β-Diiminato complexes of arsenic including the formally AsI compound [As3L3] [L = {N(C6H 3Pri2-2,6)C(H)}2CPh]

Article abstract of DOI:10.1039/b819723g

The reaction of the potassium β-diiminate KL (L = [{N(Ar)C(H)} ₂CPh]^-; Ar = C₆H₃Prⁱ ₂-2,6) with AsI₃ gave [AsI₂L] (1), which upon reduction with KC₈ produce

Full text of DOI:10.1039/b819723g

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b-Diiminato complexes of arsenic including the formally AsI compound
[As3L3] [L = {N(C6H3Pri2-2,6)C(H)}2CPh]w
Peter B. Hitchcock, Michael F. Lappert,* Gang Li and Andrey V. Protchenko
Received (in Cambridge, UK) 5th November 2008, Accepted 27th November 2008
First published as an Advance Article on the web 12th December 2008
DOI: 10.1039/b819723g
The reaction of the potassium b-diiminate KL (L
=
Crystalline complex 1 is monomeric (Fig. 1),z showing a
distorted trigonal bipyramidal environment around the As
atom with the iodine atoms apical and N1, N2 and a lone
pair occupying equatorial positions. The delocalised
AsN1C1C2C3N2 six-membered ring has a shallow boat
conformation with the As and C2 atoms being out of the
N1C1C3N2 plane by 0.661(6) and 0.155(7) A, respectively.
The two As–N bond lengths are nearly identical and are
similar to the equatorial As–N distance in the guanidinate A
at 1.915(2) A (the axial bond is much longer at 2.076(2) A).5
Reduction of 1 with an excess of KC8 in diethyl ether gave
the trinuclear As compound [(k1-L)2As–AsQAs(k2-L)] (2) in
low yield,w Scheme 1. Attempted reduction of 1 with Zn
powder or Sn{N(SiMe3)2}2 led to a brown or red-brown
precipitate, which was insoluble in common organic solvents.
The different outcome of these reductions is attributed to the
formation of K3As (in the case of KC8 as reducing agent),
which may have been the source for the central, naked As
atom in 2.
[{N(Ar)C(H)}2CPh]ꢀ; Ar = C6H3Pri2-2,6) with AsI3 gave
[AsI2L] (1), which upon reduction with KC8 produced
[(k1-L)2As–AsQAs(k2-L)] (2), having a unique bent chain of the
three arsenic atoms in the formal oxidation states +2, 0 and +1.
The chemistry of b-diiminato complexes of Group 15 elements
has recently undergone significant development, most of which
dealt with phosphorus compounds: C-centred b-diketiminates,1
N,N0-centred phosphenium salts2a–c and N,C-centred cationic
and neutral complexes.2d,e b-Diketiminatoantimony dichloride
has been prepared using the mesityl-substituted ‘‘nacnac’’
ligand (L0 = [{N(Ar)C(Me)}2CH]ꢀ, Ar = C6H2Me3-2,4,6),
while with Ar = C6H3Pri2-2,6 the products contained a tauto-
meric N,C-bonded ligand.3 Attempts to isolate a crystalline
arsenic(III) or bismuth(III) b-diketiminate were unsuccessful.2e,3
A b-diketiminato ligand was unsuitable for stabilisation of
pnictogen(I) compounds.4 It is noteworthy that guanidinates
or an amidinate M(L00)Hal2 [M = As, Sb, Bi; L00
=
{N(Ar)}2CR (Ar = C6H3Pri2-2,6; R = NCy2, NPri2 or But);
Hal = Cl or I] were readily available from MHal3 and the
appropriate lithium salt.5 Reduction of such an AsIII dichloride
yielded a diarsene; e.g., [AsCl2{(N(Ar))2C(NCy2)}] (A) was a
precursor to [As2{m-(N(Ar))2C(NCy2)}2] (B).
We have recently reported on the application of the
b-dialdiminato ligand [{N(Ar)C(H)}2CPh]ꢀ (ꢁ [L]ꢀ; Ar =
C6H3Pri2-2,6) in phosphorus2c and Group 13 metal6 chemis-
try; now we describe the successful use of this ligand for
stabilisation of both AsIII and AsI compounds.
The structural analysis (Fig. 2)z reveals that the molecule 2
has the three As atoms in a bent (ca. 901) chain arrangement
with the central As1 atom being two-coordinate and bonded
only to the other As atoms. The As2 atom is chelated by a
single ligand L with its As–N bonds slightly longer than in 1,
but shorter than even the shortest such contact in the diarsene
B (2.045(2) A).5 The average As2–N distance (1.977 A) is
close to the value calculated for the optimised neutral molecule
[AsI(nacnac)] having C2v (1.955 A, planar AsNCCCN ring)
rather than Cs (1.903 A, boat-shaped ring, as found in 2)
symmetry.4 Each of the two b-dialdiminato ligands, which is
k1-bonded to the As3 atom, has localised single and double
bonds (in contrast to the p-delocalised ligand arrangement at
As2) and thus is better treated as an eneamido ligand.
Reaction of AsI3 and KL in thf cleanly produced the
arsenic(III) b-dialdiminate [AsI2L] (1),w Scheme 1. Compound
1 was isolated in good yield as red crystals of a solvate,
1ꢂ(Et2O); it was soluble in thf or benzene and showed the
appropriate signals in its 1H and 13C NMR spectra. In
contrast, attempted synthesis of a b-diketiminatoarsenic(III)
iodide using KL0 (Ar = C6H3Pri2-2,6) led to a yellow powder
insoluble in common organic solvents preventing its further
characterisation. This difference is attributed to the ease of
deprotonation of the Me groups of the b-diketiminato ligand
backbone and the accessibility of its g-C, while Me groups are
absent in the b-dialdiminato ligand L of 1 and the g-C is
blocked by the bulky Ph substituent.
Department of Chemistry and Biochemistry, University of Sussex,
Brighton, UK BN1 9QJ. E-mail: m.f.lappert@sussex.ac.uk;
Fax: +44 (0)1273 677196; Tel: +44 (0)1273 678316
w Electronic supplementary information (ESI) available: details of
synthetic procedures and crystallographic data for 1 and 2. CCDC
708181 and 708182. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b819723g
Scheme 1 Synthesis of compounds 1 and 2.
ꢃc
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428 | Chem. Commun., 2009, 428–429

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The difference between the As1–As2 and As1–As3 bond
lengths suggests a partial double-bond character for the former;
however it is longer than in the AsI guanidinate B (2.2560(5) A)5
or the bulky aryl derivative [As2(C6H3Trip2-2,6)2] (2.285(3) A,
Trip = C6H2Pri3-2,4,6).7 This may be attributed to the zero
oxidation state for the central As1 atom in 2, in contrast to the
oxidation state +1 for both As atoms in the symmetrically
substituted diarsenes.
In conclusion, the crystalline b-dialdiminatoarsenic(III)
iodide 1 was readily available via a standard metathetical
exchange reaction of AsI3 (cf. the more complex reactivity of
phosphorus(III) halides). The compound 2, obtained in a low
yield by reduction of 1 with KC8, represents a rare example8 of
a trinuclear V-shaped As3 compound having each As atom in
a different formal oxidation state, 0 for the central non-ligated
As1 atom, +1 for the chelated As2 atom and +2 for the
bis-ligated As3 atom.
Fig. 1 Structure of complex 1 (30% thermal ellipsoids). Selected
bond lengths (A) and angles (1): As–N1 1.922(4), As–N2 1.923(4),
As–I1 2.9608(6), As–I2 2.7781(6), N1–C1 1.326(6), N2–C3 1.340(6),
C1–C2 1.403(7), C2–C3 1.374(7); N1–As–N2 92.44(17), N1–As–I2
96.66(12), N2–As–I2 96.55(11), N1–As–I1 91.05(12), N2–As–I1
90.08(11), I1–As–I2 169.59(2).
We thank the Royal Society for the award of a Sino-British
Fellowship to G. L.
Notes and references
z Crystal data. For 1ꢂ(Et2O): [C37H51AsI2N2O], M = 868.52, monoclinic,
space group P21/n, a = 11.2964(3), b = 24.7180(4), c = 14.2829(4) A,
b = 97.267(1)1, V = 3956.10(17) A3, Z = 4, T = 173(2) K, m =
2.45 mmꢀ1, 7713 independent reflections [Rint = 0.062], final R1 = 0.047
[for 6182 reflections with I 4 2s(I)], wR2 = 0.107 (all data). For
2ꢂ1.5(C6H14): [C99H123As3N6]ꢂ1.5(C6H14), M = 1751.05, triclinic, space
ꢀ
group P1, a = 15.5396(4), b = 18.7915(4), c = 19.6985(5) A, a =
101.334(2), b = 96.248(1), g = 110.643(2)1, V = 5177.5(2) A3, Z = 2,
=
T = 173(2) K, m = 1.01 mmꢀ1, 20 830 independent reflections [Rint
0.079], final R1 = 0.088 [for 13 637 reflections with I 4 2s(I)], wR2 =
0.226 (all data). CCDC 708181 and 708182. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b819723g
1 (a) P. J. Ragogna, N. Burford, M. D’Eon and R. McDonald, Chem.
Commun., 2003, 1052; (b) P. B. Hitchcock, M. F. Lappert and
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8 A CCDC search showed only one structure having an As3 unit
with a two-coordinate central As atom, in the partially alkylated
Zintl anion [K(2,2,2-crypt)][As7(CH2Ph)2]: S. P. Mattamana,
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Fig. 2 Structure of complex 2 (30% thermal ellipsoids; atom labels
for the ligand substituents are omitted for clarity). Selected bond
lengths (A) and angles (1): As1–As2 2.3328(8), As1–As3 2.4139(9),
As2–N1 1.944(5), As2–N2 2.010(5), N1–C1 1.326(7), N2–C3 1.317(8),
C1–C2 1.399(8), C2–C3 1.375(9), As3–N3 1.912(5), N3–C34 1.372(7),
C34–C35 1.367(8), C35–C36 1.455(8), N4–C36 1.255(8), As3–N5
1.919(4), N5–C67 1.368(7), C67–C68 1.363(8), C68–C69 1.449(8),
N6–C69 1.274(7); As2–As1–As3 89.88(3), N1–As2–N2 88.2(2),
N3–As3–N5 101.2(2).
Accordingly, the As3–N3 and As3–N5 bonds of 2 are shorter
than the chelated As–N bonds in 1 or 2.
ꢃc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 428–429 | 429