Reversible dioxygen binding on asymmetric dinuclear rhodium centres

Article abstract of DOI:10.1039/c3cc41815d

Electron-deficient dinuclear rhodium complexes [Rh₂Cl ₂(μ-dpmppp)(RNC)] (1), with the linear tetraphosphine ligand dpmppp, showed reversible binding of molecular oxygen to form asymmetric dirhodium η²-peroxo complexes [Rh₂Cl ₂(O₂)(μ-dpmppp)(RNC)] (2) stabilized by a Rh→Rh dative bond. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc41815d

Chemical Communications
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Volume 49 | Number 46 | 11 June 2013 | Pages 5239–5338
ISSN 1359-7345
COMMUNICATION
Takayuki Nakajima, Tomoaki Tanase et al.
Reversible dioxygen binding on asymmetric dinuclear rhodium centres
1359-7345(2013)49:46;1-Z

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Reversible dioxygen binding on asymmetric dinuclear
rhodium centres†
Cite this: Chem. Commun., 2013,
49, 5250
Takayuki Nakajima,* Miyuki Sakamoto, Sachi Kurai, Bunsho Kure and
Tomoaki Tanase*
Received 11th March 2013,
Accepted 28th March 2013
DOI: 10.1039/c3cc41815d
www.rsc.org/chemcomm
Electron-deficient dinuclear rhodium complexes [Rh2Cl2(l-dpmppp)-
(RNC)] (1), with the linear tetraphosphine ligand dpmppp, showed
reversible binding of molecular oxygen to form asymmetric
dirhodium g2-peroxo complexes [Rh2Cl2(O2)(l-dpmppp)(RNC)] (2)
stabilized by a Rh-Rh dative bond.
Scheme 1 Schematic structures of
1 and 2 and their interconversion by
Activation of molecular oxygen by metal complexes is one of the
most important chemical processes and is related to versatile
subjects ranging from biological oxygen transporting systems and
oxidase functions by metalloenzymes to oxidation of organic
compounds by metal catalysts in industrial chemistry.1 Isolation
and characterization of dioxygen complexes provide general
information on bonding structures and reactivity of coordinated
molecular oxygen. Despite the considerable literature on the
chemistry of mono-nuclear transition-metal dioxygen complexes,
asymmetric dinuclear O2 complexes are limited to a few examples,2
while they could be related to dioxygen-binding biological systems
such as hemerythrin and cytochrome c oxidase.3 Recently, we have
synthesized a linear tetraphosphine ligand, meso-1,3-bis[(diphenyl-
phosphinomethyl)phenylphosphino]propane (dpmppp), which has
proven very effective in the assembly of heterotrinuclear
MRh2(m-Cl)3 cyclic complexes, [MRh2(m-Cl)3(m-dpmppp)(CO)2]+
(M = Ni, Pd, Pt), and a homotrinuclear complex, [Rh3(m-Cl)3-
(m-dpmppp)(CO)2].4 In the present study, the dpmppp ligand was
found to support electron-deficient dinuclear rhodium complexes
[Rh2Cl2(m-dpmppp)(RNC)] (1) which further exhibited unusual rever-
sible dioxygen binding promoted by a metal-to-metal dative bond.
Reactions of dpmppp with [RhCl(cod)]2 and RNC (R = 2,6-xylyl
(Xyl), 2,6-diisopropylphenyl (Dip), 2,4,6-mesityl (Mes)) in dichloro-
methane at room temperature afforded dinuclear rhodium com-
plexes [Rh2Cl2(m-dpmppp)(RNC)] (R = Xyl (1a), Dip (1b), Mes (1c))
reversible dioxygen binding.
in moderate yields (Scheme 1). The crystal structures of 1a,b are
almost identical except for the isocyanide ligands and consist
of asymmetric dirhodium centres with a pseudo mirror plane
passing through the two rhodium centres perpendicular to the
[Rh1,Rh2,P1,P4] plane (Fig. 1).
The Rh1 centre adopts square pyramidal geometry with cis
phosphorus atoms (P2–Rh1–P3 = 95.10(5)1 (1a), 94.41(5)1 (1b)) and
cis chloride anions in the basal plane and with Rh2 occupying the
apical position. The Rh2 ion sits in a square planar environ-
ment with trans phosphorus atoms (P1–Rh2–P4 = 170.89(4)1 (1a),
167.19(4)1 (1b)) and the RNC ligand trans to the Rh1 atom. The
Rh–P distances for Rh1 are ca. 0.1 Å shorter than those for Rh2 due
to trans influence. Whereas in many cases of complexes containing
a Rh2(dppm)2 unit, the two dppm ligands arranged dimetal centres
Department of Chemistry, Faculty of Science, Nara Women’s University,
Kitauoya-nishi-machi, Nara 630-8506, Japan. E-mail: t.nakajima@cc.nara-wu.ac.jp,
tanase@cc.nara-wu.ac.jp; Fax: +81-742-20-3847
† Electronic supplementary information (ESI) available: Synthesis and spectral
data, crystal data, selected bond lengths and angles, ORTEP diagrams for 1b and
2c, and DFT calculations data. CCDC 928791 (1a), 928792 (1b), 928793 (2c) and
928794 (2d). For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3cc41815d
Fig. 1 ORTEP diagram of 1a with the atomic numbering scheme. The thermal
ellipsoids are drawn at the 40% probability level, and the hydrogen atoms are
omitted for clarity.
c
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with trans/trans- or cis/cis-P,P geometrical modes at each metal site,
the present dpmppp ligand organized a dimetal structure in an
unusual cis/trans-P,P fashion, such Rh2 examples with two dppm
ligands are quite limited to only a few compounds (dppm =
bis(diphenylphosphino)methane).5 The square planes around the
Rh1 and Rh2 atoms are approximately perpendicular with dihedral
angles of 79.51(3)1 (1a) and 79.96(2)1 (1b).
The Rh1–Rh2 distances (2.6873(10) Å (1a), 2.6949(10) Å (1b))
indicate the presence of bonding interaction which is recognized
as a Rh1-Rh2 dative bond from DFT calculations on an
optimized structure of 1a with B3LYP methods and the LANL2DZ
basis set. The HOMOÀ4 (À5.492 eV) and the LUMO (À1.300 eV)
are composed of s-bonding and s-antibonding interactions
between the two Rh d orbitals, respectively (Fig. 2a), and in
HOMOÀ4, contribution of the Rh1 d orbital is obviously large
compared to that of Rh2. The Wiberg bond index for Rh1–Rh2
(0.372) and the natural charge population of Rh1 (À0.81) and
Rh2 (À0.49) are consistent with a dative Rh1(I)-Rh2(I) bond
rather than a covalent Rh1(II)–Rh2(0) bond. Notably, the HOMO
(À4.885 eV) is a non-bonding d orbital on Rh2 (Fig. 2a), implying
a nucleophilic character of the low-coordinate Rh2 centre.
The 31P{1H} NMR spectra of 1a–c showed two sets of resonances
with characteristic Rh–P coupling at d 15.6 (1JPRh = 134 Hz) and 21.0
(1JPRh = 143 Hz) (1a), 15.6 (1JPRh = 131 Hz) and 21.0 (1JPRh = 143 Hz)
(1b), and 15.5 (1JPRh = 133 Hz) and 21.1 (1JPRh = 143 Hz) (1c) ppm,
each set being assigned to the outer and the inner P atoms using
1H–31P HMBC NMR techniques. The ESI-MS spectra of 1a–c in
dichloromethane showed the parent peaks corresponding to
[Rh2Cl(dpmppp)(RNC)]+, consistent with the X-ray and NMR results.
Complexes 1a–c readily reacted with molecular oxygen
under air in dichloromethane solution at À15 1C, which was
Fig. 3 31P{1H} NMR spectral changes of 1a in CD2Cl2 under air after (a) 0 h, (b) 6 h,
(c) 9 h, and (d) 24 h, and (e) after heating the sample (d) at 50 1C. The peaks with
asterisk as impurity are assigned to one of the P atoms of dpmppp(O)4.
monitored by 31P{1H} NMR spectra (Fig. 3). After 24 h, the peaks
for the starting materials 1 diminished and a new set of reso-
nances for [Rh2Cl2(O2)(m-dpmppp)(RNC)] (R = Xyl (2a), Dip (2b),
Mes (2c)) appeared at d 9.6 (1JPRh = 96 Hz) and 27.6 (1JPRh = 136 Hz)
(2a), 11.3 (1JPRh = 97 Hz) and 27.5 (1JPRh = 136 Hz) (2b), and 9.4
(1JPRh = 97 Hz) and 27.5 (1JPRh = 136 Hz) (2c) ppm, which were
assigned to the outer (Pout) and inner P atoms (Pin), respec-
tively. The ESI-MS spectra of 2a–c in dichloromethane showed
monovalent parent peaks corresponding to the dioxygen
adducts [Rh2Cl(O2)(dpmppp)(RNC)]+.
An analogous dioxygen complex with the tBuNC ligand, [Rh2Cl2-
(O2)(m-dpmppp)(tBuNC)] (2d), was synthesized by reacting [Rh3Cl3-
(dpmppp)(CO)2]4 with tBuNC in 29% yield (Scheme 2), although the
corresponding RhI2 dimer, [Rh2Cl2(m-dpmppp)(tBuNC)], was
not isolated yet. The structures of 2c and 2d were determined
by X-ray crystallography, which revealed that the dioxygen
coordinated to the Rh2 centre in a side-on mode (Rh2–O1 =
2.016(5) (2c), 2.039(5) (2d); Rh2–O2 = 2.026(4) (2c), 2.021(5) (2d) Å)
(Fig. 4). The O–O distances (1.436(4) (2c), 1.449(7) (2d) Å) are
typical for coordinated peroxide and similar to those reported
for other rhodium Z2-peroxo complexes.6,7 The oxidative addi-
tion of O2 to the Rh2 metal centre is in good agreement with the
1
1JRhP coupling constants since the JRhP value is known to be
dependent on the oxidation state of the connecting Rh centre.6
1
Whereas the JRhPout values for 2a–c are significantly smaller
1
than those for 1a–c, the JRhPin values for 2a–c are comparable
to those for 1a–c, suggesting the Rh1IRh2III oxidation state of 2.
Fig. 2 Representative MO diagrams of (a) 1a and (b) 2d derived from DFT
Scheme 2 Preparation of 2d from reaction of [Rh3Cl3(m-dpmppp)(CO)2] with
tBuNC.
optimization with B3LYP methods.
c
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when 2a in CH2Cl2 was treated with HCl in the presence of PPh3
(1 equiv.), a RhIRhIII complex, [Rh2Cl4(m-dpmppp)(XylNC)] (3a),
was generated quantitatively with the formation of Ph3PQO in
75% yield (Scheme 3), suggesting that the Z2-peroxo moiety was
protonated to release H2O2 which further reacted with PPh3.6a
The dioxygen complexes 2 might also be important as inter-
mediates of oxygenation processes with molecular oxygen using
metal catalysts.
In conclusion, we have synthesized the unsaturated dinuclear
rhodium complexes 1a–c with 30 valence electrons, where the
dpmppp ligand coordinated to two metal centres in an unusual
cis/trans geometrical fashion. They readily incorporated molecular
oxygen to form Z2-peroxo complexes 2a–c at À15 1C, which in turn
released dioxygen at 50 1C to restore 1a–c. Theoretical calcula-
tions indicated that the Rh-Rh dative interaction stabilized
the Rh–(Z2-peroxo) bond and is thus crucial to the reversible
dioxygen binding on the asymmetric dirhodium centre.
This work was supported by a Grant-in-Aid for Scientific
Research and that on Priority Area 2107 (no. 22108521, 24108727)
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan, and a grant from Nara Women’s University
for research projects. T.N. is grateful to the Tokuyama Science
Foundation and the Kurata Memorial Hitachi Science and
Technology Foundation.
Fig. 4 ORTEP diagram for 2d with the atomic numbering scheme. The thermal
ellipsoids are drawn at the 40% probability level, and the hydrogen atoms are
omitted for clarity.
Scheme 3 Reaction of 2a with HCl in the presence of PPh3. L = XylNC.
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c
5252 Chem. Commun., 2013, 49, 5250--5252
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