Controlling the oxidation state of arsenic in cyclic arsenic cations

Article abstract of DOI:10.1039/b602017h

Reduction of AsCl₃ with SnCl₂, followed by treatment of the "AsCl" with a 1,4-diimine results in electron transfer and formation of an arsenic(iii) salt, while treatment of this arsenic(i) reagent or AsI₃ with an α,α′-diim

Full text of DOI:10.1039/b602017h

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Controlling the oxidation state of arsenic in cyclic arsenic cations
Gregor Reeske and Alan H. Cowley*
Received (in Austin, TX, USA) 10th February 2006, Accepted 26th February 2006
First published as an Advance Article on the web 20th March 2006
DOI: 10.1039/b602017h
mode. The 1H NMR chemical shift data indicated that the
Reduction of AsCl3 with SnCl2, followed by treatment of the
MesDAD ligand remained intact. Additional insight was gained
from a single-crystal X-ray diffraction study of 28 (Fig. 1) which
revealed that the solid-state structure features two independent
anion–cation pairs in the asymmetric unit. The closest anion–
‘‘AsCl’’ with a 1,4-diimine results in electron transfer and
formation of an arsenic(III) salt, while treatment of this
arsenic(I) reagent or AsI3 with an a,a9-diiminopyridine ligand
forms an arsenic(I) salt.
˚
cation contacts of 2.246(5) and 2.336(5) A are between the arsenic
atom and one of the axial chlorides of the [SnCl5?THF]2 anion.
Collectively, the X-ray structural data indicate that the oxidation
Cyclic triphosphenium ions have been known for several years.1
Subsequently, other related ring systems incorporating P–As–P
moieties exemplified by 1 have been reported,2 as have acyclic
systems of the genre [R3PEPR3]+ (E = P, As).3 In a recent
development, it has been demonstrated that the tertiary phos-
phines in these acyclic cations can be replaced by N-heterocyclic
carbenes (NHCs).4 In general, two strategies have been employed
for the synthesis of these interesting species, namely (i) reduction of
ECl3 (E = P, As) by SnCl2 in the presence of a chelating
bis(phosphine),1,2 monodentate phosphine3 or NHC,4 and (ii)
redox reactions of a chelating bis(phosphine) with EI3 (E = P,
As).2d,e A major point of interest in each of the cations produced in
these reactions is the deduction that the central phosphorus or
arsenic atom is in the +1 oxidation state, a view that is supported
by the fact that the 31P NMR chemical shifts for this atom fall in
the range d 2210 to 2270,2b and the observation that the central
phosphorus atom of a triphosphenium ion is sufficiently basic to
undergo protonation.5 Collectively, these chemical and physical
˚
state of arsenic in 2 is +3. Thus, the C–C (av. 1.363(18) A), C–N
˚
˚
(av. 1.365(17) A) and N–As (av. 1.820(10) A) bond distances for
2 are in very good agreement with the range of literature values
of 1.338(15)–1.368(15), 1.316(18)–1.404(13) and 1.784(18)–
˚
1.8377(14) A, respectively, reported for the metrical para-
meters for the C2N2As ring of the arsenium cations of the
salts [tBuDADAs][Cl]9a and [MesDADAs][OTf].9b The
presence of the counterion [SnCl5?THF]2 in 2 is indicative
of the initial formation of ‘‘AsCl’’ as a consequence of the
reduction of AsCl3 by SnCl2.10 Although no mechanistic
information is available, a likely scenario is that the ‘‘AsCl’’
fragments are trapped by the bidentate MesDAD ligand
prior to or concomitant with abstraction of Cl2 by SnCl4 to
form [SnCl5?THF]2 and two-electron transfer from As to
the LUMO of the diimine ligand.11
properzties are suggestive of the predominance of the canonical
form D ? {E / D where D represents a phosphorus donor or
NHC and E is P or As. In each of the above cases, the P(I) or As(I)
moiety is supported by ligands that are not prone to facile
reduction. Our attention was therefore drawn to the possibility
that, by judicious choice of supporting ligand, it might be possible
to control the oxidation state of the pnicogen center, E.
z
Attention was turned next to a diimine ligand that would not
undergo easy reduction. The a,a9-diiminopyridine ligand, [a,a9-
{2,6-iPr2PhNLC(Me)}2(C5H3N)] (DppDIMPY) was selected for
this purpose.12 This ligand, well known for its usefulness in
Treatment of an equimolar mixture of AsCl3 and SnCl2 with
(2,4,6-Me3C6H2)NC(H)C(H)N(2,4,6-Me3C6H2)6
MesDAD
resulted, after work-up, in an 84% yield of brown, crystalline 2.
The CI mass spectroscopic data for 27 revealed the presence of
[SnCl5]2 in the negative mode and [MesDADAs]+ in the positive
Fig. 1 View of the [MesDADAs]+ cation of 2 showing the atom
numbering scheme and thermal ellipsoids at 50% probability (hydrogen
˚
atoms omitted for clarity). Selected bond distances (A) and angles (u) with
the corresponding values for the second molecule in the unit cell shown in
brackets. C(1)–N(1) 1.372(16) [1.357(17)], C(1)–C(2) 1.375(18) [1.351(18)],
C(2)–N(2) 1.339(16) [1.393(15)], N(1)–As(1) 1.823(10) [1.818(10)], N(2)–
As(1) 1.831(11) [1.809(10)], C(1)–N(1)–As(1) 114.2(8) [113.6(9)], C(2)–
N(2)–As(1) 113.4(9) [114.1(8)], N(1)–As(1)–N(2) 84.9(5) [85.0(5)].
Department of Chemistry and Biochemistry, The University of Texas at
Austin, 1 University Station A5300, 78712 Austin, Texas, USA.
E-mail: cowley@mail.utexas.edu; Fax: (+1) 512 471 6822;
Tel: (+1) 512 471 7484
1784 | Chem. Commun., 2006, 1784–1786
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redox reaction 4AsI3 A 2As+ + [As2I8]22 + 2I2. Compound 4 was
characterized by single-crystal X-ray diffraction and the metrical
parameters for the cation were found to be identical to those found
for 3 within experimental error (see Fig. 1 caption).
In summary, it has been found that an in situ generated
arsenic(I) intermediate reacts with a 1,4-diimine to form an
arsenium cation by a redox reaction. On the other hand, the
arsenic(I) species retains the +1 oxidation state when coordinated
to a diiminopyridine.
We are grateful to the Petroleum Research Fund (Grant 38970-
AC1) and the Robert A. Welch Foundation for financial support.
Fig. 2 View of the [DppDIMPYAs]+ cation of 3 showing the atom
numbering scheme and thermal ellipsoids at 50% probability (hydrogen
˚
atoms omitted for clarity). Selected bond distances (A) and angles (u) with
Notes and references
the corresponding values for the other half cation shown in brackets. C(2)–
N(2) 1.302(6) [1.285(6)], C(2)–C(3) 1.424(7) [1.415(7)], C(3)–N(1) 1.395(5)
[1.382(5)], N(1)–As(1) 1.862(5) [1.879(5)], N(2)–As(1) 2.095(4) [2.076(4)],
C(3)–C(2)–N(2) 113.0(4) [113.0(4)] N(2)–As(1)–N(2A) 154.2(2) [154.2(2)].
Symmetry transformations used to generate equivalent atoms: x, 2y + K,
z. The related metrical parameters for the [DppDIMPYAs]+ cation of 4
are as follows: C(2)–N(2) 1.310(12), C(2)–C(3) 1.424(15), C(3)–N(1)
1.395(13), N(1)–As(1) 1.877(8) N(2)–As(1) 2.080(9), C(3)–C(2)–N(2)
111.3(10), N(2)–As(1)–N(3) 154.9(3).
1 A. Schmidpeter, S. Lochschmidt and W. S. Sheldrick, Angew. Chem.,
Int. Ed. Engl., 1982, 21, 63.
2 (a) S. F. Gamper and H. Schmidbaur, Chem. Ber., 1993, 126, 601; (b)
A. J. Boon, H. L. Byers, K. B. Dillon, A. E. Goeta and
D. A. Longbottom, Heteroat. Chem., 2001, 12, 501; (c) M. Dotzler,
A. Schmidt, J. Ellermann, F. A. Knoch, M. Moll and W. Bauer,
Polyhedron, 1996, 15, 4425; (d) B. D. Ellis, M. Carlesimo and C. L. B.
Macdonald, Chem. Commun., 2003, 1946; (e) B. D. Ellis and C. L. B.
Macdonald, Inorg. Chem., 2004, 43, 5981.
3 (a) A. Schmidpeter, S. Lochschmidt and W. S. Sheldrick, Angew. Chem.,
Int. Ed. Engl., 1985, 24, 226; (b) M. Driess, H. Ackermann, J. Aust and
K. C. von Wu¨llen, Angew. Chem., Int. Ed., 2002, 41, 450.
4 B. D. Ellis, C. A. Dyker, A. Decken and C. L. B. Macdonald, Chem.
Commun., 2005, 1965.
polymerization catalysis,13 will, in fact, react with Li and
Li(naphthalenide) to give trilithiated products.14 However, these
reactions do not involve straightforward two-electron reduction
processes.
5 S. Lochschmidt and A. Schmidpeter, Z. Naturforsch., B: Anorg. Chem.
Org. Chem., 1985, 40b, 765.
Following the synthetic method described above for 2, ‘‘AsCl’’
was generated in THF solution prior to the addition of
DppDIMPY. After isolation and crystallization, the product (3)
was characterized on the basis of spectroscopic data7 and an X-ray
diffraction study.8 The asymmetric unit of 3 features two half
[DppDIMPYAs]+ cations and one [SnCl5?THF]2 anion (Fig. 2).
Within experimental error, the AsN3C7 skeleton is planar and the
flanking aryl groups are twisted by an average of 89.5u with respect
to this plane. The arsenic atom is bonded to the pyridine nitrogen
6 M. B. Abrams, B. L. Scott and R. T. Baker, Organometallics, 2000, 19,
4944.
7 Spectroscopic data for 2 (84% yield): 1H NMR (CD2Cl2): d 2.20 (s, 12H,
CH3), 2.40 (s, 6H, CH3), 7.14 (m, 4H, Carom–Mes), 8.22 (s, 2H,
HCLCH). MS (CI+, CH4): m/z 367 (75%, MesDADAs+); CI2 m/z 293
(100%, SnCl52); HRMS (CI, CH4) calcd for C20H25N2As+, 368.1234;
found 368.1220. Spectroscopic data for 3 (66% yield): 1H NMR
(CD2Cl2): d 1.09 (d, 12H, 3JHH = 6.9 Hz, iPr–CH3), 2.29 (sept, 4H, CH),
3
2.87 (s, 6H, CH3) 7.38–7.52 (m, 6H, Dpp–Harom), 8.53 (t, 1H, JHH
=
7.5 Hz, py–Harom), 9.78 (d, 2H, JHH = 7.5 Hz, py–Harom). MS (CI+,
CH4): m/z 556 (100%, DppDIMPYAs+); HRMS (CI, CH4) calcd for
C33H43N3As+; 556.2673; found 556.2678. Spectroscopic data for 4 (10%
yield): 1H NMR (CD2Cl2): d 1.10 (d, 12H, 3JHH = 6.6 Hz, Dpp–CH3),
1.18 (d, 12H, JHH = 6.6 Hz, Dpp–CH3), 2.31 (m, 4H, Dpp–CH), 2.87 (s,
6H, CH3), 7.38–7.49 (m, 6H, Dpp–Harom), 8.43 (t, 1H, 3JHH = 8.1 Hz,
py–Harom), 9.04 (d, 2H, 3JHH = 8.1 Hz, py–Harom). MS (CI+, CH4): m/z
(100%, DppDIMPYAs+); HRMS (CI+, CH4) calcd for C33H43N3As+,
556.2673; found 556.2678.
3
˚
and both imino nitrogen atoms. The bond to pyridine is y0.2 A
shorter than the other two; however, a survey of the Cambridge
Structural Database reveals that all three bonds fall within the
range observed for NAAs dative bonds. The designation of the
latter as dative bonds implies that intramolecular arsenicAligand
electron transfer has not taken place and hence that arsenic is in
the +1 oxidation state. Further support for this view stems from
examination of the metrical parameters for the remainder of the
cationic skeleton. Thus, the average nitrogen–carbon distance of
8 Crystal data for 2: C24H32AsCl5N2OSn (735.40), monoclinic, space
˚
group P21/c, a = 18.497(4), b = 15.947(3), c = 20.677(4) A, b = 91.97 (3)u,
3
V = 6069(2) A , Z = 8, Dc = 1.603 g cm23, m(Mo-Ka) = 2.374 mm21
,
˚
T = 153(2) K, 13 842 independent reflections (Rint = 0.1109), final R
indices (625 parameters) for 13 842 independent reflections [I , 2s(I)]
1.294(6), which is y0.02 A larger than that in the free ligand,15 is
˚
are R1
C37H51AsCl5N3OSn (924.69), orthorhombic, space group Pnma, a =
= 0.0852, wR2 = 0.1804, GOF = 1.036. For 3:
consistent with a bond order of 2. Moreover, the average C–C and
C–N bond distances for the pyridine ring of 3 are virtually
identical to those in the free ligand.15
3
˚
˚
25.951(5), b = 23.306(5), c = 17.579(4) A, V = 10 632(4) A , Z = 8, Dc =
1.155 g cm23, m(Mo-Ka) = 1.375 mm21, T = 153(2) K, 12 396
independent reflections (Rint = 0.0567), Final R indices (452 parameters)
for 12 396 independent reflections [I , 2s(I)] are R1 = 0.0593, wR2 =
0.1639, GOF = 0.907. For 4: C33H43As2I4N3 (1139.14), triclinic, space
A second arsenic(I) salt, [DppDIMPYAs]2[As2I8] (4), has been
prepared via the reaction of the free ligand with AsI3. In this case,
the counterion is [As2I8]22 and forms as a consequence of the
¯
˚
group P1, a = 10.668(2), b = 14.692(3), c = 17.074(3) A, a = 105.32(3)u,
3
23
,
˚
b = 98.94(3)u, c = 107.50(3)u, V = 2380(11) A , Z = 2, Dc = 1.590 g cm
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m(Mo-Ka) = 4.019 mm21, T = 153(2) K, 10 689 independent reflections
(Rint = 0.0762), final R indices (390 parameters) for 10 689 independent
reflections [I , 2s(I)] are R1 = 0.0588, wR2 = 0.1491, GOF = 0.812.
CCDC: 295979 (2), 295978 (3) and 295977 (4). For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b602017h.
9 (a) C. J. Carmalt, V. Lomel´ı, B. G. McBurnett and A. H. Cowley,
Chem. Commun., 1997, 2095; (b) T. Gans-Eichler, D. Gudat and
M. Nieger, Heteroat. Chem., 2005, 16, 327.
10 The formation of {PCl}n by reduction of PCl3 with
Li2[tBuNC(H)C(H)But] has been reported: M. K. Denk, S. Gupta
and R. Ramachandran, Tetrahedron Lett., 1996, 37, 9025. A similar
reaction takes place with a stannylene reducing agent: M. Veith,
H. Volker, J. P. Majoral, G. Bertrand and G. Manuel, Tetrahedron
Lett., 1983, 24, 4219.
11 Preliminary experiments indicate that a similar redox process takes place
when DppDAD [(2,6-iPr2C6H2)NC(H)C(H)N(2,6-iPr2C6H2)] is treated
with ‘‘PCl’’ as evidenced by the 31P chemical shift of d +205.0.
12 G. J. P. Britovsek, M. Bruce, V. C. Gibson, B. S. Kimberby,
P. J. Maddox, S. Mastroianni, S. J. McTavish, C. Redshaw,
G. A. Solan, S. Stro¨mberg, A. J. P. White and D. J. Williams, J. Am.
Chem. Soc., 1999, 121, 8728.
13 See, for example: L. L. Johnson, C. M. Killian and M. Brookhart,
J. Am. Chem. Soc., 1995, 117, 6414.
14 D. Enright, S. Gambarotta, G. P. A. Yap and P. H. M. Budzelaar,
Angew. Chem., Int. Ed., 2002, 41, 3873.
15 G. P. A. Yap and S. Gambarotta, private communication to CCDC
(no. 116252), 1999.
1786 | Chem. Commun., 2006, 1784–1786
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