Chemistry of M(allyl)3 (M = Rh, Ir) compounds: Structural characterization of tris(allyl)iridium complexes with phosphorus ligands

Article abstract of DOI:10.1039/a910164k

While addition of phosphorus ligands such as P(OPh)₃ to Rh(allyl)₃ gives monovalent Rh(allyl)L₂, the iridium analog gives stable mixed σ/π- tris(allyl) complexes, as evidenced by the structural characterization of mono-, b

Full text of DOI:10.1039/a910164k

Chemistry of M(allyl)3 (M = Rh, Ir) compounds: structural characterization
of tris(allyl)iridium complexes with phosphorus ligands†
Kevin D. John,a Kenneth V. Salazar,b Brian L. Scott,a R. Thomas Baker*a and Alfred P. Sattelberger*a
a Los Alamos Catalysis Initiative, Chemical Science and Technology Division, Los Alamos National Laboratory,
MS J514, Los Alamos, NM 87545, USA. E-mail: weg@lanl.gov; sattelberger@lanl.gov
b Materials Science and Technology Division, Los Alamos National Laboratory, MS E549, Los Alamos, NM 87545,
USA
Received (in Bloomington, IN, USA) 21st December 1999, Accepted 22nd February 2000
While addition of phosphorus ligands such as P(OPh)3 to
Rh(allyl)3 gives monovalent Rh(allyl)L2, the iridium analog
gives stable mixed s/p-tris(allyl) complexes, as evidenced by
the structural characterization of mono-, bi-, and tri-dentate
ligand complexes.
Recent efforts aimed at preparing and characterizing ‘single-
site’ catalysts consisting of well defined, metal–oxide surface-
bound, ligated transition metals have yielded exciting new
catalytic systems for alkane metathesis and polyolefin depoly-
merization.1,2 One particularly well-characterized class of such
sites is derived from reactions of tris(allyl)rhodium, Rh(C3H5)3,
with carefully prepared, high-purity metal oxides.3–5 The use of
these systems for catalysis, however, is hampered by weak Rh–
Scheme 1
O bonds to the metal oxide surface and the proclivity of trivalent
rhodium intermediates to undergo reductive elimination.6 We
are investigating the use of tris(allyl)iridium to prepare single-
site catalysts with better thermal and reductive stability. In
expanding the relatively unexplored7 ligand addition chemistry
of M(allyl)3 compounds, we have developed an improved
synthesis of the iridium analog and demonstrated that its
reactions with a variety of phosphorus ligands all afford
trivalent products, including a stable tris(s-allyl) adduct.
The literature procedure8 for preparing Ir(allyl)3 from
Ir(acac)3 (acac = acetylacetonate) and (allyl)MgCl affords the
desired product in ca. 20% yield. Ir(acac)3 is itself prepared in
ca. 10% yield from commercially available IrCl3·xH2O. We
have found that the crystalline yellow tetrahydrothiophene
adduct, IrCl3(tht)3, can be prepared from IrCl3·xH2O in ca. 90%
yield via a slight modification of the original synthesis.9
Reaction of IrCl3(tht)3 with allyllithium in benzene provides
Ir(allyl)3 in > 90% NMR yield [eqn 1)].10
The tris(allyl)iridium ligand adducts 1–4 were characterized
by elemental analysis, NMR and IR spectroscopy, and, for 1, 3
and 4 by single crystal X-ray diffraction (Fig. 1).§ IR analysis
revealed that all phosphine adducts possess a characteristic CNC
stretch in the region 1600–1610 cm21. 1H and 13C NMR spectra
of 1 confirmed the presence of one s-allyl and two inequivalent
p-allyl groups, even at 100 °C. The PPh3 ligand is located trans
to a methylene group of one of the p-allyl groups, as evidenced
by the large P–C coupling constant (2JPC 38 Hz). Room
temperature spectroscopic characterization of complexes 2a,b
was also consistent with an unsymmetrical structure, but a
dynamic process leads to equivalent phosphorus ligands and s-
allyl groups at 40 °C. The P–C coupling constants for 2a,b (2JPC
32.6, 47.5 Hz, respectively) indicate that the phosphines are
trans to the methylenes of the p-allyl group and the s-allyls are
thus mutually trans. By contrast, complex 3 has inequivalent s-
allyl groups, even at 100 °C, with the two phosphorus donors
trans to one methylene of the p-allyl (2JPC 39.4 Hz) and to one
of the s-allyl groups (2JPC 79.5 Hz), respectively (Fig. 2).
Adduct 4 has two sets of s-allyl groups in a 1+2 ratio, consistent
with Cs symmetry.
Removal of the solvent in vacuo followed by hexane
extraction, filtration, solvent removal and sublimation of the
brown residue affords colorless, microcrystalline Ir(allyl)3 in
ca. 45% yield. Treatment of isolated Ir(allyl)3 with PPh3 in
toluene gives the 1+1 adduct Ir(s-allyl)(p-allyl)2(PPh3) 1 in
high yield (Scheme 1).‡ Alternatively, the hexane filtrate from
the synthesis of Ir(allyl)3 can be used directly to prepare 1 with
comparable efficiency. Although complex 1 did not react
further with excess PPh3, even at 100 °C, smaller phosphorus
ligands such as PMe3 and P(OPh)3 yielded 1+2 adducts Ir(s-
allyl)2(p-allyl)L2 2a,b. The chelating bis(phosphine)
1,2-(PPh2)2C6H4 gave a similar product 3. Addition of the
tridentate phosphine PhP(CH2CH2PPh2)2 to Ir(allyl)3 gave the
tris(s-allyl) complex 4. Complexes 2–4 were prepared in > 90%
yield.
For compound 1, the p-allyl ligand trans to P is symmet-
rically bound with typical Ir–C bond distances11 (Ir–CH
2.143[5], Ir–CH2 2.201[5] Å). The other p-allyl ligand is
unsymmetrically bound as a result of the trans influence of the
s-allyl {C(1)–Ir–C(7) 161.6[2]°; Ir–C(7) 2.268[4] cf. Ir–C(9)
2.196[4] Å}. A similar effect is seen in 3 {C(1)–Ir–C(7)
162.1(1)°; Ir–C(7) 2.270(2) cf. Ir–C(9) 2.202(3) Å}.
Reaction of Rh(allyl)312 with PMe3 afforded the 1+2 adduct
5 similar to 2a. With triphenylphosphine12 or better p-accepting
ligands such as P(OPh)3, however, only monovalent complexes,
Rh(p-allyl)L2 6a,b were obtained. Analogous complexes have
been reported previously via alternative preparative routes.11,13
The reaction leading to 6b was accompanied by formation of the
C6 hydrocarbons n-hexane, hex-1-ene and 2-methylpentane.¶
In summary, our improved preparation of tris(allyl)iridium
has enabled us to demonstrate that, in contrast to its rhodium
analog, addition of a range of phosphorus ligands gives stable
trivalent adducts which show no tendency to undergo hydro-
† Electronic supplementary information (ESI) available: preparative details
and NMR data for 1–6 and crystallographic data for 1, 3 and 4. See http://
www.rsc.org/suppdata/cc/a9/a910164k/
DOI: 10.1039/a910164k
Chem. Commun., 2000, 581–582
This journal is © The Royal Society of Chemistry 2000
581

Fig. 2 Isomeric Ir(p-allyl)(s-allyl)2L2 structures.
paper is dedicated to Professor John P. Fackler, Jr. on the
occasion of his 65th birthday.
Notes and references
‡ Preparation of Ir(s-allyl)(p-allyl)2(PPh3): to a room temperature solution
of Ir(allyl)3 (0.100 g, 0.32 mmol) in toluene (10 mL) was added
triphenylphosphine (0.083 g, 0.32 mmol). The reaction mixture was stirred
for 2 h and then filtered. The filtrate was concentrated to ca. 2 mL and
allowed to sit at room temperature for 24 h to yield 1 as colorless crystals
in 92.7% yield (0.170 g, 0.24 mmol). This procedure is representative of the
syntheses of all of the phosphine complexes with the caveat that the
preparation of 3 and 4 required brief heating (80 °C, 10 min) in order to
dissolve the ligand.
§ CCDC 182/1552. See http://www.rsc.org/suppdata/cc/a9/a910164k/ for
crystallographic files in .cif format.
¶ Preparation of Rh(p-allyl)[P(OPh)3]2: to a room temperature solution of
Rh(allyl)3 (0.031 g, 0.14 mmol) in toluene (5 mL) was added triphenyl
phosphite (0.086 g, 0.28 mmol) with stirring. Approximately 30 min after
the phosphite addition, a brilliant yellow precipitate was observed. The
reaction mixture was stirred for an additional 2 h during which time the
precipitate dissolved. The filtrate was then concentrated to ca. 2 mL and
allowed to stand at 235 °C for 24 h to yield 2b as yellow crystals in 96.7%
yield (0.102 g, 0.13 mmol). GC–MS analysis of a portion of this reaction
revealed that the organic products were n-hexane, 2-methylpentane and
hex-1-ene.
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Fig. 1 Thermal ellipsoid representation of (a) 1, (b) 3 and (c) 4 shown at the
50% probability level. For 1, only one of the two molecules in the
asymmetric unit is shown. For 1 and 4, only one orientation of the
disordered allyl group is shown.§ Hydrogen atoms have been omitted for
clarity.
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carbon elimination reactions. We are currently extending these
findings to prepare trivalent, ligated iridium moieties on metal
oxide surfaces for use in hydrocarbon functionalization.
We thank the Department of Energy’s Laboratory Directed
Research and Development (LDRD) program for financial
support and Dr David Smith for preliminary experiments. This
Communication a910164k
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Chem. Commun., 2000, 581–582