[OsIII(tpy)(Cl)(NCCH3)(NSAr)]: Reversible reduction of acetonitrile by OsIII-sulfilimido complexes

Article abstract of DOI:10.1002/1521-3773(20021018)41:20<3870::AID-ANIE3870>3.0.CO;2-Q

The reversible two-electron reduction of an acetonitrile ligand to the corresponding imine has been demonstrated for the first time with the title compound. Electrochemical reduction generates the OsIII-imino product [Eq. (a)], which can be isolated as the PF₆^- salt and used in further reactions for the reduction of organic carbonyl compounds. Ar = C₆H₅, 4-MeC₆H₄, 3,5-Me₂C₆H₃.

Full text of DOI:10.1002/1521-3773(20021018)41:20<3870::AID-ANIE3870>3.0.CO;2-Q

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[OsIII(tpy)(Cl)(NCCH3)(NSAr)]: Reversible
Reduction of Acetonitrile by OsIII Sulfilimido
Complexes**
2000, 56, 9781 9790; b) G. Dewynter, M. Abdaoui, L. Toupet, J.-L.
Montero, Tetrahedron Lett. 1997, 38, 8691 9694.
[11] K. C. Nicolaou, X. Huang, S. A. Snyder, P. Bheema Rao, M. Bella,
M. V. Reddy, Angew. Chem. 2002, 114, 862 866; Angew. Chem. Int.
Ed. 2002, 41, 834 838.
My Hang V. Huynh,* R. Thomas Baker,
David E. Morris, Peter S. White, and
Thomas J. Meyer*
[12] a) G. M. Atkins, E. M. Burgess, J. Am. Chem. Soc. 1968, 90, 4744
4745; b) G. M. Atkins, E. M. Burgess, J. Am. Chem. Soc. 1972, 94,
6135 6141; c) E. M. Burgess, H. R. Penton, E. A. Taylor, J. Org.
Chem. 1973, 38, 26 31.
[13] For a review of these topics, see a) P. Taibe, S. Mobashery in
Encyclopedia of Reagents for Organic Synthesis, Vol. 5 (Ed.: L. A.
Paquette), Wiley, Chichester, 1995, pp. 3345 3347; b) S. Burckhardt,
Synlett 2000, 559. For some additionalprimary literature, see c) E. M.
Burgess, H. R. Penton, E. A. Taylor, J. Am. Chem. Soc. 1970, 92,
5224 5226; d) M. R. Wood, J. Y. Kim, K. M. Books, Tetrahedron Lett.
2002, 43, 3887 3890.
[14] Amino alcohol substrates 14, 16, and 18 were prepared by the method
of H. W. Heine, B. L. Kapur, J. L. Bove, R. W. Greiner, K. H. Klinger,
C. Mitch, J. Am. Chem. Soc. 1954, 76, 2503. For alternative approaches
to fashion these compounds, see J. T. Rudesill, R. F. Severson, J. G.
Pomonis, J. Org. Chem. 1971, 36, 3071 3076.
[15] The literature pertaining to this field is so vast that it even has its own
journalentiteld Benzodiazepine. For additionalgeneralreferences,
see information contained within: Comprehensive Medicinal Chem-
istry (Eds.: C. Hansch, P. G. Sammes, J. B. Taylor), Pergamon, Oxford,
1990.
[16] For an alternative synthesis of linear sulfamides, see F. A. Davis, M. A.
Giangiordano, W. E. Starner, Tetrahedron Lett. 1986, 27, 3957 3960.
[17] We would like to emphasize that all of our reported novel Burgess-
type reagents (including 2 and 3) are readily isolable, can be purified
through trituration with toluene, and then stored at 08C for several
months; the recent use of 3 in solution by a Merck group, therefore,[13d]
should not be taken as a sign of instability in the solid form.
[18] Notes: 1) For the synthesis of nonsymmetrical linear sulfamides
(Table 4), only 1.3 equiv of Burgess reagent is used to facilitate the
reaction process. 2) HClsatls of the starting amines and amino
alcohols do not provide good conversion into the desired product. The
free base must be isolated prior to reaction with 1 3. 3) Removalof
protecting groups was carried out by using conventionalprotocosl:
CO2Me: NaOH, MeOH:H2O (2:1), 258C, 2 h;[19] Cbz: 10% Pd/C, H2,
EtOH:EtOAc (4:1), 258C, 5 h;[20] Alloc: Pd(OAc)2 (10 mol%),
3,3’,3’’-phosphinidynetris(benzenesulfonic acid) trisodium salt
(20 mol%), HNEt2 (40 equiv), MeCN:H2O (1:1), 258C, 15 min;[21]
Bn: 20% Pd(OH)2/C, H2 (60 psi), EtOH:EtOAc (4:1), 258C, 24 h.[22]
Subsequent alkylation was carried out by using a conventional
protocol: sulfamide, NaH, DMF, 0 8C to 258C then alkyl halide,
tetrabutylammonium iodide (cat), 258C 4 h.[2d,5]
Earlier reports have revealed an extensive electron pro-
ton, reversible redox chemistry based on the OsV/IV cou-
V
þ
IV
À
ples
[Os (tpy)(Cl)2{NN(CH2)4O}] /[Os (tpy)(Cl)2{N(H)
N(CH2)4O}]þ (tpy ¼ 2,2’:6,2’’-terpyridine, N(CH2)4O is a mor-
pholide),[1]
[OsV(tpy)(Cl)2{NS(3,5-C6H3Me2)}]þ/[OsIV(tpy)-
(Cl)2{NS(H)(3,5-C6H3Me2)}]þ,[2] and [OsV(Tp)(Cl)2(NPEt2)]/
[OsIV(Tp)(Cl)2{NP(H)Et2}]
(Tp ¼ tris(pyrazolyl)borate).[3]
These couples are reminiscent of oxo/hydroxo/aqua couples
such as cis-[RuIV(bpy)2(py)(O)]2þ/cis-[RuIII(bpy)2(py)(OH)]2þ
and cis-[RuIII(bpy)2(py)(OH)]2þ/cis-[RuII(bpy)2(py)(H2O)]2þ
(bpy ¼ 2,2’-bipyridyl, py ¼ pyridyl), where the change in
proton content between oxidation states is a consequence of
differences in proton acidity of severalorders of magnitude. [4]
We report here the existence of a related electron pro-
ton chemistry, but based on the reversible two-electron
reduction of a nitrile ligand and the couples [OsIII(tpy)(Cl)-
III
ꢀ
¼
(N CCH3) (NSAr)] / [Os (tpy) (Cl) (NH CHCH3) (NSAr)]
(Ar¼ C6H5, 4-MeC6H4, 3,5-Me2C6H3). At more negative
potentials, the OsIII imino complexes undergo further reduc-
tion and solvolysis to give [OsIII(tpy)(Cl)(NCCH3)(NSAr)]
and CH3CH2NH2.
A rapid reaction occurs when cis-[OsVI(tpy)(Cl)2(N)]þ
([1]þ) and aromatic thiols (C6H5SH, 4-MeC6H4SH, or
3,5-Me2C6H3SH) are mixed in CH3CN under N2 at
room temperature [Eq. (1)]. The products are cis-
[*] Dr. T. J. Meyer
The Associate Director for Strategic Research
Los Alamos National Laboratory, MS A127
Los Alamos, NM 87545 (USA)
Fax : (þ 1)505-667-5450
E-mail: tjmeyer@lanl.gov
[19] B. Pete, I. Bitter, K. Havsµnyi, L. Tke, Heterocycles 2000, 53, 665
673.
[20] I. Levchine, P. Rajan, M. Borloo, W. Bollaert, A. Haemers, Synthesis
1994, 37 39.
[21] a) J.-P. GenÜt, E. Blart, M. Savignac, S. Lemeure, J.-M. Paris,
Tetrahedron Lett. 1993, 34, 4189 4192; b) J.-P. GenÜt, E. Blart, M.
Savignac, Synlett 1992, 715.
Dr. M. H. V. Huynh,þ Dr. R. T. Baker, Dr. D. E. Morris
Chemistry Division, C-SIC, MS J514
Los Alamos National Laboratory
Los Alamos, NM 87545 (USA)
E-mail: huynh@lanl.gov
Dr. P. S. White
Department of Chemistry
[22] R. C. Bernotas, R. V. Cube, Synth. Commun. 1990, 20, 1209 1212.
University of North Carolina at Chapel Hill
Chapel Hill, NC 27599 (USA)
[þ] Current address:
Dynamic Experiments Division
DX-2: HE Science and Technology Group, MS C920
Los Alamos National Laboratory
Los Alamos, NM 87545 (USA)
Fax : (þ 1)505-667-0500
[**] We are gratefulto the Laboratory Directed Research and Deveol p-
ment Program for support of this research. M.H.V.H. gratefully
acknowledges postdoctoral fellowship support from the Director©s
Office of Los Alamos National Laboratory. Los Alamos National
Laboratory is operated by the University of California for the U.S.
Department of Energy under Contract W-7405-ENG-36.
Supporting information for this article is available on the WWW under
http://www.angewandte.org or from the author.
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potentials compared to those for cis-[OsIV(tpy)(Cl)2{NS(H)-
C6H3Me2}]þ with þ 1.21 V for OsV/IV, À0.09 V for OsIV/III, and
À1.24 V for OsIII/II. The more negative potentials result from
the exchange of the electron-donating ClÀ ligand for CH3CN.
The corresponding couples for cis-[OsIV(tpy)(NCCH3)2-
(NSC6H3Me2)]2þ appear at þ 1.94 V for OsV/IV, þ 0.98 V for
[OsIV(tpy)(Cl)2{NS(H)Ar}]þ (Ar¼ C6H5 ([2A]þ), 4-MeC6H4
([2B]þ), 3,5-Me2C6H3 ([2C]þ)).[5]
OsIV/III, and þ 0.09 V for OsIII/II [6]
.
The cyclic voltammetric characteristics of [3C]þ in H2O/
CH3CN (1/1, v/v; I ¼ 1.0 m in NH4PF6) are strikingly different
from those in CH3CN (Figure 2).[13] Besides the electrochemi-
Addition of a small amount of water to [2]þ in CH3CN
causes solvolysis and deprotonation to give the monosolvento
complexes [OsIV(tpy)(Cl)(NCCH3)(NSAr)]þ (Ar¼ C6H5
([3A]þ), 4-MeC6H4 ([3B]þ), 3,5-Me2C6H3 ([3C]þ)) on a time
scale of minutes and the corresponding disolvento complexes
cis-[OsIV(tpy)(NCCH3)2(NSAr)]2þ on a time scale of hours.[6]
À
The mononitrile complexes were isolated as their PF6 salts
and characterized by elemental analysis and cyclic voltam-
1
metry as well as H NMR, infrared, and UV/Vis spectros-
copies.[5]
Crystals of [3B]PF6 were grown by slow diffusion of Et2O
into a solution in CH3CN.[7] The structure shows that the
distorted octahedralarrangement of ilgands at Os in the cis
parent nitrido complex is retained (Figure 1).[7] The Os
N(tpy) bond lengths range from 1.948(6) to 2.078(7) ä with
À
À
À
the shortest one trans to the Os N(CH3CN) bond. The Os
À
N(sulfilimido) and N4 S1 bond lengths of 1.899(8) and
1.535(9) ä, respectively, are consistent with multiple bond-
ing.[8 The angle N4-S1-C16 is 107.9(6)8 and thus indicates
10]
Figure 2. A) Cyclic voltammogram of [3C]þ in CH3CN/H2O (1/1, v/v) at
pH 0.48 and I ¼ 1.0 m in NH4PF6 (V versus SSCE). B) The OsIV/III couple in
0.2m CH3CN/NBu4PF6. C) The OsIII/II couple under the same conditions as
in (B). Inset: The pH dependence of E1/2 for the OsIV/III couple of [3C]þ in
CH3CN/H2O from pH 0.48 to 8.50. A line of slope 57 mV/pH unit is shown
drawn through the data.
pseudo sp3 hybridization at the S atom of the sulfilimido
À
ligand. The Os N(sulfilimido) bond length, the bent angle
Os1-N4-S1 of 129.3(5)8, and the diamagnetism of the com-
plexes (as shown by 1H NMR spectroscopy) are all consistent
with d4 spin-paired OsIV. There are structuralsimailrities
with related complexes such as [OsIV(tpy)(Cl)(NCCH3)-
{NN(CH2)4O}]þ,[11] cis-[OsIV(tpy)(NCCH3)2{NN(CH2)4O}]2þ,[12]
and trans-[OsIV(tpy)(NCCH3)2(NSC6H4Me)]2þ.[7]
cally reversible OsIV/III wave at 0.67 V, a reversible, pH-
dependent two-electron wave appears at E1/2 ¼ þ 0.29 V at
pH 0.48 and an irreversible, multiple-electron reduction wave
is seen at Epc ¼ À0.54 V. The peak-to-peak separation for the
two-electron, reversible wave is 37 mV, which is half that of
the OsIV/III wave (73 mV) and consistent with a two-electron
process. Comparison of relative peak currents confirms
that the couple at E1/2 ¼ 0.29 V is a two-electron couple.
Measurements of E1/2 as a function of pH value show
that in the case of [3C]þ the E1/2 value for the two-
electron wave decreases by 57 mV/pH unit from
pH 0.48 to 8.50 (Figure 2, inset). This is consistent with
the chemically and electrochemically reversible 2eÀ/
2Hþ couple shown in Equation (2). Similar results have
Cyclic voltammograms of [3C]þ in dry CH3CN (5.0 î
10À3 m) containing 0.1m NBu4PF6 show the following couples:
OsV/IV at þ 1.56 V, OsIV/III at þ 0.65 V, and OsIII/II at À0.79 V
(versus SSCE). These couples are shifted to more positive
½ðtpyÞðClÞðNSC6H3Me2ÞOsIIIÀNꢀCCH3Š þ 2 eÀ þ 2 Hþ
! ½ðtpyÞðClÞðNSC6H3Me2ÞOsIIIÀNH¼CHCH3Š
ð2Þ
been obtained for [3A]þ and [3B]þ with the 2eÀ/2Hþ
waves appearing at 0.31 V and 0.30 V, respectively, at
pH 0.48.
The reduced OsIII imino product shown in Equa-
tion (2) was electrochemically generated in CH3CN/
H2O (1/1, v/v; pH 2.5, I ¼ 0.5m NH4PF6) at Eapp
¼
À0.35 V with n ¼ 2. Once formed, [OsIII(tpy)(Cl)-
¼
Figure 1. ORTEP diagram (30% ellipsoids) for [3B]PF6.
(NH CHCH3)(NSC6H3Me2)] undergoes air oxidation
Angew. Chem. Int. Ed. 2002, 41, No. 20
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IV
þ
¼
to [Os (tpy)(Cl)(NH CHCH3)(NSC6H3Me2)] within mi-
complex acts as an electron proton donor in CH3CN/H2O
nutes and then to [3C]þ in about 1 h.[14] The imine complex
was isolated as the PF6 salt and characterized by elemental
(3/1, v/v; pH 1.8, I ¼ 0.5m in NH4PF6) toward 1) benzaldehyde
À
with reduction to benzylaclohol[
kPhCHO(25.0 ꢁ 0.18C) ¼
1
analysis as well as UV/Vis, H NMR, and infrared spectros-
(1.03 ꢁ 0.01) î 10À2 mÀ1 sÀ1, Eq. (4)], 2) benzoquinone with
copies.[5] We have so far been unsuccessfulat
growing a suitable crystal for X-ray structure
analysis.
The pH dependence of the irreversible,
multiple-electron wave was also investigated
in CH3CN/H2O (1/1, v/v). The Epc values for this
wave decrease by 55 mV/pH unit from Epc ¼
À0.57 V at pH 0.48 to Epc ¼ À0.69 V at
pH 2.5.[5] Above pH 2.5, the peak current falls
by a factor of 4, and Epc becomes pH-independ-
ent. Electrolysis of [3C]þ at Eapp ¼ À0.85 V in
CH3CN/H2O at pH 2.2 at 58C to n ¼ 4 followed
by investigation with GC-MS reveals ethyl-
amine as a product. The retention of the initial
cyclic voltammogram at the end of the elec-
trolysis and the appearance of ethylamine are
consistent with 2eÀ/2Hþ reduction followed by
rapid solvolysis as shown in Equation (3). Reduction of the
reduction to hydroquinone [kQ(25.0 ꢁ 0.18C) ¼ (3.63 ꢁ
0.01) î 102 mÀ1 sÀ1, Eq. (5)], and 3) dipyridylketone with
½ðtpyÞðClÞðNSC6H3Me2ÞOsIIIÀNH¼CHCH3Š þ 2 eÀ þ 3 Hþ
reduction to the corresponding alcohol [kPy CO(25.0 ꢁ
2
0.18C) ¼ (1.07 ꢁ 0.01) î 10À2 mÀ1 sÀ1, Eq. (6)]. The reduced
products in the CH3CN/H2O reaction mixtures were quanti-
tatively determined by GC-MS.[25]
CH3CN=H2
O
þ CH CN
½ðtpyÞðClÞðNSC H Me ÞOsIIIÀNꢀCCH3Š
ð3Þ
ƒƒƒƒƒƒ!
3
6
3
2
pH 2:2
þ CH3CH2NHþ2
Received: May 13, 2002 [Z19288]
nitrile complex is catalytic at À0.85 V, but changes occur in
the complex upon electrolysis for extended periods of time, as
shown by electrochemical measurements. The underlying
chemistry remains to be investigated in detail.
[1] M. H. V. Huynh, P. S. White, T. J. Meyer, J. Am. Chem. Soc. 1999, 121,
4530 4531.
[2] M. H. V. Huynh, P. S. White, T. J. Meyer, Angew. Chem. 2000, 112,
4267 4270; Angew. Chem. Int. Ed. 2000, 39, 4101 4104.
[3] M. H. V. Huynh, T. J. Meyer, Angew. Chem. Int. Ed. 2002, 114, 1453
1456; Angew. Chem. Int. Ed. 2002, 41, 1395 1398.
There is literature precedence for the reduction of coordi-
2
À
ꢀ
À
¼
nated acetonitrile ( Na Cb CH3) to acetimidoyl( h -N(H)
2
À
¼
C CH3; bonded through Na and Cb), ethylidenimido (h -N
À
CH CH3; bonded to M through Na and to M’ through the
[16]
double bond),[15] azavinylidenyl ( N CH CH3),
imino
À ¼
À
[4] R. A. Binstead, M. E. McGuire, A. Dovletoglou, W. K. Seok, L. E.
Roecker, T. J. Meyer, J. Am. Chem. Soc. 1992, 114, 173 186.
[5] See the Supporting Information.
[16 17]
[18]
À
¼
À
ꢀ À
À
( NH CH CH3),
ethylimido ( N CH2 CH3), ethyl-
[16]
3
À
À
À
À
À
amido ( NH CH2 CH3),
and aza-allylic (h -CH2 CH
[6] In CH3CN/H2O (1/1, v/v), solvolysis of [3]þ occurs on a time scale of 4
5 h to give cis-[OsIV(tpy)(NCCH3)2(NSAr)]2þ. Details will be reported
elsewhere.
NH2)[19] ligands. In these reactions, hydrogen[20] and/or
hydride (HÀ,[21] NaBH4,[22] LiHBEt3,[16] and LiAlH4[22]) are
the reductants.
[7] CCDC-184082 ([OsIV(tpy)(Cl)(NCCH3)(NSC6H4Me)]þ), CCDC-
184083 (cis-[OsVI(tpy)(Cl)2(N)]þ), and CCDC-184084 (trans-
There is also evidence for the protonation of neutral
acetonitrile complexes, either diprotonation at Cb by HClor
monoprotonation at Na and Cb by HBF4¥Et2O. At electron-
rich metalcenters, protonation is accompanied by oxidation
[OsIV(tpy)(NCCH3)2(NSC6H4Me)]2þ
) contain the supplementary
crystallographic data for this paper. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/conts/retrieving.html(or from the
Cambridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB21EZ, UK; fax: (þ 44)1223-336-033; or deposit@ccdc.ca-
m.ac.uk).
[23]
of the metalcenter to form ethyilmido
or imino com-
plexes.[19] There are also examples of hydride formation at the
metal.[24]
[8] M. H. V. Huynh, P. S. White, T. J. Meyer, Angew. Chem. 2000, 112,
4267 4270; Angew. Chem. Int. Ed. 2000, 39, 4101 4104.
[9] M. H. V. Huynh, P. S. White, T. J. Meyer, J. Am. Chem. Soc. 2001, 123,
9170 9171.
[10] M. H. V. Huynh, E.-S. El-Samanody, K. D. Demadis, P. S. White, T. J.
Meyer, Inorg. Chem. 2000, 39, 3075 3085.
[11] M. H. V. Huynh, E.-S. El-Samanody, P. S. White, T. J. Meyer, Inorg.
Chem. 1999, 38, 3760 3761.
[12] M. H. V. Huynh, P. S. White, T. J. Meyer, Inorg. Chem. 2001, 40, 5231
5235.
Our result is the first example of reversible 2eÀ/2Hþ
reduction of the acetonitrile ligand to the corresponding
imine. It may presage an extensive proton-coupled electron
transfer or hydride transfer chemistry analogous to that of
2þ
II
2þ
RuIV O /Ru OH2 couples but with proton and electron
¼
À
acceptor sites on the two-atom nitrile ligand (OsIII N CCH3/
À ꢀ
OsIII NH CHCH3).
À
¼
[13] In these measurements, all buffer solutions from pH 0.0 to pH 14.0
have an ionic strength of 1.0m in NH4PF6: HNO3 for pH 0 2, KHP for
pH 3 5, KH2PO4 and Na2HPO4 for pH 6 8, Na2B4O7¥10H2O for
pH 9 10, Na2HPO4 for pH 11, NaOH for pH 12 14. The pH values
III
¼
Initialexperiments show that [Os
(tpy)(Cl)(NH
CHCH3)(NSC6H3Me2)] undergoes hydrogen transfer with
reduction of an organic carbonylcompound. The imino
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COMMUNICATIONS
reported are those measured in the mixed solvents and are the same to
in the organometallic transformations of metal centers.
Alkylating agents may attack not only the imino function[2]
but also all of the pyridine ring carbon atoms[3,4] and even the
nitrogen atom.[5] In addition, dimerization may be achieved by
imine reductive coupling[4] or by the cycloaddition of two
pyridine rings to form a tricyclic system.[4] In parallel with all
of these transformations, either one or both of the CH3 groups
attached to the imine functions may be partly deprotonat-
ed.[4,5] In this unusualvariety of transformation, the metal
center coordinated to the ligand may engage in redox
reactions and may either lower or even increase its oxidation
state.[3,6,7] This behavior illustrates the unique ability of this
ligand system to a) accept negative charge with preferential
spin-density localization on the imine groups and on the
pyridine N and Cpara atoms, and b) to engage in internalredox
processes with the coordinated metal. The ability of the ligand
to accept or to donate negative charge to the metalis
paramount to the fine-tuning of the redox potentialof the
metal, and of its Lewis acidity which, in turn, determines the
catalytic behavior of the metal complex.
within experimentalerror of those measured in CH CN/H2O.
3
[14] In the absence of air, [OsIV(tpy)(Cl)(NH CHCH3)(NSC6H3Me2)]
þ
¼
does not undergo ligand substitution to form [3C]þ in CH3CN/H2O (1/
1, v/v).
[15] a) M. A. Andrews, H. D. Kaesz, J. Am. Chem. Soc. 1979, 101, 7238
7244; b) M. A. Andrews, H. D. Kaesz, J. Am. Chem. Soc. 1979, 101,
7255 7259; c) M. A. Andrews, G. Vanbuskirk, C. B. Knobler, H. D.
Kaesz, J. Am. Chem. Soc. 1979, 101, 7245 7254; d) M. A. Andrews,
H. D. Kaesz, J. Am. Chem. Soc. 1979, 101, 7260 7264.
[16] S. G. Feng, J. L. Templeton, J. Am. Chem. Soc. 1989, 111, 6477 6478.
[17] W. Y. Yeh, C. S. Ting, S. M. Peng, G. H. Lee, Organometallics, 1995,
14, 1417 1422, and references therein.
[18] a) S. J. Anderson, F. J. Wells, G. Wilkinson, B. Hussain, M. B. Hurst-
house, Polyhedron 1988, 7, 2615 2626; b) A. R. Barron, J. E. Salt, G.
Wilkinson, M. Motevalli, M. B. Hursthouse, J. Chem. Soc. Dalton
Trans. 1987, 2947 2954; c) M. Bakir, P. E. Fanwick, R. A. Walton,
Inorg. Chem. 1988, 27, 2016 2017.
[19] B. S. McGilligan, T. C. Wright, G. Wilkinson, M. Motevalli, M. B.
Hursthouse, J. Chem. Soc. Dalton Trans. 1988, 1737 1742.
[20] a) A. A. Sutyagina, T. V. Kulchitskaya, G. D. Vovchenko, Sov. Electro-
chem. 1975, 11, 1335 1337; b) A. A. Sutyagina, T. V. Kulchitskaya,
T. L. Balabontseva, G. D. Vovchenko, Vestn. Mosk. Univ. Ser. 2: Khim.
1976, 17, 332 335.
[21] L. F. Rhodes, L. M. Venanzi, Inorg. Chem. 1987, 26, 2692 2695.
[22] J. March, Advanced Organic Chemistry, 3rd ed., Wiley Interscience,
New York, 1985, p. 809, and reference [1] on p. 815.
Given the above scenario, we became interested in clarify-
ing the ability of this remarkable ligand system to accept
negative charge. For this purpose, we have carried out the
[23] H. Seino, Y. Tanabe, Y. Ishii, M. Hidai, Inorg. Chim. Acta 1998, 280,
163 171.
¼
reduction of [a,a’-{2,6-(iPr)2PhN C(Me)}2(C5H3N)] with
[24] a) J. J. R. F. Dasilva, M. F. C. G. Dasilva, R. A. Henderson, A. J. L.
Pombeiro, R. L. Richards, J. Organomet. Chem. 1993, 461, 141 145;
b) A. J. L. Pombeiro, D. L. Hughes, R. L. Richards, J. Chem. Soc.
Chem. Commun. 1988, 1052 1053; c) A. J. L. Pombeiro, Inorg. Chim.
Acta 1992, 200, 179 186; d) A. J. L. Pombeiro, New J. Chem. 1994, 18,
163 174.
[25] The organic products from the reduction reactions were analyzed by
GC-MS by using calibration curves and procedures similar to those
found in ref. [12]. A typicalproduct anaylsis is described in the
Supporting Information.
strong reductants such as Li and [Li(naphthalenide)], in the
absence of transition metals. Herein we describe our findings.
The reactions were carried out by treating a solution of
¼
[a,a’-{2,6-(iPr)2PhN C(Me)}2(C5H3N)] with either metallic Li
under argon or [Li(naphthalenide)] under nitrogen in THF.
Regardless of the stoichiometric ratio, the reduction with
¼
Li afforded
a
mixture of [a,a’-{2,6-(iPr)2PhN C(Me)}2-
¼
(C5H3N)][Li(thf)]3 (1) and [{a-[2,6-(iPr)2PhN C(Me)]}-
¼
{a’-[2,6-(iPr)2PhN-C( CH2)]}C5H3N][Li(thf)]2[Li(thf)2] (2)
through a readily reproducible process. Both species contain
a trianionic ligand (Scheme 1). Separation of the two ex-
The Ability of the a,a’-Diiminopyridine Ligand
Systemto Accept Negative Charge: Isolation of
Paramagnetic and Diamagnetic Trianions**
David Enright, Sandro Gambarotta,*
Glenn P. A. Yap, and Peter H. M. Budzelaar*
Recent work, both in this research group and elsewhere, has
shown that the a,a’-diiminopyridine ligand [a,a’-{2,6-
¼
(iPr)2PhN C(Me)}2(C5H3N)], well known for its ability to
form highly active polymerization catalysts,[1] can be involved
[*] Prof. S. Gambarotta, D. Enright, Dr. G. P. A. Yap
Department of Chemistry
Scheme 1.
University of Ottawa
Ottawa, Ontario, K1N 6N5 (Canada)
Fax : (þ 1)613-5672-5170
tremely air-sensitive species was possible because of their
relative solubilities in hexane. However, we observed that
reactions carried out at low temperature with 3 equiv of
[Li(naphthalenide)] afforded 2 as the only isolated compound
(67%) and no evidence for the presence of 1.
The connectivity of 1 was elucidated by an X-ray crystal
structure (Figure 1). The complex contains the intact ligand,
which adopts the usualchealting tridentate conformation
E-mail: sgambaro@science.uottawa.ca.
Dr. P. H. M. Budzelaar
Department of Inorganic Chemistry
University of Nijmegen
Toernooiveld 1, 6525 ED Nijmegen (The Netherlands)
Fax : (þ 31)24-355-3450
E-mail: budz@sci.kun.nl
[**] This work was supported by the NaturalSciences and Engineering
Councilof Canada (NSERC).
Angew. Chem. Int. Ed. 2002, 41, No. 20
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0044-8249/02/4120-3873 $ 20.00+.50/0
3873