Selective synthesis of isomeric heterodinuclear complexes with switched metal arrangements via proton-induced reversible metal migration

Article abstract of DOI:10.1039/b409514f

Pairs of isomeric heterodinuclear complexes, [(cod)Ir(μ-PNNN)M(L)] BF₄ and [(L)M(μ-PNN)Ir(cod)]BF₄, with switched metal arrangements are prepared in a specific manner by simply changing the addition order of the reagents.

Full text of DOI:10.1039/b409514f

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Selective synthesis of isomeric heterodinuclear complexes with switched
metal arrangements via proton-induced reversible metal migration{
Christian Dubs, Akiko Inagaki and Munetaka Akita*
Chemical Resources Laboratory, Tokyo Institute of Technology, R1-27, 4259 Nagatsuta, Midori-ku,
Yokohama 226-8503, Japan. E-mail: makita@res.titech.ac.jp
Received (in Cambridge, UK) 24th June 2004, Accepted 31st August 2004
First published as an Advance Article on the web 21st October 2004
Pairs of isomeric heterodinuclear complexes, [(cod)Ir(m-
reagent and the base exclusively brought about formation of the
isomeric species 4?BF4,{ where the metal arrangement is switched
as also determined for 4a?BF4 by X-ray crystallography
(Scheme 1).§
Low solubility of 2?BF4 in organic solvents hampered full
spectroscopic characterization but upfield shift of the H6 signal,
which was a sign of NN-coordination,{ led to tentative assignment
of the structure shown in Scheme 1. It is notable that deprotonation
of 2?BF4 with NEt3 afforded the neutral species 5, where the
iridium atom was incorporated into the PN-site as confirmed by (i)
PNNN)M(L)]BF4 and [(L)M(m-PNNN)Ir(cod)]BF4, with
switched metal arrangements are prepared in a specific
manner by simply changing the addition order of the
reagents.
Polynuclear heterometallic complexes are expected to display
unique reactivity through cooperation of the metal centers with
different characters.1 But putting different metal fragments on
particular coordination sites on a polydentate ligand in a specific
manner is not always facile. Doubly donor-armed heteroaromatics
with two chelating coordination sites are versatile dinucleating
ligands.2 Although a variety of such derivatives have been prepared
so far, attempts to prepare heterometallic complexes have been
few.3 We are now carrying out a synthetic study on heterodinuclear
complexes with the PNNN ligand 1 (3-diphenylphosphinomethyl-
5-pyridylpyrazolate; Scheme 1),4 which follows that on homo-
metallic polynuclear complexes with a symmetric PNNP ligand.5,6
The PNNN ligand 1 has one PN- and one NN-coordination site
and herein we wish to report the selective synthesis of pairs of
isomeric heterodinuclear complexes with
a reversed metal
arrangement from the same sources by simply changing the
addition procedure of the reagents.
Scheme 2
Scheme 1 All reactions were carried out in CH2Cl2.
Treatment of PNNN-H (1-H) with [Ir(g4-cod)2]BF4 in CH2Cl2
gave a pale yellow precipitate 2?BF4{ (Scheme 1). Subsequent addi-
tion of another labile metal component [M(L)(cod)]BF4 [M(L) ~
Pd(g3-allyl), Rh(g4-cod)] followed by a base (NEt3) furnished the
heterodinuclear complexes 3?BF4.{ In 3?BF4 the Ir and Pd
fragments occupied the NN- and PN-coordination site, respec-
tively, as determined for 3a?BF4 by X-ray crystallography.§ To our
surprise, simply reversing the addition order of the second metal
the deshielded 31P signal [dP 30.1; cf. 1-H: 220.1], (ii) downfield
shift of the doublet H6 signal and (iii) X-ray crystallography
(Scheme 2).§ Protonation of 5 with HBF4?OEt2 immediately
caused quantitative precipitation of 2?BF4.
These results suggest that the deprotonation–protonation
processes bring about reversible migration of the Ir fragment
between the NN- and PN-coordination sites, which can account for
the selective formation of 3?BF4 and 4?BF4. Coordination of the P
atom in 2?BF4 to the second metal species followed by
deprotonation leads to 3?BF4, whereas a deprotonation–migra-
tion–coordination sequence from 2?BF4 affords the other isomer
4?BF4 by way of 5. Accordingly, treatment of 5 with [Pd(g3-
allyl)(cod)]BF4 gave 4a?BF4 quantitatively.
{ Electronic supplementary information (ESI) available: experimental
details, spectroscopic and crystallographic data. See http://www.rsc.org/
suppdata/cc/b4/b409514f/
2 7 6 0
C h e m . C o m m u n . , 2 0 0 4 , 2 7 6 0 – 2 7 6 1
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Few metal migration reactions on a polynucleating ligand have
been reported and their reaction mechanisms remain to be clarified.
For example, Oro et al.7 reported a s-1,2-metallotropic shift on
dirhodium m-pyrazolato complexes, which was interpreted in terms
of a m–g1:g2-pyrazolato intermediate. Further examination of the
reaction mechanism reveals that the present migration process is
not unimolecular but that metal migration occurs in a dimeric
intermediate. The 31P NMR signal for 2?BF4 appears at dP 25.3,
which is distinct from those of the free ligand 1-H as well as
chelated species (e.g. 5: dP 31.2), but suggests some coordination.
No dynamic behavior is applicable, because solid 31P NMR
measurements also give a single resonance in the same region
(dP 0.6). On the basis of the ESI-MS data containing peaks at
m/z ~ 1376 [2 6 2 1 BF4 (193Ir)] and 1288 [2 6 2 – H (193Ir)], we
propose an oligomeric structure, most likely a dimeric one
[22?(BF4)2],8 and the present intriguing selective formation of the
isomeric species can be explained by the mechanisms shown in
Scheme 3.
We are grateful to the Japanese Government for financial
support of the present research (Grant-in-Aid for Scientific
Research on Priority Area, No. 16033219) and a Monbukaga-
kusho scholarship (C.D.). We also thank Dr Hiroki Fukumoto for
X-ray data collection.
Notes and references
{ Selected spectroscopic data: 1-H: dH 6.48 (pz), 8.56 (H6). 2?BF4: dH 5.10
(pz), 7.98 (H6). 3a?BF4: dP 34.0; dH 6.76 (pz), 7.73 (H6). 3b?BF4: dP 38.8 (d,
J
P–Rh ~ 154.6 Hz); dH 6.71 (pz), 7.87 (H6). 4a?BF4: dP 37.7; dH 6.83 (pz),
8.53 (H6). 4b?BF4: dP 38.1; dH 6.80 (pz). 5?BF4: dP 31.2; dH 6.73 (pz), 8.50
(H6). For cod complexes, the NN-coordination can be confirmed by a
combination of dH(H6) and dP data. The H6 signal appears in higher field.
§ X-Ray data collections were carried out with a Rigaku RAXIS-IV
imaging plate area detector at 260 uC. 3a?BF4: C32H34BN3F4PPdIr,
¯
˚ ˚
w ~ 877.04, triclinic, space group P1, a ~ 10.898(6) A, b ~ 11.791(8) A,
M
˚
c ~ 12.960(8) A, a ~ 96.74(2)u b ~ 11.63(2)u, c ~ 95.51(2)u, V ~
1519(1) A , Z ~ 2, dcalcd ~ 1.916 g cm23, R1 ~ 0.073 (refined on F2) for
3
˚
4754 data (I
C
w
32.5H35BN3F4PPdIr, Mw ~ 919.47, triclinic, space group P1, a ~
2s(I)) and 388 parameters. 4a?BF4?(CH2Cl2)0.5:
¯
˚
˚
˚
13.877(9) A, b ~ 14.786(13) A, c ~ 16.189(14) A, a ~ 83.74(4)u b ~
79.66(3)u, c ~ 88.57(3)u, V ~ 3248(5) A , Z ~ 2, dcalcd ~ 1.88 g cm23
,
3
˚
R1 ~ 0.063 (refined on F2) for 3480 data (I w 2s(I)) and 737 parameters.
5: C29H62N6P2Ir2, Mw ~ 642.72, triclinic, space group P1, a ~ 9.341(5) A,
¯
˚
˚
˚
b ~ 9.958(5) A, c ~ 13.571(9) A, a ~ 92.45(5)u, b ~ 102.39(4)u, c ~
98.18(2)u, V ~ 1216.9(13) A , Z ~ 2, dcalcd ~ 1.754 g cm23, R1 ~ 0.058
3
˚
(refined on F2) for 2686 data (I w 2s(I)) and 307 parameters. 8?(CH2Cl2)2:
C60H62N6P2Cl4Ir2, Mw ~ 1455.30, triclinic, space group P1, a ~
¯
˚
˚
˚
˚
11.130(18) A, b ~ 14.56(2) A, c ~ 18.75(2) A, a ~ 78.51(6)u b ~ 85.37(6)u,
c ~ 71.38(6)u, V ~ 2823(7) A , Z ~ 2, dcalcd ~ 1.712 g cm23, R1 ~ 0.0727
3
(refined on F2) for 2858 data (I w 2s(I)) and 627 parameters. CCDC
243413–243416. See http://www.rsc.org/suppdata/cc/b4/b409514f/ for crys-
tallographic data in .cif or other electronic format.
1 R. D. Adams, in Comprehensive Organometallic Chemistry II, ed.
E. W. Abel, F. G. A. Stone and G. Wilkinson, Pergamon, Oxford, 1995,
vol. 10.
2
Scheme 3 BF4 is omitted for clarity.
2 A. L. Gavrilova and B. Bosnich, Chem. Rev., 2004, 104, 349.
3 See for example M. Konrad, S. Wuthe, F. Meyer and E. Kaifer, Eur.
J. Inorg. Chem., 2001, 2233; J. C. Ro¨der, F. Meyer, R. F. Winter and
E. Kaifer, J. Organomet. Chem., 2002, 641, 113; S. Baitalik, U. Flo¨rke and
K. Nag, Inorg. Chim. Acta, 2002, 337, 439.
4 The PNNN-H ligand (1-H), which was a mixture of PN- and NN-
protonated tautomers, was prepared following the conventional reaction
sequence. (i) (COOEt)2/NaOEt. (ii) N2H4?H2O. (iii) LiAlH4. (iv) SOCl2.
(v) LiPPh2. (See ESI.)
Coordination of 1 to [Ir(cod)2]BF4 gives the NN-coordinated
species 2?BF4, which is in equilibrium with the five-coordinate
dimeric form 22(BF4)2, and the equilibrium is shifted to the dimer,
which precipitates out. This can be explained by the Lewis acidity
of the cationic Ir center, which should prefer five-coordination to
four-coordination. The second metal species interacts with 2?BF4
present as a minor component in the solution to give the adduct
6(BF4)2, which is converted to 3?BF4 upon deprotonation. On the
other hand, initial deprotonation of 2?BF4 should give the
intermediate 7, which dissociates into the mononuclear species 5,
because the metal center in a neutral species is less Lewis acidic and
may not be coordinated by a fifth donor. Subsequent NN-
coordination to the second metal species furnishes the other isomer
4?BF4. It should be noted that, in the metal migration process (7 A
5), the iridium fragments are transferred to the other ligand and as
a result, the coordination site is switched. Intervention of a five-
coordinated species is supported by formation of 8, a dimer of 5
(Scheme 2).9
.
5 PNNP ~ 3,5-bis(diphenylphosphinomethyl)pyrazolate: T. G. Schenck,
J. M. Downes, C. R. C. Milne, P. B. Mackenzie, H. Boucher, J. W. Whelan
and B. Bosnich, Inorg. Chem., 1985, 24, 2334.
6 S. Tanaka and M. Akita, Angew. Chem., Int. Ed., 2001, 40, 2865;
S. Tanaka, C. Dubs, A. Inagaki and M. Akita, Organometallics, 2004, 23,
317.
Notable features of the present selective isomer formation are
that (i) the flexible coordination structure (four- vs. five-coordina-
tion), charge (neutral vs. cationic) and Lewis acidity of the metal
center are switched by the deprotonation–protonation process and
(ii) the supramolecular intermediate 7 undergoes interligand metal
migration accompanying the shift from the NN- to the PN-
coordination site.
Thus pairs of regioisomers of heterodinuclear species are
synthesized with perfect selectivity from the same sources by
simply changing the addition procedure of the reagents and the
present study provides a new concept of combinatorial approach to
a variety of transition metal complexes. The reactivity of the
isomeric heteronuclear complexes with the two organic ligands
being located in close proximity is now being studied.
7 Y. Yuan, M. V. Jime´nez, E. Sola, F. J. Lahoz and L. A. Oro, J. Am.
Chem. Soc., 2002, 124, 752; C. Tejel, J. M. Villoro, M. A. Ciriano,
J. A. Lo´pez, E. Eguiza´bal, F. J. Lahoz, V. I. Bakhmutov and L. A. Oro,
Organometallics, 1996, 15, 2967.
8 Recently we found that the symmetrical PNNP ligand also gave an
intermediate analogous to 22?(BF4)2, which showed two doublet 31P-
NMR signals coupled with each other suggesting coordination of two
phosphorous atoms in different environments to the iridium center.
C. Dubs, A. Inagaki and M. Akita, unpublished data.
9 While only monomer 5 was detected by spectroscopic analyses of a
solution sample, two forms of single crystals (5 and 8) were isolated from a
solid sample and characterized by X-ray crystallography.§ The formation
of 8 suggests that the present system consists of a network of equilibria
and another metal migration process should be involved in the formation
of 8 from 5.
C h e m . C o m m u n . , 2 0 0 4 , 2 7 6 0 – 2 7 6 1
2 7 6 1