Diazabutadiene complexes of selenium as Se2+ transfer reagents

Article abstract of DOI:10.1039/b917841d

Alkyl and aryl substituted diazabutadiene ligands are shown to support a highly electrophilic Se²⁺ synthon, which can be utilized in ligand exchange reactions to generate Se centred dicationic coordination complexes.

Full text of DOI:10.1039/b917841d

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Diazabutadiene complexes of selenium as Se2+ transfer reagentsw
Jason L. Dutton, Taylor L. Battista, Michael J. Sgro and Paul J. Ragogna*
Received (in Cambridge, UK) 1st September 2009, Accepted 2nd December 2009
First published as an Advance Article on the web 23rd December 2009
DOI: 10.1039/b917841d
Alkyl and aryl substituted diazabutadiene ligands are shown to
support a highly electrophilic ‘‘Se2+’’ synthon, which can be
utilized in ligand exchange reactions to generate Se centred
dicationic coordination complexes.
The isolation of new main group element synthons is critical to
the development of highly novel structure, bonding and
reactivity for the p-block elements. At the forefront of current
efforts is the study of low-oxidation state species, where recent
examples include the synthesis of stable P(I),1,2 P(0),3–5
As(I),1,6 Si(II),7,8 Si(0),9–11 Ge(II),12,13 B(I),14 S(II),15 Te(II),16,17
and C(0)18,19 complexes (e.g. 1–3).
diazabutadiene ligands, as well as results from combustion
analysis, the powder was assigned as being the Cy2DAB
chelate of SeCl2 (4).21 The addition of two stoichiometric
equivalents of TMS–OTf to a CH2Cl2 slurry of 4 resulted in
a colour change from yellow to nearly colourless within
40 minutes. The supernatant was then decanted and the
colourless powder washed with Et2O and dried in vacuo. A
1
sample of the powder was dissolved in CD3CN for H NMR
spectroscopy, which revealed a set of resonances consistent
with a single product containing the Cy2DAB framework. The
singlet arising from the protons on the ligand backbone was
shifted far downfield (d = 9.82 ppm) from the free ligand
(d = 7.92 ppm), which is a typical feature when a large
positive charge is imparted onto the ring.22 X-Ray diffraction
studies on single crystals grown from the bulk powder revealed
the dicationic Se–Cy2DAB chelate paired with two triflate
anions (5) (Fig. 1). The metrical parameters of the five
The common theme for these achievements is the use
of a formally neutral 2-electron donor ligands to stabilize
otherwise highly transient species. Although a variety of
donors can be utilized (i.e. phosphine, imine), a universal
player is the N-heterocyclic carbene, as it has been utilized
in a large majority of the examples cited above. Once
stabilized, the compounds can, in principle, be stored for later
use, while still retaining the unique reactivity of a low-oxidation
state central core. In this context, we report the isolation
and comprehensive characterization of an electrophilic Se2+
synthon (5) supported by a diazabutadiene (DAB) ligand.z
The Se(II) centre can be forced from the chelate using NHC or
4-DMAP to generate trapped Se2+ dications (6, 7). These
transformations indicate that the central element in 5 is
uniquely available for further chemistry and can be utilized
as the synthetic equivalent of a naked Se2+
.
The 1 : 1 stoichiometric reaction between dicyclohexyl-
diazabutadiene (Cy2DAB) and SeCl2 (generated in situ from
SeCl4 and SbPh3)20 in THF resulted in the immediate
precipitation of a yellow powder. The supernatant was decanted
and the powder washed with Et2O to remove the SbPh3Cl2
byproduct. The isolated material was found to be highly
insoluble in all organic solvents, precluding NMR spectro-
scopy, and the growth of single crystals. Nevertheless,
based on studies of the reaction of SeCl2 with aryl substituted
Fig. 1 Solid state structure of 5. One of the two formula units within
the asymmetric unit is represented. Ellipsoids are drawn to 30%
probability and hydrogen atoms are omitted for clarity. Selected bond
lengths (A) and angles (1): Se(1)–N(1) 1.843(3), Se(1)–N(2) 1.845(3),
N(1)–C(1) 1.294(5), C(1)–C(2) 1.415(5), Se(1)–O(11) 2.570(3),
Se(1)–O(23) 2.711(3), S(11)–O(11) 1.432(3), S(11)–O(12) 1.433(3),
S(11)–O(13) 1.429(3), N(1)–Se(1)–N(2) 84.2(1).
Department of Chemistry, The University of Western Ontario, 1151
Richmond St., London, Ontario, Canada. E-mail: pragogna@uwo.ca;
Fax: +1-519-661-3022; Tel: +1-519-661-2111 ext. 87048
w CCDC 738651–738653. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b917841d
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membered ring are in almost perfect agreement with
previously calculated values.23 The Se–N bonds average 1.84 A,
and the N–C bonds average 1.30 A, reflecting retainment of
the diimine framework. Long Se–O contacts of 2.57–2.71 A
are observed in the solid state; in solution the compound is
distinctly ionic, based on the 19F{1H} chemical shift of the
triflates (d = ꢁ78.4 ppm, cf. MeOTf d = ꢁ75.4 ppm;
[NOct]4[OTf] d = ꢁ79.0 ppm).24
We have previously synthesized a related Se dication, with
diisopropylphenyl R-groups on the DAB ligand.22 However,
this dication was paired with the highly reactive [SnCl6]
dianion, precluding our ability to study the chemistry. Having
the dication in hand paired with inert triflate anions, we
sought to demonstrate that the Se2+ centre is labile, therefore
we explored its reactivity in ligand exchange reactions.
The 2
: 1 stoichiometric reaction of 5 with the NHC
2,5-diisopropylimidazole-3,4-dimethyl-2-ylidene (iPr2NHC)
in THF resulted in a yellow solution. After workup a light
yellow powder was obtained. A sample of the solid was
redissolved in CDCl3 for 1H NMR spectroscopy, which
revealed a set of resonances indicative of a single product
containing the iPr2NHC and the absence of Cy2DAB.
Single crystals were grown from a CH2Cl2 solution of
the bulk powder via vapour diffusion of n-pentane at
30 1C, and subsequent X-ray diffraction experiments revealed
Fig. 3 Solid state structure of 7. Ellipsoids are drawn to 30%
probability. Triflate anions, hydrogen atoms and CH2Cl2 solvate
are omitted for clarity. One of the two independent dications
within the asymmetric unit is represented. The Se atom sits on an
inversion centre. Selected bond lengths (A) and angles (1): Se(1)–N(11)
2.149(5), Se(1)–N(21) 2.159(5), N(11)–Se(1)–N(21) 85.4(2),
N(11)–Se(1)–N(21A) 94.6(2).
The reaction of a Se(II) synthon supported by the aryl
substituted Dipp2BIAN ligand (8) with an excess of AgOTf
and four equivalents of 4-DMAP (4-methylaminopyridine)
resulted in a yellow slurry.21,25 Upon removal of the silver
salts and precipitation with n-pentane a nearly colourless
powder was obtained, the 1H NMR spectrum showed only
signals arising from a 4-DMAP derivative. Single crystals were
grown from a CH2Cl2 solution of the bulk powder via vapour
diffusion of Et2O at ꢁ30 1C. X-Ray diffraction studies
revealed a Se centred dication (5) bearing four 4-DMAP
substituents in a pinwheel motif, approaching D4h symmetry
(Fig. 3). The closest Se–OTf contact is 4.02 A, well outside the
sum of the van der Waals radii for Se and O (3.4 A). The
presence of the Dipp2BIAN ligand is critical to the reaction for
the stabilization of the N-chelated Se(OTf)2 intermediate (9);
the same reaction using free SeBr2 does not give 7 as the
product.
i
a Se centred dication bound by two Pr2NHC ligands (6)
(Fig. 2).
The Se–C bonds are 1.915(3) A and 1.920(3) A for
Se(1)–C(10) and Se(1)–C(20), respectively, and the
C(10)–Se(1)–C(20) bond angle is 96.3(1)1. These two pieces
of data point to purely single Se–C bonds, unlike the partial
allene character observed in the C(0) analogue 3 (C–C–C =
1341), similar to the cationic P(I) species (1) (C–P–C = 97.41),
and our work with the dicationic Te(II) congener (C–Te–C =
91.51).2,16,18 Unlike our tellurium work the addition of two
further equivalents of NHC does not give a square planar
complex, but rather results in decomposition.
We have shown that it is possible to stabilize a highly
electrophilic Se(II) source utilizing diazabutadiene ligands.
The Se centre is sufficiently labile to allow for the delivery
of the Se2+ fragment in ligand exchange reactions, and we
are currently exploring its reactivity in small molecule
activations.
Fig. 2 Solid state structure of 6. Ellipsoids are drawn to 30%
probability hydrogen atoms and methyl groups on the isopropyl
substituents are omitted for clarity. Selected bond lengths (A) and
angles (1): Se(1)–C(10) 1.915(3), Se(1)–C(20) 1.920(3), Se(1)–O(12)
2.969(2), Se(1)–O(21) 2.755(3), C(10)–Se(1)–C(20) 96.3(1).
We thank Natural Sciences and Engineering Research
Council (NSERC), The Canada Foundation for Innovation,
and The University of Western Ontario for their financial
support.
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Yield 0.070 g, 43%; dp 135–138 1C; 1H NMR (CDCl3, d ppm): 8.40 (d,
3JHꢁH 6.8 Hz, 2H), 6.55 (d, 3JHꢁH 7.2 Hz, 2H), 3.05 (s, 6H); 13C{1H}
NMR (CH2Cl2, d ppm): 155.1, 148.0, 107.0, 39.0; 19F{1H} NMR
(CH2Cl2, d ppm): ꢁ78.6; FT-Raman (cmꢁ1, ranked intensity ()):
111(3), 253(2), 312(11), 348(8), 571(12), 655(13), 760(1), 947(5),
1032(4), 1058(6), 1222(7), 1444(15), 1611(9), 1563(14), 2932(10);
FT-IR (cmꢁ1, ranked intensity ()): 520(12), 572(14), 636(7), 755(15),
808(11), 1002(5), 1031(4), 1052(8), 1154(6), 1219(3), 1269(2), 1396(9),
1444(13), 1561(10), 1616(1). Crystal data: C31.5H43Cl3F6N8O6S2Se1,
moiety formula [Se1N8C28H40][CF3S1O3]2ꢃ1.5CH2Cl2; M = 993.16 g
Notes and references
z All manipulations were performed under an N2 atmosphere in a
glovebox. Synthesis of 4: a solution of Ph3Sb (0.160 g, 0.455 mmol;
THF 3 mL) was added to a solution of SeCl4 (0.100 g, 0.455 mmol;
THF 5 mL) giving an orange solution. A solution of Cy2DAB (0.100 g,
0.455 mmol; THF 3 mL) was added resulting in the immediate
generation of a yellow slurry. The mixture was centrifuged and the
supernatant was decanted. The precipitate was washed with Et2O
(3 ꢂ 5 mL) and dried in vacuo giving 4 as a yellow powder. Yield 0.134 g,
80%; dp 130–131 1C; FT-Raman (cmꢁ1, ranked intensity ()): 85(2),
123(1), 213(6), 252(3), 330(9), 410(8), 513(13), 587(12), 796(15),
1297(7), 1445(14), 1483(10), 2854(5), 2940(4), 3014(11); FT-IR
(cmꢁ1, ranked intensity ()): 435(13), 520(5), 583(14), 869(8), 894(15),
1009(11), 1027(10), 1059(7), 1277(9), 1298(12), 1445(6), 1481(2),
2853(4), 2932(1), 3014(3); elemental analysis, found (calcd): C 45.79
(45.40), H 6.28 (6.54), N 7.19 (7.57)%. Synthesis of 5: neat TMS–OTf
(91 mL, 0.500 mmol) was added to a slurry of 4 (0.099 g, 0.250 mmol;
CH2Cl2 5 mL) resulting in a colour change from yellow to nearly
colourless over 40 minutes. Diethyl ether (10 mL) was added and the
solids were allowed to settle. The supernatant was decanted and the
powder washed with Et2O (3 ꢂ 5 mL). The solid was then dried
in vacuo giving 5 as a colourless powder. Yield 0.147 g, 95%; dp
141–143 1C; 1H NMR (CD3CN, d ppm): 9.82 (s, 2H), 5.18 (m, 2H,
cyclohexyl C-H), 2.31–1.28 (cyclohexyl CH2); 13C{1H} NMR
(CD3CN, d ppm): 156.7, 71.1, 35.0, 25.0, 24.0; 19F{1H} NMR
(CH3CN, d ppm): ꢁ78.4; 77Se{1H} NMR (CD3CN, d ppm): 1335;
FT-Raman (cmꢁ1, ranked intensity ()): 111(2), 252(1), 349(9), 521(14),
645(15), 761(6), 799(7), 1034(3), 1147(8), 1224(11), 1250(12), 1356(13),
1482(4), 2870(10), 2953(5); FT-IR (cmꢁ1, ranked intensity ()): 515(8),
576(12), 640(5), 752(14), 875(15), 1029(3), 1172(4), 1237(1), 1280(2),
1430(13), 1455(9), 1481(10), 2870(11), 2952(7), 3068(6); elemental
analysis, found (calcd): C 32.05 (32.16), H 3.56 (4.05), N 4.12
(4.69)%; ESI-MS (m/z+): [M ꢁ H] 299, [M ꢁ C6H11] 217. Crystal
data: C16H24F6N2O6S2Se1, M = 597.45 g molꢁ1, monoclinic, C2/c,
a = 20.755(4), b = 20.753(4), c = 22.493(5) A, b = 102.06(3)1, V =
9474(3) A3, T = 150(2) K, Z = 16, DC = 1.675 Mg mꢁ3, measured
reflections = 20 705, unique = 10 832 (Rint = 0.0655), refined
parameters = 596, R[I 4 2s(I)] = 0.0551, wR2(F2) = 0.1208, R1
(all data) = 0.1213, wR2 (all data) = 0.1456. CCDC 738651. Synthesis
of 6: a solution of NHC (0.102 g, 0.565 mmol; THF 5 mL) was added
dropwise at ꢁ30 1C to a slurry of 5 (0.175 g, 0.283 mmol; THF 5 mL)
resulting in a light yellow solution. n-Pentane (3 mL) was added
resulting in the deposition of an orange gel. The supernatant was
decanted and a further 10 mL of n-pentane was added resulting in the
precipitation of a light yellow powder. The mixture was cooled to
–30 1C and the supernatant decanted. The powder washed with Et2O
(3 ꢂ 5 mL). The solids were dried in vacuo giving 6 as a light yellow
powder. Yield 0.104 g, 50%; dp 102–105 1C; 1H NMR (CD3CN,
molꢁ1, triclinic, P1, a = 11.951(2), b = 13.706(3), c = 14.440(3), a =
ꢀ
67.84(3)1, b = 86.92(3)1, g = 78.47(3)1, V = 2145.8(7) A3, T = 150(2) K,
Z = 2, DC = 1.536 Mg mꢁ3, measured reflections = 14 477, unique =
7641 (Rint = 0.0380), refined parameters = 536, R[I 4 2s(I)] =
0.0815, wR2(F2)
=
0.2215, R1 (all data)
= 0.1191, wR2
(all data) = 0.2526. CCDC 738653.
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25 Attempts at utilizing compound 5 for this reaction gave protonated
4-DMAP as the main product owing to the acidic protons found
on the ligand ‘‘backbone’’. Use of the BIAN ligand circumvents
this problem. For the synthesis of 8, see ref. 21.
3
3
d ppm): 5.13 (sept, JHꢁH 6.8 Hz, 4H), 2.45 (s, 12H), 1.60 (d, JHꢁH
6.8 Hz, 24H); 19F{1H} NMR (CH2Cl2, d ppm): ꢁ78.4; FT-Raman
(cmꢁ1, ranked intensity ()): 112(12), 146(11), 253(13), 313(3), 347(10),
573(15), 753(7), 886(14), 1031(1), 1278(4), 1367(9), 1406(5), 1450(8),
1615(6), 2947(2); FT-IR (cmꢁ1, ranked intensity ()): 516(15), 571(11),
752(14), 781(4), 903(8), 1221(9), 1380(6), 1400(7), 1453(13), 1616(12),
2305(1), 2354(3), 2944(5), 2988(10), 3446(2); ESI-MS (m/z+):
[M]2[OTf]3 1327, [M][OTf] 588, [M
C24H40F6N4O6S2Se1, 737.68
ꢁ
iPr] 398. Crystal data:
M
=
g
molꢁ1
,
monoclinic, P21/c,
a = 12.181(2), b = 17.678(4), c = 15.178(3) A, b = 95.10(3)1, V =
3256(1) A3, T = 150(2) K, Z = 4, DC = 1.505 Mg mꢁ3, measured
reflections
= 12 210, unique = 6449 (Rint = 0.0326), refined
parameters = 400, R[I 4 2s(I)] = 0.0423, wR2(F2) = 0.0930, R1
(all data) = 0.0616, wR2 (all data) = 0.1022. CCDC 738652. Synthesis
of 7: a solution of 8 (0.139 g, 0.188 mmol; CH2Cl2 5 mL) was added to
solid AgOTf (0.120 g, 0.469 mmol; CH2Cl2 5 mL) and was allowed to
stir for 10 minutes, giving a dark orange mixture. A solution of
4-DMAP (0.091 g, 0.752 mmol; CH2Cl2 3 mL) was added giving a
light orange slurry. The mixture was centrifuged and the supernatant
decanted. n-Pentane (15 mL) was added resulting in a light yellow
precipitate. The supernatant was decanted and the precipitate washed
with Et2O (3 ꢂ 5 mL), then dried in vacuo giving 7 as a beige powder.
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 1041–1043 | 1043