Carbene stabilization of diarsenic: From hypervalency to allotropy

Article abstract of DOI:10.1002/chem.200902840

A new class of carbene-stabilized diphosphorus molecules, [L:P-P:L] (L:=N-heterocyclic carbenes (NHCs)) was reported. AsCl₃ was added to a stirred slurry of the NHC carbene ligand and hexane at ambient temperature. After stirring for a further 21 h, the solvent was removed in vacuo leading to an off white powder. X ray quality colorless crystals of 1 were isolated from a solution in toluene/THF at ambient temperature. THF was added to a flask containing [L¹:AsCl₃] and KC₃. After stirring the mixture vigorously for 15h at ambient temperature, the solvent was removed in vacuo and the residue was extracted with hexane. Concentrating to 4 mL gave X-ray quality single red crystals of 2 overnight at ambient temperature.

Full text of DOI:10.1002/chem.200902840

DOI: 10.1002/chem.200902840
Carbene Stabilization of Diarsenic: From Hypervalency to Allotropy
Mariham Y. Abraham, Yuzhong Wang, Yaoming Xie, Pingrong Wei,
Henry F. Schaefer, III, P. von R. Schleyer,* and Gregory H. Robinson*[a]
In contrast to nitrogen, the lightest yet most ubiquitous
Group 15 element, phosphorus and arsenic exhibit extensive
allotropy. Along with the more stable polymeric forms, both
white phosphorus and yellow arsenic are well-known tetra-
hedral allotropes. Indeed, P4 and As4 readily pyrolyze to
afford P2 and As2,[2,3] respectively, which are well-charac-
[1]
terized energetically and spectroscopically in the gas
phase,[4] but are only persistent at high temperatures. Cum-
mins and co-workers demonstrated that it is possible to gen-
erate complexes of diphosphorus in condensed phases by
mild thermal extrusion of P2 from niobium diphosphaazide
complexes.[5] We subsequently reported a new class of car-
À
bene-stabilized diphosphorus molecules, [LDP PDL] (LD=N-
heterocyclic carbenes (NHCs)).[6] Most recently, Bertrand
and co-workers synthesized cyclicACTHNUGTRNENUG(alkyl)ACHTUNGTNER(NUGN amino)carbene-sta-
Scheme 1. Bonding modes for diatomic compounds of Group 15 elements
(E=N, P, or As).
bilized P1 and P2 complexes by P4 fragmentation.[7]
The bond energy of N2 (226 kcalmolÀ1) is nearly twice
that of P2 (116 kcalmolÀ1)[8] and thrice that of As2
(83 kcalmolÀ1),[9] thus suggesting a diminished importance of
p–p bonding among third and fourth period elements.[10]
Consequently, unlike N2, which usually[11] exhibits bonding
mode A (Scheme 1) in its complexes, the E2 cores in the cor-
responding diphosphorus and diarsenic complexes may
assume either bonding mode A (triply bonded dipnictogen)
or B (singly bonded dipnictinidene; Scheme 1). For example,
diarsenic, like diphosphorus, has been known to function as
a four-, six-, or eight-electron-donor ligand (C–G, Scheme 1)
in transition-metal (M)–carbonyl complexes.[1,2,12,13] An E2
(E=P or As) fragment with an A-type structure may be ex-
tracted from C–F,[1] whereas the dipnictinidene core (B) has
been observed in G.[2,13] Is it possible to prepare a carbene-
stabilized diarsenic analogue of H (Scheme 1)?
We recently demonstrated that NHCs can effectively sta-
bilize highly reactive molecules.[14–17] Prominent examples of
carbene-stabilized complexes not only include diphospho-
rus,[6] diborenes ([LD(H)B=B(H)DL]),[18,19] and disilicon
([LDSi=SiDL]),[20] but this strategy also succeeded in the
recent preparation of a neutral metalloaromatic Ga6 octahe-
dron ([LDGaACTHNUTRGNEUNG
(Ga4Mes4)GaDL]).[21] Herein, we now report the
syntheses, X-ray structures, and computations of carbene-
stabilized arsenic derivatives of AsCl3 ([L1DAsCl3] (1)), and
As2 ([L DAs AsDL ] (2), L D=DC{N(2,6-iPr2C6H3)CH}2). Com-
pound 2 is the first example of a Lewis base stabilized dia-
rsenic complex.
The hypervalent carbene-stabilized arsenic chloride com-
pound 1 was prepared, in nearly quantitative yield, by the
treatment of AsCl3 with the carbene ligand (L1D)[22]
(Scheme 2). Subsequent potassium/graphite reduction of 1
affords the carbene-stabilized diarsenic complex, 2, as air-
sensitive red crystals (Scheme 2). The 1H NMR imidazole
[a] M. Y. Abraham, Dr. Y. Wang, Dr. Y. Xie, Dr. P. Wei,
Prof. H. F. Schaefer, III, Prof. P. v. . R. Schleyer, Prof. G. H. Robinson
Department of Chemistry and
1
1
1
À
The Center for Computational Chemistry
The University of Georgia
Athens, GA 30602-2556 (USA)
Fax : (+1)706-542-9454
E-mail: robinson@chem.uga.edu
schleyer@chem.uga.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902840.
432
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Chem. Eur. J. 2010, 16, 432 – 435

COMMUNICATION
Figure 1. Molecular structure of 1 (thermal ellipsoids represent 30%
probability; hydrogen atoms omitted for clarity). Selected bond lengths
À
À
À
[ꢁ] and angles [8]: As(1) C(1) 2.018(3), As(1) Cl(1) 2.171(1), As(1)
À
Cl(2) 2.484(2), As(1) Cl(3) 2.359(2); C(1)-As(1)-Cl(1) 99.92(10), C(1)-
As(1)-Cl(2) 89.09(13), C(1)-As(1)-Cl(3) 90.35(14), Cl(1)-As(1)-Cl(2)
88.23(10), Cl(1)-As(1)-Cl(3) 90.65(8), Cl(2)-As(1)-Cl(3) 178.65(13).
Scheme 2. Syntheses of carbene-stabilized AsCl3 (1) and diarsenic 2.
resonances of 1 and 2 are d=6.47 and 6.12 ppm, respective-
ly.
Scheme 3. Structures of [LDAsCl3] complexes.
The X-ray structure confirms the four coordination of ar-
senic in 1 as well as the truncated trigonal bipyramidal
À
AsCl4 -like “see-saw” geometry (Figure 1). The C(1) and
Cl(1) atoms of 1 occupy two equatorial positions; the re-
maining two Cl atoms are axial (Scheme 3a). These structur-
À
al features of 1 not only resemble AsCl4 , but also the
ACTHNUTRGNEUNG
[L2DAsCl3] adduct (L2D=SC{N(CH3)CH}2, 3).[23] In contrast,
the DNACHTUNGTRENNUNG
(CH3)3 Lewis base ligand (L3) in [L3DAsCl3] (4),[24] oc-
cupies one of the two axial positions as shown in Scheme 3b.
This structural distinction may be ascribed to the relative-
ly high electronegativity and small size of the nitrogen
center.[23] Model computations on [L4DAsCl3] (L4D=
DC{N(H)CH}2) 1-H not only confirm the preference of the
parent L4D ligand for the equatorial (Scheme 3a) over the
axial (Scheme 3b) position, but also reveal the steric effects
of the isopropyl groups in 1. The simplified carbene ligand
in 1-H lines up with the axial chlorines rather than with the
equatorial chlorine, as in 1. The 178.65(13)8 axial Cl(2)-
As(1)-Cl(3) bond angle of 1 equals the 178.78 N-As-Cl angle
of 4, but is larger than the 171.88 Cl(2)-As(1)-Cl(3) angle of
3, which may be caused by the weak axial chlorine bridging
interactions between two neighboring molecules of 3 in the
solid state.[23]
Figure 2. Molecular structure of 2 (thermal ellipsoids represent 30%
probability; hydrogen atoms omitted for clarity). Selected bond lengths
À
À
[ꢁ] and angles [8]: As(1) C(1) 1.881(2), As(1) As(1A) 2.442(1); C(1)-
As(1)-As(1A) 101.11(5), N(1)-C(1)-As(1) 118.98(12), N(2)-C(1)-As(1)
137.63(13).
Compound 2, with Ci symmetry, is isostructural with com-
pound H (E=P; LD=L1, 5)[6] and adopts a trans-bent geome-
À
try around the As As bond (the C(1)-As(1)-As(1A)-C(1A)
torsion angle=1808; Figure 2). The 2.442(1) ꢁ central As
senic single-bonded covalent radii.[26] The 101.11(5)8 C(1)-
As(1)-As(1A) bond angle is about 28 less than the
103.19(6)8 C(1)-P(1)-P(1A) bond angle in 5, whereas the
À
As bond length is the same as the bond length of 2.44 ꢁ[25]
in gaseous As4 and corresponds to the 2.42 ꢁ sum of the ar-
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www.chemeurj.org
433

G. H. Robinson, P. v. R. Schleyer et al.
À
À
As As bond in 2 is about 0.24 ꢁ longer than the P P bond
À
of 5 (2.205(1) ꢁ). The 1.881(2) ꢁ As CNHC bond length in 2
compares well to the reported values for carbene–arsinidene
adducts (1.899(3)–1.902(7) ꢁ).[27] These distances indicate
partial As=C double-bond character because they are short-
er than the 2.018(3) (ꢁ) value in 1, but are longer than the
typical 1.816–1.827 ꢁ As=C double-bond lengths in acyclic
arsaalkenes.[27] The imidazole rings and the central As As
À
bond are essentially coplanar in 2. The 2.08 N(2)-C(1)-
As(1)-As(1A) torsion angle is the same as the 2.38 N(2)-
C(1)-P(1)-P(1A) torsion angle in 5, but contrasts with the
90.88 N(2)-C(1)-Si(1)-Si(1A) torsion angle in [L1DSi=SiDL1]
(L1D=DC{N(2,6-iPr2C6H3)CH}2).[20]
À
In contrast to trans-bent 2, DAs AsD lone pair repulsion in
the fully B3LYP/DZP DFT-optimized structure of the sim-
4
4
4
plified [L DAs AsDL ] (L D=DCACHTNUGRTNEUNG
(NHCH)2) model 2-H[28] favors
À
a gauche conformation with C2 symmetry and a 93.98 C-As-
À
As-C torsion angle (Figure 3). However, the central As As
Figure 4. Localized molecular orbitals (LMOs) of 2-H in Ci symmetry
À
(based on the X-ray coordinates of 2). a) As As s-bonding orbital;
À
b) As C s-bonding orbital; c) lone-pair orbital (mainly p character) with
p–p back-donation to the empty p orbital of CNHC; d) lone-pair orbital
(mainly s character).
Figure 3. The optimized geometry of the 2-H model in C2 symmetry.
23.3% p, 0.0% d). Compound 2 is not only isostructural to
the carbene-stabilized diphosphinidene 5,[6] but is also struc-
turally similar to carbene–arsinidene adducts.[27] Thus, com-
pound 2 can be regarded as a diarsinidene complex.
The utilization of NHCs is a viable means to stabilize
highly reactive molecules of nonmetal and metalloid main
group elements. Carbene-stabilized diarsenic 2 represents
the first Lewis base stabilized diatomic molecule of the
Group 13–15 elements, in the formal oxidation state of zero,
in the fourth period or higher of the periodic table.
single bond rotational barrier in 2-H is small. The torsion
angle discrepancy between 2 and 2-H, which we also found
[6]
À
in the analogous [LDP PDL] systems, may be attributed to
the substantial steric repulsion between the bulky carbene li-
gands in 2. The other computed structural parameters (dAs
À
As =2.464 ꢁ; dAsÀC =1.928 ꢁ; C-As-As angle=95.48) of 2-H
(C2) agree with the experimental values for 2.
The localized molecular orbitals (LMOs) of 2-H were
computed in Ci symmetry by using the X-ray coordinates of
2.[28] These LMOs (Figure 4) and natural bond orbital
(NBO) analysis support the bonding description (H).
Experimental Section
À
LMO a (Figure 4), depicting the As As single bond in 2-H
(Wiberg Bond Index (WBI)=1.009), has predominately 4p
As character (8.9% s, 90.7% p, 0.4% d). The As–CNHC in-
teraction (WBI=1.341) has partial double-bond character:
Synthesis of 1: AsCl3 (1.63 g, 8.99 mmol) was added to a stirred slurry of
the NHC carbene ligand (DL1, 3.50 g, 9.01 mmol) and hexane (50 mL) at
ambient temperature. After stirring for a further 21 h, the solvent was re-
moved in vacuo leading to an off-white powder 1 (4.95 g, 96.5%). X-ray
quality colorless crystals of 1 were isolated from a solution in toluene/
THF at ambient temperature. These gradually decompose and melt at
À
LMO b (Figure 4) depicts the As C s bond resulting from
DCNHC lone-pair donation to As, whereas LMO c (Figure 4)
displays the p–p back-donation of the As lone-pair orbital
with pure p character (0.0% s, 99.8% p, 0.2% d) to the
3248C. 1H NMR (400 MHz, C6D6): d=0.93 (d, 12H; CH
12H; CH(CH3)2), 3.30 (m, 4H; CH(CH3)2), 6.47 (s, 2H; NCH), 7.08 (d,
ACHTUGNTREN(UNNG CH3)2), 1.54 (d,
A
ACHTUNGTRENNUNG
À
4H; Ar-H), 7.18 (t, 2H; Ar-H). Crystal data for 1: C27H36AsCl3N2, Mr =
569.85, monoclinic, P21/n (No. 14), a=11.0692(7) ꢁ, b=13.8303(9) ꢁ,
empty p orbital of CNHC. Although the As C s-bond polari-
zation is 67.8% carbon and 32.2% arsenic (As has 15.4% s,
84.3% p, and 0.3% d character), the modest p–p back-don-
ation exhibited in LMO c (Figure 4) has 66.3% As and
33.7% C polarization. The remaining As lone-pair orbital
(LMO d, Figure 4) has predominant s character (76.6% s,
c=19.2882(12) ꢁ,
a=90.008,
b=104.3760(10)8,
g=90.008,
V=
2860.4(3) ꢁ3, Z=4, R1=0.0465 for 3682 data (I>2s(I)), wR2 =0.1169
(all data).
Synthesis of 2: THF (60 mL) was added to a flask containing [L1DAsCl3]
(4.95 g, 8.69 mmol) and KC8 (3.52 g, 26.07 mmol). After stirring the mix-
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Chem. Eur. J. 2010, 16, 432 – 435

Carbene Stabilization of Diarsenic
COMMUNICATION
ture vigorously for 15 h at ambient temperature, the solvent was removed
in vacuo and the residue was extracted with hexane. Concentrating to
4 mL gave X-ray quality single red crystals of 2 overnight at ambient
temperature (0.77 g, 19.2%). M.p. 216.58C; 1HNMR (400 MHz, C6D6):
[16] D. Scheschkewitz, Angew. Chem. 2008, 120, 2021–2023; Angew.
Chem. Int. Ed. 2008, 47, 1995–1997.
[17] C. A. Dyker, G. Bertrand, Science 2008, 321, 1050–1051.
[18] Y. Wang, B. Quillian, P. Wei, C. S. Wannere, Y. Xie, R. B. King, H. F.
Schaefer III, P. v. R. Schleyer, G. H. Robinson, J. Am. Chem. Soc.
2007, 129, 12412–12413.
[19] Y. Wang, B. Quillian, P. Wei, Y. Xie, C. S. Wannere, R. B. King, H. F.
Schaefer III, P. v. R. Schleyer, G. H. Robinson, J. Am. Chem. Soc.
2008, 130, 3298–3299.
[20] Y. Wang, Y. Xie, P. Wei, R. B. King, H. F. Schaefer III, P. v. R.
Schleyer, G. H. Robinson, Science 2008, 321, 1069–1071.
[21] B. Quillian, P. Wei, C. S. Wannere, P. v. R. Schleyer, G. H. Robinson,
J. Am. Chem. Soc. 2009, 131, 3168–3169.
d=1.15 (d, 24H; CH
ACHTUNGTRENNUNG(CH3)2), 1.49 (d, 24H; CHACHTUNGTRNE(NUGN CH3)2), 3.06 (m, 8H;
CH(CH3)2), 6.12 (s, 4H; NCH), 7.05 (d, 8H; Ar-H), 7.19 (t, 4H; Ar-H).
ACHTUNGTRENNUNG
¯
Crystal data for 2: C54H72As2N4, Mr =927.0, triclinic, P1 (No.2), a=
10.842(4) ꢁ, b=11.160(4) ꢁ, c=12.592(5) ꢁ, a=95.640(5)8, b=
112.601(5)8, g=108.741(5)8, V=1288.6(9) ꢁ3, Z=1, R1=0.0349 for 4685
data (I>2s(I)), wR2 =0.0890 (all data).
CCDC-748496 (1) and CCDC-748495 (2) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[22] A. J. Arduengo III, R. Krafczyk, R. Schmutzler, H. A. Craig, J. R.
Goerlich, W. J. Marshall, M. Unverzagt, Tetrahedron 1999, 55,
14523–14534.
[23] D. J. Williams, C. O. Quicksall, K. J. Wynne, Inorg. Chem. 1978, 17,
2071–2073.
[24] M. Webster, S. Keats, J. Chem. Soc. A 1971, 836–838.
[25] L. R. Maxwell, S. B. Hendricks, V. M. Mosley, J. Chem. Phys. 1935,
3, 699–709.
[26] P. Pyykkç, M. Atsumi, Chem. Eur. J. 2009, 15, 186–197.
[27] A. J. Arduengo III, J. C. Calabrese, A. H. Cowley, H. V. Rasika Dias,
J. R. Goerlich, W. J. Marshall, B. Riegel, Inorg. Chem. 1997, 36,
2151–2158.
[28] DFT computations: The 2-H model was optimized at the B3LYP/
DZP level with the Gaussian 94 and Gaussian 03 programs: Gaussi-
an 94, Revision B.3; M. J. Frisch, G. W. Trucks, H. B. Schlegel,
P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R, Cheeseman, T. Keith,
G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-
Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski,
A. Nanavakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen,
M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L.
Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart,
M. Head-Gordon, C. Gonzalez, J. A. Pople, Gaussian, Inc., Pitts-
burgh, PA, 1995; Gaussian 03, Revision C.02; M. J. Frisch, G. W.
Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman,
Acknowledgements
We are grateful to the National Science Foundation for support of this
work: CHE-0608142 (G.H.R.), CHE-0716718 (P.v.R.S.), and CHE-
0749868 (H.F.S.).
Keywords: allotropy · arsenic · carbenes · density functional
calculations · hypervalent compounds
[1] F. A. Cotton, G. Wilkinson, M. Bochmann, C. Murillo, Advanced In-
organic Chemistry, 6th ed., Wiley, New York, 1998.
[2] G. Huttner, B. Sigwarth, O. Scheidsteger, L. Zsolnai, O. Orama, Or-
ganometallics 1985, 4, 326–332.
[3] Gmelin Handbuch der Anorganischen Chemie, Arsen, Vol. 17,
Verlag Chemie, Weinheim, 1952.
[4] K. P. Huber, G. Herzberg, Molecular Spectra and Molecular Struc-
ture. IV. Constants of Diatomic Molecules, Van Nostrand, Reinhold,
New York, 1979.
J. A. MontgomACTHNUGRTNEUNGery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M.
Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M.
Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima,
Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P.
Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gom-
perts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Po-
melli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Sal-
vador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Dan-
iels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Ragha-
vachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford,
J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Ko-
maromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y.
Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson,
W. Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc.,
Wallingford CT, 2004 (see the Supporting Information for details).
The single-point computation of 2-H model (Ci symmetry) was per-
formed using the X-ray coordinates of 2.
[5] N. A. Piro, J. S. Figueroa, J. T. McKellar, C. C. Cummins, Science
2006, 313, 1276–1279.
[6] Y. Wang, Y. Xie, P. Wei, R. B. King, H. F. Schaefer III, P. v. R.
Schleyer, G. H. Robinson, J. Am. Chem. Soc. 2008, 130, 14970–
14971.
[7] O. Back, G. Kuchenbeiser, B. Donnadieu, G. Bertrand, Angew.
Chem. 2009, 121, 5638–5641; Angew. Chem. Int. Ed. 2009, 48, 5530–
5533.
[8] W. E. Dasent, Inorganic Energetics: An Introduction. 2nd Ed, Cam-
bridge University Press, Cambridge, 1982.
[9] Y. Mochizuki, K. Tanaka, Chem. Phys. Lett. 1997, 274, 264–268.
[10] W. Kutzelnigg, Einfꢀhrung in die Theoretische Chemie, Vol. 2,
Wiley-VCH, Weinheim, 1978.
[11] For a range of exceptions, see: D. Roy, A. Navarro-Vazquez, P. v. R.
Schleyer, J. Am. Chem. Soc. 2009, 131, 13045–13053.
[12] Advances in Organometallic Chemistry, Vol. 42, (Eds.: F. G. A.
Stone, R. West), Academic Press, San Diego, 1998.
[13] O. J. Scherer, M. Ehses, G. Wolmershauser, Angew. Chem. 1998,
110, 530–533; Angew. Chem. Int. Ed. 1998, 37, 507–510.
[14] Y. Wang, G. H. Robinson, Chem. Commun. 2009, 5201–5213.
[15] R. Wolf, W. Uhl, Angew. Chem. 2009, 121, 6905–6907; Angew.
Chem. Int. Ed. 2009, 48, 6774–6776.
Received: October 14, 2009
Published online: November 24, 2009
Chem. Eur. J. 2010, 16, 432 – 435
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