Complexes of Stiboranium Mono-, Di-, and Trications

Article abstract of DOI:10.1002/chem.201800196

Reaction of Ph₂SbCl₃ with 2,2′-bipyridine and Me₃SiOSO₂CF₃ releases chlorobenzene, which is interpreted as a reductive (Sb^V/Sb^III) elimination from a complex of a stiboranium cation. Conversely, reactions of Ph₂SbCl₃ with 4-methylpyridine-N-oxide and AgOSO₂CF₃ give redox-resistant complexes with the generic formulae [Ph₂SbCl_3?xL_x+1][OTf]_x, including a compound containing a pnictogen(V) trication.

Full text of DOI:10.1002/chem.201800196

A Journal of
Accepted Article
Title: Complexes of Stiboranium Mono-, Di-and Tri-cations
Authors: Neil Burford, Christopher Frazee, Robert McDonald, Michael
Ferguson, Brian Patrick, and Andreas Decken
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To be cited as: Chem. Eur. J. 10.1002/chem.201800196
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10.1002/chem.201800196
Chemistry - A European Journal
Complexes of Stiboranium Mono-, Di- and Trications
Christopher Frazee,a Neil Burford,*a Robert McDonald,b Michael J. Ferguson,b Andreas Deckenc
and Brian O. Patrickd
aDepartment of Chemistry,University of Victoria, P.O. Box 1700, Stn CSC, Victoria, BC V8W
2Y2, Canada, nburford@uvic.ca, Fax: +1 250 721 7147; Tel: +1 250 721 7150
bDepartment of Chemistry, University of Alberta, Edmonton, Alberta, T6G 2 T2,
Canada
cDepartment of Chemistry, University of New Brunswick P.O. Box 4400, Fredericton, NB, E3B
5A3, Canada
dDepartment of Chemistry, University of British Columbia, Vancouver, BC, V6T 1Z1
Abstract: Reaction of Ph2SbCl3 with 2,2'-bipyridine and Me3SiOSO2CF3 releases
chlorobenzene, which is interpreted as a reductive (SbV/SbIII) elimination from a cationic
complex of a stiboranium cation. Conversely, reactions of Ph2SbCl3 with 4-methylpyridine-N-
oxide and AgOSO2CF3 give redox resistant complexes with the generic formulae [Ph2SbCl3-
xLx+1][OTf]x, including a compound containing the first example of a pnictogen(V) trication.
Coordination complexes involving p-block element Lewis acceptor centers offer
opportunities for the discovery of new reactivites that are distinct from those of transition metal
complexes. While p-block centers classically behave as Lewis donors, examples of compounds
containing p-block centers as Lewis acceptors have been developed only in recent years by
introducing a cationic charge on the p-block center.[1] In most cases, the cationic complexes are
synthesized by replacing anionic halide substituents (X) on a p-block center with classical L
donors. Complexes of antimony represent the most diverse series,[2] including examples of
mono-[3], di-[4], and tri-[5] cations featuring Sb(III). Complexes featuring Sb(V) as the Lewis
acceptor center have been reported for cations of the general formulae [Ph4SbL]+ and
[Ph3SbL2]2+, where L = 4-dimethylaminopyridine, Ph3PO or 4-methylpyridine-N-oxide
(OPyrMe).[6],[7],[8] Recognizing the potential to enhance reactivity at the Lewis acceptor site with
the presence of a higher cationic charge, we envisage complexes of the generic formulae
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10.1002/chem.201800196
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[R2SbL3]3+, [RSbL4]4+ and [SbL5]5+ through the sequential abstraction of halide substituents from
X3SbR2, X4SbR and X5Sb, respectively, in the presence of L.[9] Here we report the
characterization of a series of mono-, di- and tri-cations featuring an Sb(V) center, and provides
evidence for the redox instability of cations bearing higher cationic charges.
Reaction of Ph2SbCl3[10],[11] with two equivalents of 2,2'-bipyridine (bipy) followed by
three equivalents of trimethylsilyltrifluoromethanesulfonate (TMSOTf) results in formation of
[PhSb(bipy)][OTf]2 (1), which was isolated from a mixture of products by crystallization and has
been comprehensively characterized (Figure 1). Signals corresponding to chlorobenzene were
observed in the 13C NMR spectrum of the reaction mixture, indicating that reduction of Sb(V) to
Sb(III) is associated with the oxidative coupling of Ph and Cl. These observations are consistent
with the previously reported disproportionation of R3SbI2 to give RI and R2SbI (R =
Me3SiCH2and Me3CCH2) through thermal decomposition.[12],[13] Moreover, cationic phosphine
complexes of Sb(III) and Sb(V) centers have been observed to oxidatively couple R3P to form
diphosphonium cations (R =Me, Et, Pr).[6],[14],[15] The redox behavior of Ph2SbCl3 in the presence
of bipy and TMSOTf contrasts the apparent redox stability of the previously reported
[Ph3Sb(bipy)][OTf]2,[6],[7] which contains a formal dication. We speculate that the imposition of a
tricationic charge is avoided by reductive elimination of PhCl from [Ph2Sb(bipy)Cl][OTf]2.
Figure 1: Solid-state structure of [PhSb(bipy)][OTf]2 (1). All hydrogen atoms omitted for clarity.
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In contrast to reactions of Ph2SbCl3 with TMSOTf and bipy, redox behaviour is
suppressed in reactions of Ph2SbCl3 with AgOTf in the presence of OPyrMe. A study of the
sequential stoichiometric combination according to Scheme 1 has provided compounds 2, 3, 4
and 5, each of which has been isolated in high yield. In addition, appropriate stoichiometric
combinations of compounds 2-4 with an additional equivalent of AgOTf and OPyrMe give
compounds 3-5, respectively.
Equation 1: Generic equation for the formation of compounds 2 (m=1, n=0), 3 (m=2, n=1), 4
(m=3, n=2) and 5 (m=4, n=3)
Figure 2: General structure of compounds 2-5
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The solid-state structures of compounds 2, 3 and 5 are shown in Figure 3 with selected
structural parameters listed in Table 1. It has not been possible to determine the solid-state
structure of compound 4. In each of the compounds the antimony center adopts a distorted
octahedral geometry with the two phenyl substituents in a trans (C-Sb-C=168.62-177.45°)
configuration. Attempts to form the five coordinate species have resulted only in the isolation of
the six coordinate species. This observed preference for an octahedral configuration contrasts the
3+
speculated trigonal bipyramidal structures of R2SbL3 , that were previously proposed on the
basis of elemental analysis and 1H NMR.[16]
The Sb-O{PyrMe} bonds in 2, 3 and 5 are slightly longer than the sum of the covalent
radii of Sb and O (ƩCRSb---O = 2.03Å), while the Sb---O{Tf} contacts are outside the sum of the
van der Waals radii (ƩvdWSb---O = 3.58Å), indicating that all compounds are ionic. The Sb-
O{PyrMe} bonds in 2, 3 and 5 are all shorter than that in [Ph4Sb(OPyrMe)][OTf][7]. The Sb-O
bonds in 2 and 3 are longer than that in [Ph3Sb(OPyrMe)2][OTf]2,[7] and the shortest Sb-O bonds
are observed in 5, consistent with the magnitude of the cationic charge. The Sb-Cl bonds in 2 are
similar to those in Ph3SbCl2 (average Sb-Cl = 2.463(2) Å),[17]and the Sb-Cl bond trans to
OPyrMe is the shortest, indicating that the trans influence of Cl- is greater than that of OPyrMe.
[13]
This observation is consistent with the bond distances observed for the hydrate of Ph2SbCl3
[Sb-Cl(trans to OH2) = 2.331(4)Å, Sb-Cl(trans to Cl) = 2.490(4)Å]. The cis configuration and
bond lengths of the Sb-Cl bonds in 3 are similar to those in Ph2SbCl2(acac) [Sb-Cl =
2.418(3)Å].[18] The degree of twisting of the phenyl rings increases from 2 to 5, likely due to
interactions between the phenyl and pyridine rings.
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10.1002/chem.201800196
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2)
3)
5)
Figure 2: Solid state structures of 2) Ph2SbCl3(OPyrMe), 3) The cation in
[Ph2SbCl2(OPyrMe)2][OTf] and 5) The cation in [Ph2Sb(OPyrMe)4][OTf]3. All hydrogen atoms
and counter-anions omitted for clarity.
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Table 1: Selected bond lengths (Å) and angles (°) of compounds 2, 3 and 5 in comparison with
those in [Ph4Sb(OPyrMe)][OTf] and [Ph3Sb(OPyrMe)2][OTf]2
[Ph4Sb(OPyrMe)][OTf] [Ph3Sb(OPyrMe)2][OTf]2
2
3
5
2.1779(12)
2.164(2)
2.117(4)
2.4486(12)
2.1381(15)
Sb-O1
Sb-O2
-
-
-
2.160(2)
2.098(3)
-
2.1392(15)
-
-
2.135(4)
-
-
Sb-O3
2.123(4)
-
-
Sb-O4
2.4561(5) 2.4086(10)
2.4380(5) 2.4187(10)
-
-
-
Sb-Cl1
Sb-Cl2
Sb-Cl3
Sb---OTf
C-Sb-C
-
-
-
2.4449(5)
-
-
-
-
-
5.6392(12)
4.920(4)
5.185(2)
123.22(6)
115.31(7)
114.83(7)
97.22(7)
98.18(6)
100.60(6)
-
4.656(2)
118.07(8)
120.70(8)
121.23(8)
174.39(6) 168.62(12) 177.45(19)
12.4
32.7
51.8
-
Ph-Ph
Twist
angle
In conclusion, a comprehensive series of Sb(V) complexes with the generic formula
[Ph2SbCl3-xLx+1][OTf]x (L = OPyrMe) has been prepared by chloride ion abstraction in the
presence of the ligand, including the first example of a Pn(V)3+ trication. In contrast, attempts to
prepare [Ph2Sb(bipy)]3+ resulted in reduction of antimony(V) to antimony(III) with concommital
elimination of PhCl. We are expanding the ligand scope for these cations with a view to
increasing structural diversity and redox reactivity, as well as targeting tetra- and pentacations
that are expected to exhibit more diverse redox chemistry as dictated by the increased cationic
charge.
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