ori-
ented syn to Cl(2), a structural feature common to both 3 and
OsCl2(NHNO)(CO)(PPh3)2.5b The spectroscopic and structural
data clearly show that the nitrosyl ligand has undergone
protonation at nitrogen, but it is noteworthy that O-protonation
of the nitrosyl ligand of (C5Me5)W(NO)(R)2 (R = CH2Ph,
CH2SiMe3) to give hydroxylimido complexes has recently been
reported.10
a GAANN Fellowship from the U. S. Department of Education
(to R. M.). We thank Dr. Daniel Mindiola for crystallographic
assistance.
Notes and references
† To a sample of 1 (0.36 g, 0.47 mmol) dissolved in 8 mL of CH2Cl2 and
cooled to 278 °C was added 3 equiv. of anhydrous HCl via a calibrated
volume. The red solution was stirred for 20 min, the volume was reduced to
3 mL, and a toluene/pentane mixture was added to induce crystallisation.
The resulting pink powder was purified by recrystallisation from dichloro-
methane/pentane, yielding pure 3 (0.24 g, 0.28 mmol, 60%). For 3: 1H NMR
(CD2Cl2, 298 K) d 22.75 (s, 1 H, Ir–NHNO), 7.1–7.3 (m, 30 H, Ph), 218.90
(t, 1 H, Ir–H, 2JPH = 27.8 Hz). 31P{1H} NMR (CD2Cl2, 298 K) d 21.73 (s).
IR (CaF2/fluorolube mull): 2810 (w, nNH), 1493 (s, nNO cm21). Anal. Calcd.
for C36H32NCl2IrOP2: C, 52.75; H, 3.93; N, 1.71. Found: C, 52.53; H, 3.80;
N, 1.80%.
Reaction of 3 with NaN(SiMe3)2 in THF solution results in its
dehydrohalogenation to give the five-co-ordinate nitrosyl
hydride complex IrHCl(NO)(PPh3)2 (4, eqn. 2). The Ir–H
resonance for 4 appears at d 218.65 (triplet, 2JPH = 30.6 Hz) in
1
the H NMR spectrum, and equivalent phosphine ligands are
indicated by a singlet resonance at d 18.85 in the 31P{1H}
spectrum. The infrared spectrum shows a strong band for nNO at
1548 cm21 (which compares well with the literature value of
1545 cm21 for 4 prepared by an independent route).8 The
spectroscopic similarity between 4 and the crystallographically
characterised complex IrCl2(NO)(PPh3)2 (nNO = 1560 cm21; d
Ir–PPh3 = 11.6)11 gives further support to the square-pyramidal
structure of 4 shown in eqn. 2. Related conversions of nitroxyl
ligands to nitrosyls by base have been described.4b,5a
‡ Crystal data: For 3·4(C4H8O), C51H55Cl2IrNO4P2, M
= 1071.00,
¯
triclinic, P1, a = 9.751(11), b = 11.1216(12), c = 23.281(3) Å, a =
82.508(2), b = 84.582(2), g = 64.976(2)°, Z = 2, V = 2318.2(4) Å3, T =
100 K, m(Mo-Ka) = 3.110 mm21. Of 13810 total reflections (red needle,
1.77 = q = 28.33°), 9458 were independent and 7746 (Rint = 7.21%) were
observed with I > 2s(I). A semi-empirical absorption correction was
performed using psi-scans. Patterson methods were used to locate the heavy
atoms Ir, Cl, P, and other heteroatoms. No anomalous bond lengths or
thermal parameters were noted except for one disordered tetrahydrofuran
molecule of solvation which resided at an inversion centre. Three of its
carbon atoms (C20s, C21s, C22s) were located and refined, but the
complete THF molecule could not be resolved. All non-hydrogen atoms
were refined anisotropically, and hydrogen atoms were refined isotropically
and fit to idealised positions. R(F) = 7.33% and R(wF) = 16.35%. CCDC
reference number 178606. See http://www.rsc.org/suppdata/cc/b1/
b111645b/ for crystallographic data in CIF or other electronic format.
§ Characterisation of 2 followed by comparison with an authentic sample
made by the literature route.8 For 2: 1H NMR (CD2Cl2, 298 K) d 8.00 (s, 1
H, OH), 7.7–7.2 (m, 30 H, Ph), 5.38 (s, 2 H, NH2). 31P{1H} NMR (CD2Cl2,
298 K) d 230.44 (s).
In a key experiment, treatment of CH2Cl2 solutions of
isolated samples of 4 with anhydrous HCl cleanly gives 3 (eqn.
2). This indicates that the mechanism for formation of 3 from 1
likely involves initial oxidative addition of HCl to form 4. The
subsequent reaction of 4 with the second equivalent of acid to
give 3 must be fast relative to the first step since no intermediate
is observed in the transformation of 1 to 3.
(2)
Finally, a comment on the relevance of 3 as an intermediate
in the reduction of the nitrosyl ligand of 1 to the hydroxylamine
ligand of IrCl3(NH2OH)(PPh3)2 (2) is in order. Interestingly,
addition of anhydrous HCl to a pure sample of 3 in methylene
chloride does not result in further reduction of the nitroxyl
ligand. (In fact, in our synthesis of 3, addition of a slight excess
of acid gives the best yields, vide supra.) However, addition of
EtOH to solutions of 3 containing HCl results in the rapid
conversion of 3 to the hydroxylamine derivative 2 (eqn. 3).§
The exact role of EtOH in this reaction is unclear, but it
probably involves acid solvation. Reduction of the nitroxyl
ligand is formally effected by the delivery of H2 (from the Ir–H)
and H+ (from the acid). In Roper’s original reports of the
preparation of IrCl3(NH2OH)(PPh3)2, aqueous HCl was used to
carry out protonation of 1 (alternatively, anhydrous HCl was
used along with ethanol as a co-solvent),5a,8 and it was the
aqueous conditions and ethanolic workup that prevented the
isolation of the iridium nitroxyl intermediate.
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(3)
This research was supported by grants from the National
Science Foundation and the Petroleum Research Fund, admin-
istered by the American Chemical Society (to G. L. H.), and by
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