Capture of phosphorus(I) and arsenic(I) moieties by a 1,2-bis(arylimino) acenaphthene (aryl-BIAN) ligand. A case of intramolecular charge transfer

Article abstract of DOI:10.1021/ja058459m

The reaction of PCl₃ with SnCl₂ in THF solution, followed by treatment with dpp-BIAN (dpp = 2,6-i-Pr₂C₆H₃), affords the phosphenium complex [(dpp-BIAN)P][SnCl₅·THF]. The ³¹P chem

Full text of DOI:10.1021/ja058459m

Published on Web 02/11/2006
Capture of Phosphorus(I) and Arsenic(I) Moieties by a
1,2-Bis(arylimino)acenaphthene (Aryl-BIAN) Ligand. A Case of Intramolecular
Charge Transfer
Gregor Reeske, Clint R. Hoberg, Nicholas J. Hill, and Alan H. Cowley*
Department of Chemistry and Biochemistry, The UniVersity of Texas at Austin, 1 UniVersity Station A5300,
Austin, Texas 78712-0165
Received January 2, 2006; E-mail: cowley@mail.utexas.edu
The presence of an extensive π-system and two Lewis basic sites
enables the 1,2-bis(arylimino)acenaphthene (aryl-BIAN) class of
ligands (1, Chart 1) to function as both electron and proton sponges.
This desirable combination of properties has resulted in the
widespread use of aryl-BIAN-supported transition metal derivatives
as versatile catalysts for a variety of important reactions.1 However,
considerably less information is available regarding aryl-BIAN
complexes of the main group elements.2 To date, this ligand class
has not been used in the context of group 15 chemistry.
Chart 1
Figure 1. ORTEP view of the [(dpp-BIAN)P]+ cation of 3 showing the
atom numbering scheme. Selected bond distances [Å] and angles [°] with
corresponding values for 6 in parentheses: P-N(1) 1.694(4) [1.700(4)],
P-N(2) 1.689(4) [1.685(5)], N(1)-C(1) 1.351(5) [1.354(7)], N(2)-C(12)
1.366(5) [1.361(7)], C(1)-C(12) 1.395(5) [1.380(8)], N(1)-P-N(2) 90.23-
(17) [89.75(2)], P-N(1)-C(1) 113.1(3) [113.2(4)], N(1)-C(1)-C(12)
112.3(4) [111.8(5)], C(1)-C(12)-N(2) 110.9(4) [111.7(5)], C(12)-N(2)-P
113.4(3) [113.5(4)].
It has been known for several years that the reduction of PCl3
with SnCl2 in the presence of chelating bis(phosphines) results in
the formation of cyclic triphosphenium ions (2, Chart 1).3 Use of
a similar protocol with AsCl3 permits the isolation of an arsenic
analogue of 2 with a six-membered C3P2As ring.4 Subsequently,
several other cyclic triphosphenium cations featuring a variety of
ring sizes and types have been reported.5 Although the mechanism
of formation of these cations has not been established, it is
reasonable to assume that the SnCl2 reduction of ECl3 (E ) P, As)
results initially in “ECl” and SnCl46 and that the former is trapped
by the chelating bis(phosphine) prior to or concomitant with
abstraction of Cl- by SnCl4. In more recent work, it has been
discovered that 2 (R ) Ph) can be isolated as the iodide salt from
the redox reaction of PI3 with bis(diphenylphosphinoethane).7
Acyclic cations of the types [R3PPPR3]+ and [R3PAsPR3]+ are also
known8,9 as are some N-heterocyclic carbene (NHC) analogues.10
Typically, the 31P chemical shifts of the central phosphorus atom
of cyclic triphosphenium cations fall in the range of δ -210 to
-270.5 Moreover, in the case of 2 (R ) Ph), this atom is sufficiently
basic to undergo protonation,11 hence these salts are best regarded
as adducts of phosphorus(I), namely, the predominant canonical
form is D+ f P- r D+ (D ) phosphine, NHC).
region observed for phosphenium cations.12 Further insight was
gained from a single-crystal X-ray diffraction study of 3 (Figure
1).13
The most noteworthy structural features concern the C-C and
C-N bond distances within the planar PN2C2 ring. Specifically,
the C(1)-C(12) bond distance (1.395(5) Å) is considerably shorter
than the corresponding distance in the uncoordinated dpp-BIAN
ligand (1.527 Å)14 and indicative of double bond character.
Moreover, the C-N bond distances in 3 (av. 1.385(5) Å) are longer
than those in free dpp-BIAN (1.272 Å)14 and commensurate with
a bond order of approximately one. Overall, the metrical parameters
for the PN2C2 ring are very similar to those found for cyclic
phosphenium cation 4 (Chart 1).15 Moreover, the structure of the
dpp-BIAN ligand in 3 also bears a close resemblance to that of the
complex [(dtb-BIAN)Mg(THF)2] which was prepared via the
reaction of activated Mg metal with the neutral dtb-BIAN ligand
(1; Ar ) 2,5-di-tert-butylphenyl).16 Thus, akin to Mg metal, the
“PCl” molecule functions as a two-electron reductant toward the
aryl-BIAN ligand. Accordingly, and in contrast to 2 and related
triphosphenium cations, the dicoordinate phosphorus atom of 3 is
in the +3 rather than the +1 oxidation state. The fact that internal
redox takes place in the case of 3 but not 2 is attributable to the
presence of a low-lying LUMO in the neutral dpp-BIAN ligand
and the aromaticity of the resulting [(dpp-BIAN)]2- anion. The
[SnCl5‚THF]- counteranion is essentially octahedral and the closest
P+‚‚‚Cl contacts are to Cl(1) (3.374(5) Å) and Cl(2) (3.328(5) Å).
We have also investigated the ambient temperature reaction of
equimolar quantities of dpp-BIAN and PI3 in CH2Cl2 solution in
Given the foregoing, we became interested in exploring the
consequences of trapping the putative ECl molecules with ligands
other than phosphines and carbenes. Treatment of an equimolar
mixture of PCl3 and SnCl2 with dpp-BIAN (1; Ar ) 2,6-i-Pr2C6H3)
in THF solution at ambient temperature resulted, after workup, in
a 66% yield of a dark green salt of composition [(dpp-BIAN)P]-
[SnCl5‚THF] (3). The most obvious feature of the NMR spectral
data for 3 is the 31P chemical shift of δ +232.5, which falls in the
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J. AM. CHEM. SOC. 2006, 128, 2800-2801
10.1021/ja058459m CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
complexes, the phosphorus or arsenic atoms in these cations adopt
the +3 oxidation state.
Acknowledgment. We are grateful to the Petroleum Research
Fund (Grant 38970-AC1) and the Robert A. Welch Foundation
(Grant F-135) for support.
Supporting Information Available: Experimental details, spec-
troscopic data, and X-ray crystallographic data for 3, 6, and 7 (CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
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Figure 2. ORTEP view of the [(dpp-BIAN)As]+ cation of 7 showing the
atom numbering scheme. Selected bond distances [Å] and angles [°]: As-
N(1) 1.839(3), As-N(2) 1.857(4), N(1)-C(1) 1.348(5), N(2)-C(12) 1.348-
(5), C(1)-C(12) 1.399(6), N(1)-As-N(2) 84.89(15), As-N(1)-C(1)
114.7(3), N(1)-C(1)-C(12) 112.6(4), C(1)-C(12)-N(2) 115.3(4), C(12)-
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P-As-P cation.
the absence of a reducing agent. 31P NMR spectroscopic assay of
the resulting dark brown reaction mixture revealed the exclusive
presence of a sharp singlet at δ +234.5. That virtually quantitative
formation of [(dpp-BIAN)P][I3] (6) had occurred was confirmed
on the basis of 1H and 13C NMR spectroscopic data along with an
X-ray diffraction study of a single crystal grown from THF
solution.13 Comparison of the metrical parameters for the phos-
phenium cations of 3 and 6 reveals that they are identical within
experimental error. The closest contact between P+ and I3- is 3.883-
(6) Å. Although we have no mechanistic information, it is plausible
that the interaction of dpp-BIAN with PI3 results in the initial
formation of I2 and “[(dpp-BIAN)PI]”, from which I- is abstracted
by I2. As in the case of 3, subsequent or concomitant intramolecular
charge transfer affords the final product, 6.
The arsenium salt [(dpp-BIAN)As][SnCl5‚THF] (7) has been
prepared by a similar procedure to that employed for the synthesis
of 3. The green crystalline product was examined by single-crystal
X-ray diffraction (Figure 2).13 The AsN2C2 ring is planar, and the
average C-N and As-N bond distances are very similar to those
in 515 and subsequently reported17 cyclic arsenium cations, thus
supporting the view that arsenic is in the +3 oxidation state. The
fact that the C(1)-C(12) bond distances in 3, 6, and 7 are ∼0.06
Å longer than the corresponding distances in 4 and 5 is presumably
due to the constraints of the somewhat rigid dpp-BIAN framework.
Finally, we note that the [(dpp-BIAN)As]+ cation is isoelectronic
with [(dpp-BIAN)Ge].18 As expected, the N-E-N bond angle and
E-N bond distances are smaller for the arsenic cation than the
germylene due to the fact that the ionic radius of As3+ is less than
that of Ge2+. As in the case of 3, the shortest cation-anion contacts
involve As+‚‚‚Cl(1) (3.298(5) Å) and As+‚‚‚Cl(2) (3.215(5) Å).
In summary, we have prepared the salts [(dpp-BIAN)P][SnCl5‚
THF], [(dpp-BIAN)P][I3], and [(dpp-BIAN)As][SnCl5‚THF] which
represent the first examples of group 15 complexes supported by a
BIAN ligand. On the basis of NMR and X-ray structural data, it is
concluded that, in contrast to the corresponding bis(phosphine)
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(13) All X-ray data sets were collected at 153 K on a Nonius-Kappa CCD
diffractometer. Crystal data for 3: C40H48Cl5N2OPSn, monoclinic, space
group P21/n, a ) 13.189(5), b ) 22.335(5), c ) 14.637(5) Å, â ) 104.102-
(5)°, V ) 4182(2) Å3, Z ) 4, Fcalcd ) 1.429 g cm-3, 2θmax ) 52.00, Mo
KR (λ ) 0.71073 Å), total reflections collected ) 15 498, unique
reflections ) 8180 (Rint ) 0.0562), absorption coefficient µ ) 1.002 mm-1
,
final R indices R1 ) 0.0471, wR2 ) 0.0995, GOF ) 0.979. Crystal data
for 6: C72H80I6N4P2, monoclinic, P21/c, a ) 17.625(4), b ) 15.265(3), c
) 17.014(3) Å, â ) 114.03(3)°, V ) 4180.5(15) Å3, Z ) 2, Fcalcd ) 1.450
g cm-3, 2θmax ) 54.96, Mo KR (λ ) 0.71073 Å), total reflections collected
) 17 430, unique reflections ) 9546 (Rint ) 0.0437), absorption coefficient
µ ) 2.304 mm-1, final R indices R1 ) 0.0478, wR2 ) 0.1202, GOF )
0.927. Crystal data for 7: C40H48AsCl5N2OSn, monoclinic, P21/n, a )
13.317(5), b ) 22.302(5), c ) 14.546(5) Å, â ) 103.936(5)°, V ) 4193.5-
(2) Å3, Z ) 4, Fcalcd ) 1.495 g cm-3, 2θmax ) 54.96, Mo KR (λ ) 0.71073
Å), total reflections collected ) 16 547, unique reflections ) 9460 (Rint
) 0.0710), absorption coefficient µ ) 1.744 mm-1, final R indices R1 )
0.0512, wR2 ) 0.0831, GOF ) 0.985.
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