Reactions of Ph3Sb=S with copper(I) complexes supported by N-donor ligands: Formation of stable adducts and S-transfer reactivity

Article abstract of DOI:10.1021/ic102449m

In the exploration of sulfur-delivery reagents useful for synthesizing models of the tetracopper-sulfide cluster of nitrous oxide reductase, reactions of Ph₃Sb=S with Cu(I) complexes of N,N,N′,N′-tetramethyl- 2R,3-R-cyclohexanediamine (TMCHD) and 1,4,7-trialkyltriazacyclononanes (R ₃tacn; R = Me, Et, iPr) were studied. Treatment of [(R ₃tacn)Cu(NCCH₃)]SbF₆ (R = Me, Et, or iPr) with 1 equiv of S=SbPh₃ in CH₂Cl₂ yielded adducts [(R₃tacn)Cu(S=SbPh₃)]SbF₆ (1-3), which were fully characterized, including by X-ray crystallography. The adducts slowly decayed to [(R₃tacn)₂Cu₂(μ- η²:η²-S₂)]²⁺ species (4-6) and SbPh₃, or more quickly in the presence of additional [(R ₃tacn)Cu(NCCH₃)]SbF₆ to 4-6 and [(R ₃tacn)Cu(SbPh₃)]SbF₆ (7-9). The results of mechanistic studies of the latter process were consistent with rapid intermolecular exchange of S=SbPh₃ between 1-3 and added [(R ₃tacn)Cu(NCCH₃)]SbF₆, followed by conversion to product via a dicopper intermediate formed in a rapid pre-equilibrium step. Key evidence supporting this step came from the observation of saturation behavior in a plot of the initial rate of loss of 1 versus the initial concentration of [(Me₃tacn)Cu(NCCH₃)]SbF₆. Also, treatment of [(TMCHD)Cu(CH₃CN)]PF₆ with S=SbPh₃ led to the known tricopper cluster [(TMCHD)₃Cu₃(μ₃-S) ₂](PF₆)₃ in good yield (79%), a synthetic procedure superior to that previously reported.

Full text of DOI:10.1021/ic102449m

ARTICLE
pubs.acs.org/IC
Reactions of Ph3SbdS with Copper(I) Complexes Supported by
N-Donor Ligands: Formation of Stable Adducts and S-Transfer
Reactivity
Lei Yang, Jacqui Tehranchi, and William B. Tolman*
Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, 207 Pleasant Street SE, Minneapolis,
Minnesota 55455, United States
S Supporting Information
b
ABSTRACT: In the exploration of sulfur-delivery reagents useful for synthesiz-
ing models of the tetracopper-sulfide cluster of nitrous oxide reductase, reactions
of Ph3SbdS with Cu(I) complexes of N,N,N0,N0-tetramethyl-2R,3R-cyclohex-
anediamine (TMCHD) and 1,4,7-trialkyltriazacyclononanes (R3tacn; R = Me,
Et, iPr) were studied. Treatment of [(R3tacn)Cu(NCCH3)]SbF6 (R = Me, Et, or
iPr) with
1
equiv of SdSbPh3 in CH2Cl2 yielded adducts
[(R3tacn)Cu(SdSbPh3)]SbF6 (1-3), which were fully characterized, including
by X-ray crystallography. The adducts slowly decayed to [(R3tacn)2Cu2(μ-η2:η2-
S2)]2þ species (4-6) and SbPh3, or more quickly in the presence of additional
[(R3tacn)Cu(NCCH3)]SbF6 to 4-6 and [(R3tacn)Cu(SbPh3)]SbF6 (7-9). The results of mechanistic studies of the latter
process were consistent with rapid intermolecular exchange of SdSbPh3 between 1-3 and added [(R3tacn)Cu(NCCH3)]SbF6,
followed by conversion to product via a dicopper intermediate formed in a rapid pre-equilibrium step. Key evidence supporting this
step came from the observation of saturation behavior in a plot of the initial rate of loss of 1 versus the initial concentration of
[(Me3tacn)Cu(NCCH3)]SbF6. Also, treatment of [(TMCHD)Cu(CH3CN)]PF6 with SdSbPh3 led to the known tricopper
cluster [(TMCHD)3Cu3(μ3-S)2](PF6)3 in good yield (79%), a synthetic procedure superior to that previously reported (Brown, E.
C.; York, J. T.; Antholine, W. E.; Ruiz, E.; Alvarez, S.; Tolman, W. B. J. Am. Chem. Soc. 2005, 127, 13752-13753).
’ INTRODUCTION
sulfide complexes.10,11 The utility of Ph3SbS likely stems from
the thermodynamic instability of the Sb-S bond, which is
weaker than the related bonds in R3EdS (E = P or As)
congeners.12 Herein, we report the results of an investigation
of the reactivity of Ph3SbdS with selected Cu(I) complexes of N,
The environmentally important reduction of N2O to N2 is
catalyzed enzymatically by nitrous oxide reductase,1 which has
been shown on the basis of X-ray crystallography2 and other
spectroscopic techniques3 to contain a unique tetracopper-
sulfide cluster supported by multiple histidine residues in its
active site. The novel structure of this cluster and hypotheses for
the mechanism of N2O coordination and reduction at its copper
centers4 have stimulated extensive efforts to create copper-
sulfur model complexes supported by N-donor ligands.5,6 These
efforts have focused on the use of relatively few types of sulfur-
containing reagents (e.g., S8 and Na2S2) in reactions with Cu(I)
and Cu(II) species. Studies of the copper-sulfur compounds
characterized so far have raised interesting bonding questions,7,8
and, in one instance, have led to the discovery of reactivity with
N2O.6 Nonetheless, only a limited array of N-donor ligated
copper-sulfur motifs have been characterized, and an accurate
model of the nitrous oxide active site has yet to be constructed.
In seeking to broaden the scope of available copper-sulfur
complexes as a means to address mechanistic and electronic
structural issues relevant to the enzyme active site, we are
exploring reactions of copper precursors with an expanded array
of sulfur-containing reagents. Several studies have shown that
triphenyl antimony sulfide (Ph3SbdS) is useful for sulfur transfer
reactions,9 including for the preparation of transition metal
N,N0,N0-tetramethyl-2R,3R-cyclohexanediamine
(TMCHD)
and 1,4,7-trialkyltriazacyclononanes (R3tacn; R = Me, Et, iPr).
Key findings include the isolation and structural characterization
of novel LCu(I)-SdSbPh3 (L = R3tacn; R = Me, Et, iPr) adducts,
which are the first examples of transition metal complexes bound
to Ph3SbdS to be structurally characterized by X-ray
crystallography.13 These complexes subsequently decay cleanly
to [Cu2(μ-η2:η2-S2)]2þ species, particularly when treated with
additional [(R3tacn)Cu(CH3CN)]SbF6, and mechanistic in-
sights for this process were obtained through kinetics studies.
In addition, by using, Ph3SbdS an improved synthetic route to
the [Cu3S2]3þ core8 was discovered.
’ EXPERIMENTAL SECTION
General Considerations. All solvents and reagents were
obtained from commercial sources and used as received unless otherwise
Received: December 7, 2010
Published: February 21, 2011
r
2011 American Chemical Society
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Inorganic Chemistry
ARTICLE
noted. The solvents CH2Cl2, pentane, and Et2O were dried over
CaH2 and distilled under vacuum or passed through solvent
purification columns (Glass Contour, Laguna, CA). The complexes
Et2O into the dark brown filtrate at room temperature afforded the
product as deep green crystals (16 mg, 46%). 1H NMR (300 MHz, d7-
DMF): δ = 7.32 (s, 16H), 6.96 (t, 16H), 6.81 (t, 8H), 3.16-3.09 Hz (m,
36H), 1.36 (t, J = 6.0 Hz, 18H) ppm; 13C{1H} NMR (75 MHz, d7-
DMF): δ = 137.11, 135.71, 126.33, 122.62, 93.84, 56.04, 54.64, and
12.39 ppm. UV-vis [λmax, nm (ε, M-1 cm-1) in DMF]: 410 (7400),
378 (7600). Anal. Calcd for C72H94B2Cu2N6S2: C, 68.83; H, 7.54; N,
6.69. Found: C, 68.45; H, 7.42; N, 6.70. FT-IR: 3052.7, 2978.9, 1579.6,
1478.5, 1465.9, 1386.9, 1270.4, 1258.5, 1144.8, 1122.9, 1069.9, 1033.3,
1018.3, 999.5, 921.0, 862.4, 842.3, 821.8, 792.9, 771.5, 742.3, 732.0,
[(R3tacn)Cu(CH3CN)]SbF6
(R
=
Me,14
Et,15
iPr16),
[(R3tacn)Cu(CH3CN)]BPh4 (R = Et15 or iPr17), [(Me3tacn)2Cu2-
18
(S2)](SbF6)2,14 and [(TMCHD)Cu(CH3CN)]PF6 were prepared
according to published procedures. All metal complexes were prepared
and stored in a glovebox under a dry N2 atmosphere. Triphenylantimony
sulfide (Ph3SbdS) and 2,3-dimethylbutadiene were purchased from
Strem and Aldrich, respectively, and were used without purification.
NMR spectra were recorded on either Varian VI-300 or VXR-300
spectrometers at ∼20 °C. Chemical shifts (δ) were referenced to
residual solvent signal and integrated intensities compared to 1,3,5-
trimethyloxybenzene added as an internal standard. UV-vis spectra
were recorded on an HP8453 (190-1100 nm) diode-array spectro-
photometer. Elemental analyses were performed by Robertson Microlit
Laboratory (Ledgewood, NJ). Electrospray ionization mass spectra
(ESI-MS) were recorded on a Bruker BioTOF II instrument. IR spectra
were obtained using a ThermoNicolet Avatar 370 FT-IR.
710.1, 700.6, 668.2 cm-1
.
[(iPr3tacn)2Cu2(μ-S2)](BPh4)2 (6). A similar procedure to that
used for the preparation of 5 was followed, except THF was used as the
reaction solvent and the product was isolated as dark red crystals from
CH2Cl2 at -20 °C (37% yield). 1H NMR (300 MHz, CD2Cl2): δ = 7.33
(s, 15H), 7.04 (t, J = 6.0 Hz, 14H), 6.89 (t, J = 6.0 Hz, 11H), 3.15-3.06
Hz (m, 5H), 2.95-2.74 (m, 4H), 2.65-2.46 (m, 11H), 2.33-2.20 (m,
10H), 1.26-1.08 (d, J = 6.0 Hz, 36H) ppm; 13C{1H} NMR (75 MHz,
CD2Cl2): δ = 136.44, 126.22, 122.42, 60.47, 51.75, 45.10, 19.79, and
18.46 ppm. UV-vis [λmax, nm (ε, M-1 cm-1) in CH2Cl2]: 476 (7200),
380 (11000). Anal. Calcd for C78H106B2Cu2N6S2: C, 69.88; H, 7.97; N,
6.27. Found: C, 69.91; H, 7.78; N, 6.24. FT-IR: 3053.5, 2978.0, 1579.8,
1480.7, 1467.0, 1451.0, 1426.7, 1390.1, 1369.2, 1347.1, 1291.9, 1268.7,
1166.3, 1141.9, 1129.4, 1067.3, 1046.6, 1033.5, 962.4, 943.1, 841.7,
[(R3tacn)Cu(SdSbPh3)]SbF6 (R = Me (1), Et (2), or iPr
(3)). All three complexes were prepared according to this illustrative
procedure: In an inert atmosphere, to a solution of the SdSbPh3
(32 mg, 0.083 mmol) in CH2Cl2 (4 mL) was added
[(Me3tacn)Cu(NCCH3)]SbF6 (43 mg, 0.083 mmol) in CH2Cl2
(1 mL). After stirring for 1 h, the mixture was filtered and the volume
of the filtrate was reduced to ∼1 mL, and Et2O (15 mL) was added,
resulting in formation of a yellow precipitate. The supernatant was
decanted, and the yellow powder was washed with Et2O (3 ꢀ 6 mL).
The product was isolated in crystalline form by layering Et2O onto a
concentrated tetrahydrofuran (THF) solution at -20 °C (65 mg, 92%).
Analogous procedures were used to isolate 2 and 3 as yellow crystals in
41% and 32% yields, respectively. 1: 1H NMR (300 MHz, CD2Cl2): δ =
7.77-7.61 (m, 15H), 2.59-2.53 Hz (m, 12H), 2.38 (s, 9H) ppm;
13C{1H} NMR: (75 MHz, CD2Cl2): δ = 134.37, 133.44, 130.97, 55.03,
49.02 ppm. UV-vis [λmax, nm (ε, M-1 cm-1) in CH2Cl2]: 356 (2100).
Anal. Calcd for C27H36CuF6N3SSb2: C, 37.90; H, 4.24; N, 4.91. Found:
C, 37.77; H, 4.24; N, 4.99. ESI-MS: [Cu(Me3tacn)(SdSbPh3)]þ calc.
m/z 620.0, found 620.3. FT-IR: 2859.2, 1480.5, 1460.1, 1437.5, 1363.4,
1300.0, 1152.8, 1130.2, 1089.4, 1065.5, 1017.0, 966.3, 984.1, 889.5,
773.5, 751.3, 735.4, 692.4, 656.4 cm-1. 2: 1H NMR (300 MHz,
CD2Cl2): δ = 7.74-7.60 (m, 15H), 2.60-2.53 Hz (m, 18H), 1.10 (t,
J = 6.0 Hz, 9H) ppm; 13C{1H} NMR (75 MHz, CD2Cl2): δ = 134.6,
133.4, 130.9, 55.3, 53.3, 13.0 ppm. UV-vis [λmax, nm (ε, M-1 cm-1) in
CH2Cl2]: 356 (2300). Anal. Calcd for C30H42CuF6N3SSb2: C, 40.13; H,
4.72; N, 4.68. Found: C, 39.76; H, 4.70; N, 4.65. ESI-MS:
[Cu(Et3tacn)(SdSbPh3)]þ calc. m/z 662.0, found 662.2. FT-IR:
2972.4, 1479.3, 1437.6, 1378.9, 1347.9, 1315.0, 1136.6, 1124.9,
1067.4, 1041.1, 1031.4, 995.7, 928.8, 897.9, 876.2, 860.3, 851.7, 827.1,
812.7, 796.8, 751.0, 739.1 cm-1. 3: 1H NMR (300 MHz, CD2Cl2): δ =
7.73-7.59 (m, 15H), 2.66-2.61 Hz (m, 9H), 2.47-2.44 Hz (m, 6H),
1.13 (d, J = 6.0 Hz, 18H) ppm; 13C{1H} NMR (75 MHz, CD2Cl2): δ =
761.0, 749.3, 734.1, 705.6, 680.6, 668.1 cm-1
.
[(R3tacn)Cu(SbPh3)]SbF6 (7, R = Me; 8, R = Et; 9, R =
iPr). These complexes were prepared similarly, according to the
following representative procedure for 7. In an inert atmosphere, to a
solution of SbPh3 (40.0 mg, 0.113 mmol) in CH2Cl2 (4 mL) was added
[(Me3tacn)Cu(NCCH3)]SbF6 (58.0 mg, 0.113 mmol) in CH2Cl2 (1
mL). The mixture was stirred 3 h, filtered, and the volume of the filtrate
was reduced to ∼1 mL under reduced pressure. A portion of Et2O (15
mL) was added to yield a white precipitate. The supernatant solution
was decanted, and the white powder was washed three times with Et2O
(3 ꢀ 6 mL). The white product was recrystallized by diffusion of Et2O
into a concentrated CH2Cl2 solution at room temperature to generate
the product as colorless crystals (71 mg, 76%). 7: 1H NMR (300 MHz,
CD2Cl2): δ = 7.48-7.43 (m, 15H), 2.90 Hz (s, 12H), 2.73 (s, 9H) ppm;
13C{1H} NMR (75 MHz, CD2Cl2): δ = 135.87, 130.89, 130.27, 55.98,
50.69 ppm. Anal. Calcd for C27H36CuF6N3Sb2: C, 39.37; H, 4.41; N,
5.10. Found: C, 38.87; H, 4.61; N, 4.86. ESI-MS:
[Cu(Me3tacn)(SbPh3)]þ calc. m/z 588.0, found 588.1. FT-IR:
1463.1, 1434.1, 1363.8, 1299.2, 1153.8, 1127.8, 1085.8, 1069.8,
1057.1, 1015.0, 1000.3, 985.9, 768.3, 737.6, 697.7 cm-1. 8 (68% yield):
1H NMR (300 MHz, CD2Cl2): δ = 7.48-7.41 (m, 15H), 2.90-2.85 Hz
(m, 18H), 1.15 (t, J = 6.0 Hz, 9H) ppm; 13C{1H} NMR (75 MHz,
CD2Cl2): δ = 135.85, 130.88, 130.26, 56.87, 54.68, 14.46 ppm. Anal.
Calcd for C30H42CuF6N3Sb2: C, 41.62; H, 4.89; N, 4.85. Found: C,
41.50; H, 5.20; N, 4.92. ESI-MS: [Cu(Et3tacn)(SdSbPh3)]þ calc. m/z
662.0, found 662.2. FT-IR: 1434.2, 1375.8, 1338.5, 1121.2, 1067.2,
1
1037.6, 999.7, 931.38, 768.2, 737.3, 699.67 cm-1. 9 (70% yield): H
138.75, 134.69, 133.38, 130.91, 58.29, 50.83, 19.70 ppm. UV-vis [λmax
,
NMR (300 MHz, CD2Cl2): δ = 7.49-7.41 (m, 15H), 3.15-3.11 (m,
3H), 2.93-2.89 Hz (m, 6H), 2.80-2.75 Hz (m, 6H), 1.13 (d, J = 6.0 Hz,
18H) ppm; 13C{1H} NMR (75 MHz, CD2Cl2): δ = 135.9, 130.85,
130.25, 59.69, 51.77, 20.13 ppm. Anal. Calcd for C33H48CuF6N3Sb2: C,
43.66; H, 5.33; N, 4.63. Found: C, 43.30; H, 5.72; N, 4.62. ESI-MS:
[Cu(iPr3tacn)(SbPh3)]þ calc. m/z 672.1, found 672.2. FT-IR: 1434.4,
1389.4, 1367.0, 1347.1, 1161.7, 1123.2, 1066.5, 997.3, 964.31, 756.2,
nm (ε, M-1 cm-1) in CH2Cl2]: 356 (2300). Anal. Calcd for
C33H48CuF6N3SSb2: C, 42.17; H, 5.15; N, 4.47. Found: C, 42.11; H,
5.23; N, 4.46. ESI-MS: [Cu(iPr3tacn)(SdSbPh3)]þ calc. m/z 704.1,
found 704.3. FT-IR: 2965.7, 1491.5, 1478.4, 1437.0, 1386.8, 1368.5,
1351.5, 1299.1, 1265.1, 1167.0, 1129.4, 1067.2, 1020.3, 995.7, 968.1,
856.7, 841.0, 750.3, 737.9, 721.3, 692.6, 656.5 cm-1
.
739.2, 720.6, 699.6 cm-1
.
[(Et3tacn)2Cu2(μ-S2)](BPh4)2 (5). Elemental sulfur (1.8 mg,
0.007 mmol) was added to a solution of [(Et3tacn)Cu(NCCH3)]-
BPh4 (36 mg, 0.056 mmol) in CH2Cl2 (4 mL). After stirring for 2 h,
solvent was removed under reduced pressure to yield a brown solid. The
brown solid was washed with Et2O (2 ꢀ 6 mL), extracted with
dimethylformamide (DMF, 2 mL), and then filtered. Slow diffusion of
[(TMCHD)3Cu3(S)2](PF6)3 (12). In an inert atmosphere, to a
solution of the [(TMCHD)Cu(NCCH3)]PF6 (31 mg, 0.073 mmol)
in CH2Cl2 (3 mL) was added SdSbPh3 (29 mg, 0.049 mmol) in CH2Cl2
(1 mL). After the mixture was stirred for 2 h, it was filtered, and the
solvent was removed from the filtrate under vacuum to yield a deep
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ARTICLE
Scheme 1. Reactions of Cu(I)-SdSbPh3 Adducts
Figure 1. Representation of the X-ray crystal structure of 1, showing the
cationic portion with all non-hydrogen atoms as 50% thermal ellipsoids
(H atoms omitted for clarity, only heteroatoms labeled).
Table 1. Selected Bond Distances (Å) and Angles (deg) for
the X-ray Structures of 1-3a
1
2
3
Cu1-N1
Cu1-N2
Cu1-N3
Cu1-S1
S1-Sb1
2.160(2)
2.107(2)
2.199(2)
2.1671(8)
2.2832(7)
3.411
2.006(6)
2.030(8)
2.157(4)
2.1813(10)
2.2735(9)
3.490
2.178(2)
2.161(2)
2.166(2)
2.2027(7)
2.2812(7)
3.547
Cu1 Sb1
green solid, which was washed with Et2O (3 ꢀ 6 mL). The deep green
powder obtained was crystallized from CH2Cl2 at -20 °C to form dark
amber crystals of the product (28 mg, 79%). The product was identified
by its X-ray crystal structure and by the similarity of its UV-vis spectrum
to previously reported data.8a
3 3 3
N1-Cu1-N2
N1-Cu1-N3
N2-Cu1-N3
N1-Cu1-S1
N2-Cu1-S1
N3-Cu1-S1
Cu1-S1-Sb1
84.96(9)
83.41(8)
91.3(3)
71.1(2)
84.52(8)
84.08(8)
83.96(9)
82.2(3)
84.39(8)
General Procedures for NMR Kinetics. In a glovebox, appro-
priate volumes of starting materials in CD2Cl2 were mixed in a vial and
the volumes were quickly adjusted to 1 mL so that the concentrations of
adducts 1-3 and [(R3tacn)Cu(CH3CN)]SbF6 were 4.7 mM and 47
mM, respectively. The solution was then quickly transferred to a J.
Young NMR tube that was removed from glovebox and placed in the
spectrometer probe. The progress of the reaction was monitored by 1H
NMR spectroscopy at room temperature with 1,3,5-trimethoxybenzene
as an internal standard. The initial rates were determined in experiments
in which the first 5-10% of the reaction was followed; the rate constants
were obtained by linear fitting of the initial rate change. In the
experiments with 2,3-dimethylbutadiene, identical procedures were
used except 20 equiv of 2,3-dimethylbutadiene was added to the mixture.
Data analysis and graphical representations were performed using the
program Kaleidagraph.
114.17(6)
148.91(7)
120.92(6)
100.06(3)
115.8(2)
150.2(2)
117.09(10)
103.14(4)
127.05(6)
143.48(6)
113.59(6)
104.55(3)
a Estimated standard deviations indicated in parentheses.
and (c) parent ions [(R3tacn)Cu(SSbPh3)]þ with appropriate
isotope patterns in ESI mass spectra (Supporting Information,
Figure S2).
The X-ray crystal structures of complexes 1-3 are shown in
Figure 1 (1) and Supporting Information, Figure S3 (2 and 3),
with selected bond distances and angles listed in Table 1. To our
knowledge, they represent the first such structures with
Ph3SbdS coordinated to a metal center.19 They contain similar
four-coordinate Cu(I) centers with highly distorted tetrahedral
geometries characterized by τ4 values: 0.640 (1), 0.657 (2), and
0.634 (3).20 Essentially, the distortion involves perturbation of
the Cu-S bonds from the idealized trigonal axis toward copla-
narity with two of the N-donor atoms on the supporting ligand
(N2 and N3 for 1, Figure 1), presumably as a result of steric
interactions between the N-donor ligand substituents and the
phenyl rings of the coordinated Ph3SbdS moiety. These steric
’ RESULTS AND DISCUSSION
Synthesis and Characterization of LCu(I)-SdSbPh3 Ad-
ducts and [L2Cu2(S2)]2þ Decay Products. Reaction of
[(R3tacn)Cu(NCCH3)]SbF6 (R = Me, Et, or iPr) with 1 equiv
of SdSbPh3 in CH2Cl2 yielded adducts 1-3, respectively, as
yellow crystalline solids (Scheme 1). The complexes are stable in
the solid state when stored under nitrogen, but in solution they
decompose slowly (see below). The formulations of 1-3 are
based on NMR, UV-vis, and FT-IR spectroscopy, CHN anal-
ysis, ESI-MS, and X-ray crystallography. Notable identifying
effects also appear to influence the Cu Sb distance, which
3 3 3
increases as the size of the R group of the ligand increases from
3.411 Å (1), 3.490 Å (2), to 3.547 Å (3). The Cu-S-Sb moiety
adopts a “bent” conformation with Cu-S-Sb bond angles
between 100.1 and 104.6°. The average Cu-N bond lengths
range between 2.06 and 2.17 Å, comparable to those in other
copper(I) complexes of R3tacn ligands.17,21 The Cu-S distances
of complexes 1-3 (2.167-2.203 Å) are shorter than typical
1
features include (a) H NMR spectra with sharp peaks in the
diamagnetic region and multiplets for the Ph3SbdS hydrogen
atoms shifted ∼0.2-0.3 ppm downfield from uncomplexed
Ph3SbdS (Supporting Information, Figure S1), (b) a shoulder in
UV-vis spectra with λmax at ∼350 nm (ε=∼2100-2300 M-1cm-1),
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-
Figure 2. Representation of the X-ray structure of 5 (BPh4 salt),
showing the dicationic portion with all non-hydrogen atoms as 50%
thermal ellipsoids (H atoms omitted for clarity, only heteroatoms
labeled). Selected bond distances (Å) and angles (deg) are as follows:
Cu1-N1, 2.202(3); Cu1-N2, 2.017(2); Cu1-N3, 2.003(2); Cu1-S1,
Figure 3. Plot of the initial rate of decay (1H NMR) of 1 vs
[[(Me3tacn)Cu(CH3CN)]SbF6]0 for the reaction of
1 with
[(Me3tacn)Cu(CH3CN)]SbF6 to yield 4 and 7 in CD2Cl2 at 20 °C.
Each data point is an average of data for 3 replicate runs. Error bars span
the range of values for the replicate runs. The line is a fit of the data to
eq 1 (R = 0.95).
2.2152(8); S1-S10, 2.1501(14); Cu1 Cu10, 3.876; N1-Cu1-N2,
3 3 3
86.17(10); N1-Cu1-N3, 86.13(10); N2-Cu1-N3, 88.50(10); N1-
Cu1-S1, 108.49(7); N2-Cu1-S1, 160.58(7); N3-Cu1-S1,
104.76(8); S1-Cu1-S10, 58.03(3).
The adducts [(R3tacn)Cu(SbPh3)]SbF6 (7, R = Me; 8, R = Et;
9, R = iPr) formed in the reactions of 1-3 with
[(R3tacn)Cu(CH3CN)]SbF6 also were identified by comparison
copper(I)-thioether sulfur interactions (∼2.30 Å),22 and longer
than copper(I)-thiolate interactions (2.13-2.18 Å),23 but com-
parable to known Cu(I)-SdPPh3 complexes (2.21-2.27 Å).24
Complexation of Ph3SbdS to the copper center induces little if
any change in the S-Sb bonding, as the S-Sb distances in 1-3
(2.281-2.283 Å) are only slightly longer than that in free
SdSbPh3 (2.244(1) Å).25
1
of H NMR spectra with data obtained from independently
prepared samples. These samples were synthesized in good yield
(∼70%) from the reaction of SbPh3 with [(R3tacn)-
Cu(CH3CN)]SbF6. They were isolated as colorless crystalline
solids and were fully characterized by CHN analysis, NMR and
FTIR spectroscopy, and ESI-MS. Notably, the mass spectrum for
each complex exhibits a parent ion with the appropriate isotope
pattern for [(R3tacn)Cu(SbPh3)]þ (illustrated for R = Me in
Supporting Information, Figure S5).
By monitoring solutions of 1 in CD2Cl2 by 1H NMR
spectroscopy with an internal standard (Supporting Information,
Figure S4) at 20 °C the slow decay (t1/2 ∼12 h) of 1 to SbPh3 and
the disulfido-dicopper(II) species [(Me3tacn)2Cu2(μ-η2:η2-
S2)](SbF6)2 (4) was observed in accordance with the stoichi-
ometry shown in Scheme 1. A similar decay reaction of 2
occurred to yield the respective disulfido-dicopper(II) complex
(5, with SbF6- counterion), but at a significantly slower rate (t1/2
∼2 d). Complex 3 decayed at a similar rate as 2, but the nature of
the product(s) in this case was not clear. The disulfido-dicopper-
(II) complexes 4-6 formed much more rapidly and with higher
yields/conversions upon addition of [(R3tacn)Cu(CH3CN)]-
SbF6 to solutions of 1-3, but in these reactions the adducts
[(R3tacn)Cu(SbPh3)]SbF6 (7, R = Me; 8, R = Et; 9, R = iPr)
formed instead of free SbPh3 (Scheme 1).
Mechanistic Studies. A series of experiments were per-
formed to gain insight into the reactions of the adducts 1-3
with [(R3tacn)Cu(CH3CN)]SbF6 to yield the disulfido-
dicopper(II) complexes 4-6 and the SbPh3 adducts 7-9
(Supporting Information, Figure S6). First, the reactions of
the complexes with identical supporting ligands under pseudo-
first-order conditions (i.e., 1 þ 10 equiv of [(Me3tacn)-
Cu(CH3CN)]SbF6, 2 þ 10 equiv of [(Et3tacn)Cu(CH3CN)]-
SbF6, and 3 þ 10 equiv of [(iPr3tacn)Cu(CH3CN)]SbF6) in
1
CD2Cl2 were monitored as a function of time by H NMR
spectroscopy. At initial concentrations [1-3]0 = 4.7 mM at
20 °C, the consumption of 1-3 followed first-order kinetics
(Supporting Information, Figures S6 and S7). The rates mea-
sured for the reactions of 2 and 3 are similar, with both being
>∼10 times slower than that of 1, as reflected by the measured
kobs values (averages from 3 replicate runs) of 6.6(5) ꢀ 10-4 s-1
(1), 8.4(1) ꢀ 10-5 s-1 (2), and 6.0(4) ꢀ 10-5 s-1 (3). The
results are roughly consistent with the relative steric profiles of
the reactant pairs (1 < 2 ∼ 3) and support a mechanism wherein
steric interactions among the reactant pairs influence the rate
(e.g., involving interaction of the Cu(I)-SdSbPh3 adduct with
the added Cu(I) reactant).
The disulfido-dicopper(II) complexes 4-6 were identified by
1
comparison of H NMR spectra with data reported previously
(4)14 or obtained from independently prepared samples of the
-
variants 5 and 6. These latter complexes were isolated as BPh4
salts by treating [(R3tacn)Cu(CH3CN)]BPh4 with S8 and were
fully characterized by CHN analysis and NMR, FTIR, and UV-
vis spectroscopy. Complexes 4-6 share similarly sharp 1H NMR
features in the diamagnetic region, consistent with singlet ground
states arising from disulfide-mediated antiferromagnetic cou-
2-
pling between the Cu(II) ions. They also share diagnostic S2
f Cu(II) LMCT transitions in UV-vis spectra at λmax 380-400
nm (ε ∼8000-14,000).26,7b In addition, the X-ray structure of 5
was solved (Figure 2). The X-ray structure of 5 is similar to that
previously reported for 4,14 as illustrated by analogous S-S (4:
2.165(4) Å; 5: 2.150(1) Å) and Cu-Cu (4: 3.84 Å; 5: 3.88 Å)
distances and square pyramidal coordination geometries (τ5 =
0.03 for 4 and 0.01 for 5).27
To further test this hypothesis, reactions of [(Me3tacn)-
Cu(CH3CN)]SbF6 with 1 were examined by measuring initial
reaction rates of disappearance of 1 as a function of
[[(Me3tacn)Cu(CH3CN)]SbF6]0 (Figure 3). The initial rate
saturates as the initial concentration of the added Cu(I) reagent
increases. This finding is consistent with a mechanism
(Scheme 2) involving an initial rapid pre-equilibrium (Keq)
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Scheme 2. Proposed Mechanism for the Formation of 4 from
the Reaction of 1 with [(Me3tacn)Cu(CH3CN)]SbF6
Scheme 3. Decay of SdSbPh3 to S2 as Determined by
Trapping with 2,3-Dimethylbutadiene
Scheme 4. Equilibration of Cu(I)-SdSbPh3 and Cu(I)-
NCCH3 Complexes
involving formation of a dicopper intermediate (A) followed by a
rate-determining product formation step (k2). The fit of the data
to the corresponding eq 1 is shown in Figure 3 (solid line),
yielding Keq = 200 M-1 and k2 = 1.2 ꢀ 10-4 s-1. The proposed
structure for A is speculative, as it was not observed directly.
While the product formation process from A (k2) must involve
multiple steps, including a step in which a second sulfur atom is
added, the decay of 1 is first order in [1] (see above) which
implies that a unimolecular reaction of A is rate-controlling (e.g.,
cleavage of the S-Sb bond).
approximately 10 times faster than that of the reaction of 1 to
yield 10. The rate of decay of 1 to yield 10 in the presence of 2,3-
dimethylbutadiene is similar to that observed for the decay of 1 to
4 in its absence, suggesting that S2 formation cannot be ruled out
in the pathways of both reactions. However, these reactions are
significantly slower than that for the reaction of 1 with
[(Me3tacn)Cu(CH3CN)]SbF6 to give 4 and 7. On this basis
and in view of the kinetic data described above, it appears unlikely
that S2 formation is important in the reaction of 1 with
[(Me3tacn)Cu(CH3CN)]SbF6.
This latter reaction is further complicated by exchange of the
SdSbPh3 unit between the Cu(I) centers. This exchange was
identified by 1H NMR spectroscopy and ESI-MS analysis of the
reaction of 1 with [(R3tacn)Cu(CH3CN)]SbF6 (R = Et or iPr).
Shortly after mixing, the 1H NMR spectrum showed peaks due to
all four species in the equilibrium shown in Scheme 4. From the
relative integrations after equilibrium was reached (∼10 min),
K0eq values of 0.41 (R = Et) or 0.054 (R = iPr) were measured.
These results were corroborated by ESI-MS (Supporting Infor-
mation, Figure S8), where parent ion peak envelopes for the
cationic portions of 1 and 2 (R = Et, ∼1:1 ratio) or 1 and 3 (R =
iPr, ∼6:1 ratio) were observed immediately after mixing of the
respective reagents. The trend in K0eq values correlates inversely
with the degree of steric interactions between the ligand substit-
uents and the bound SdSbPh3 moiety (K0eq decreases as the
k2½1ꢁ½CuðIÞꢁ
rate ¼
ð1Þ
-1
K
þ ½CuðIÞꢁ
eq
We also considered the possibility that the reactions of 1-3 to
form the disulfido-dicopper(II) complexes 4-6 might involve
decomposition of SdSbPh3 to S2.9a This decomposition was
reported to occur in CS2 via a second-order process with a rate
constant of 0.014(2) M-1 s-1 at 35 °C, with the release of S2
suggested by the formation of cyclic sulfides 10 and 11 when 2,3-
dimethylbutadiene was used as a trapping reagent (Scheme 3).
We found that mixtures of SdSbPh3 (4.7 mM) and 2,3-
dimethylbutadiene (20 equiv) in CD2Cl2 at 20 °C were un-
changed after ∼3 d, with no evidence for formation of 10 or 11.
In a second experiment, 1H NMR spectroscopic monitoring of a
mixture of 1 and 2,3-dimethylbutadiene (20 equiv) revealed slow
generation of 10 and loss of 1 via a first-order process with a rate
constant equal to 1.4(4) ꢀ 10-5 s-1 (t1/2 = ∼14 h). There was
no evidence for the presence of the disulfido-dicopper(II)
complex 4 during this process. Interestingly, under identical
conditions 4 also decayed in the presence of 2,3-dimethylbuta-
diene to yield 10. This reaction of 4 is characterized by a first-
order rate constant of 1.7(3) ꢀ 10-4 s-1, corresponding to a rate
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steric interactions increase, R = Et < iPr). Importantly, equilibra-
tion is rapid relative to the decay to the disulfido-dicopper(II)
complexes, and thus occurs prior to the suggested pathway for
the decay reaction shown in Scheme 2.
’ REFERENCES
(1) Zumft, W. G.; Kroneck, P. M. H. Adv. Microb. Phys. 2007,
52, 107–227.
(2) (a) Brown, K.; Tegoni, M.; Prud^encio, M.; Pereira, A. S.; Besson,
S.; Moura, J. J.; Moura, I.; Cambillau, C. Nat. Struct. Biol. 2000,
7, 191–195. (b) Brown, K.; Djinovic-Carugo, K.; Haltia, T.; Cabrito,
I.; Saraste, M.; Moura, J. J.; Moura, I.; Tegoni, M.; Cambillau, C. J. Biol.
Chem. 2000, 275, 41133–41136. (c) Paraskevopoulos, K.; Antonyuk,
S. V.; Sawers, R. G.; Eady, R. R.; Hasnain, S. S. J. Mol. Biol. 2006,
362, 55–65.
(3) (a) Rasmussen, T.; Berks, B. C.; Sanders-Loehr, J.; Dooley, D. M.;
Zumft, W. G.; Thomson, A. J. Biochemistry 2000, 39, 12753–12756. (b)
Chen, P.; Cabrito, I.; Moura, J. J. G.; Moura, I.; Solomon, E. I. J. Am.
Chem. Soc. 2002, 124, 10497–10507. (c) Alvarez, M. L.; Ai, J.; Zumft, W.;
Sanders-Loehr, J.; Dooley, D. M. J. Am. Chem. Soc. 2001, 123, 576–587.
(d) Ghosh, S.; Gorelsky, S. I.; DeBeer George, S.; Chan, J. M.; Cabrito, I.;
Dooley, D. M.; Moura, J. J. G.; Moura, I.; Solomon, E. I. J. Am. Chem. Soc.
2007, 129, 3955–3965. (e) Oganesyan, V. S.; Rasmussen, T.; Fairhurst,
S.; Thomson, A. J. Dalton Trans. 2004, 996–1002.
(4) (a) Chen, P.; Gorelsky, S. I.; Ghosh, S.; Solomon, E. I. Angew.
Chem., Int. Ed. 2004, 43, 4132–4140. (b) Solomon, E. I.; Sarangi, R.;
Woertink, J. S.; Augustine, A. J.; Yoon, J.; Ghosh, S. Acc. Chem. Res. 2007,
40, 581–591.
Reaction of SdSbPh3 with [(TMCHD)Cu(NCCH3)]PF6. In
contrast to the reactions with the Cu(I) complexes of R3tacn
ligands that led to isolable Cu(I)-SdSbPh3 adducts, no such
adducts were identified when the Cu(I) complex of the bidentate
diamine TMCHD was treated with SdSbPh3. Instead, the
known tricopper cluster [(TMCHD)3Cu3(S)2](PF6)3 (12)
was isolated cleanly in good yield (79%). This procedure for
the synthesis of 12 is superior to that previously reported
involving use of S8,8a facilitating advanced spectroscopic studies
of the cluster aimed at addressing contentious bonding and
oxidation state issues.8 Presumably, an initial Cu(I)-SdSbPh3
adduct forms in the reaction, but because of its lower coordina-
tion number is more prone to oligomerization than the analogues
supported by the tridentate R3tacn ligands.
’ SUMMARY AND CONCLUSIONS
In explorations ultimately aimed at preparing copper-sulfur
complexes to model the active site of nitrous oxide reductase, we
have found that Ph3SbdS forms stable adducts [(R3tacn)-
Cu(SdSbPh3)]SbF6 (1-3), the first examples of which have
been structurally characterized by X-ray crystallography.
These adducts undergo slow decay in solution to form
[(R3tacn)2Cu2(μ-η2:η2-S2)]2þ species (4-6) and SbPh3.
Conversion to 4-6 is accelerated by addition of
[(R3tacn)Cu(NCCH3)]SbF6 to 1-3, and yield [(R3tacn)-
Cu(SbPh3)]SbF6 (7-9) as coproduct instead of free SbPh3.
Mechanistic studies of this reaction revealed rapid exchange of
Ph3SbdS between the Cu(I) sites and pre-equilibrium formation
of a dicopper intermediate. We speculate that the dicopper
intermediate contains a bridging Ph3SbdS moiety and that the
rate-controlling step in the reaction involves loss of Ph3Sb from
that intermediate. Subsequent more rapid events that ultimately
result in [Cu2(μ-η2:η2-S2)]2þ core formation remain unclear.
Reaction of [(TMCHD)Cu(CH3CN)]PF6 with SdSbPh3 did
not lead to an observable adduct, and instead led to the known
tricopper cluster [(TMCHD)3Cu3(μ3-S)2](PF6)3 in good yield.
Overall, the results demonstrate the utility of Ph3SbdS for
delivering sulfur to Cu(I) centers supported by N-donor ligands,
cleanly yielding thermodynamically stable [Cu2(μ-η2:η2-S2)]2þ
and [Cu3S2]3þ cores.
(5) York, J. T.; Bar-Nahum, I.; Tolman, W. B. Inorg. Chim. Acta
2008, 361, 885–893, and references cited therein.
(6) Bar-Nahum, I.; Gupta, A. K.; Huber, S. M.; Ertem, M. Z.;
Cramer, C. J.; Tolman, W. B. J. Am. Chem. Soc. 2009, 131, 2812–2814.
(7) (a) Sarangi, R.; York, J. T.; Helton, M. E.; Fujisawa, K.; Karlin,
K. D.; Tolman, W. B.; Hodgson, K. O.; Hedman, B.; Solomon, E. I. J. Am.
Chem. Soc. 2008, 130, 676–686. (b) Chen, P.; Fujisawa, K.; Helton,
M. E.; Karlin, K. D.; Solomon, E. I. J. Am. Chem . Soc. 2003,
125, 6394–6408.
(8) (a) Brown, E. C.; York, J. T.; Antholine, W. E.; Ruiz, E.; Alvarez,
S.; Tolman, W. B. J. Am. Chem. Soc. 2005, 127, 13752–13753. (b) Mealli,
C.; Ienco, A.; Poduska, A.; Hoffmann, R. Angew. Chem., Int. Ed. 2008,
47, 2864–2868. (c) Alvarez, S.; Hoffmann, R.; Mealli, C. Chem.—Eur. J.
2009, 15, 8358–8373. (d) Berry, J. F. Chem.—Eur. J. 2010,
16, 2719–2724.
(9) (a) Jason, M. E. Inorg. Chem. 1997, 36, 2641–2646. (b) Baechler,
R. D.; Stack, M.; Stevenson, K.; Vanvalkenburgh, V. Phosphorus, Sulfur
Silicon Relat. Elem. 1990, 48, 49–52.
(10) (a) Bauer, A.; Capps, K. B.; Wixmerten, B.; Abboud, K. A.; Hoff,
C. D. Inorg. Chem. 1999, 38, 2136–2142. (b) Zuo, J.-L.; Zhou, H.-C.;
Holm, R. H. Inorg. Chem. 2003, 42, 4624–4631. (c) Groysman, S.;
Holm, R. H. Inorg. Chem. 2007, 46, 4090–4102. (d) Groysman, S.;
Wang, J.-J.; Tagore, R.; Lee, S. C.; Holm, R. H. J. Am. Chem. Soc. 2008,
130, 12794–12807.
(11) Donahue, J. P. Chem. Rev. 2006, 106, 4747–4783.
(12) Capps, K. B.; Wixmerten, B.; Bauer, A.; Hoff, C. Inorg. Chem.
1998, 37, 2861–2864.
(13) Lobana, T. S. Prog. Inorg. Chem. 1989, 37, 495–588.
(14) Bar-Nahum, I.; York, J. T.; Young, V. G., Jr.; Tolman, W. B.
Angew. Chem., Int. Ed. 2008, 47, 533–536.
’ ASSOCIATED CONTENT
S
Supporting Information. Illustrative experimental pro-
b
cedures, spectra, and kinetics results, and representions of the
X-ray crystal structures of complexes 2 and 3 (PDF); X-ray
structural data (CIFs). This material is available free of charge via
the Internet at http://pubs.acs.org.
(15) See Supporting Information.
(16) (a) Haselhorst, G.; Stoetzel, S.; Strassburger, A.; Walz, W.;
Wieghardt, K.; Nuber, B. J. Chem. Soc., Dalton Trans. 1993, 83–90. (b)
Cahoy, J.; Holland, P. L.; Tolman, W. B. Inorg. Chem. 1999,
38, 2161–2168.
(17) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.; Pan, G.; Wang,
X.; Young, V. G., Jr.; Cramer, C. J.; Que, L., Jr.; Tolman, W. B. J. Am.
Chem. Soc. 1996, 118, 11555–11574.
(18) Mahadevan, V.; Hou, Z.; Cole, A. P.; Root, D. E.; Lal, T. K.;
Solomon, E. I.; Stack, T. D. P. J. Am. Chem. Soc. 1997,
119, 11996–11997.
(19) Transition metal complexes of SdSbPh3 have been reported,
although none have been characterized structurally by X-ray crystal-
lography: (a) King, M. G.; McQuillan, G. P. J. Chem. Soc., A
1967, 898–901. (b) Kuhn, N.; Schumann, H. J. Organomet. Chem.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: wtolman@umn.edu.
’ ACKNOWLEDGMENT
We thank the NIH (R37 GM47365 to W.B.T.) for financial
support of this research and Victor G. Young, Jr., for assistance
with the X-ray crystallography.
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ARTICLE
1986, 304, 181–193. (c) Hieber, W.; John, P. Chem. Ber. 1970,
103, 2161–2177.
(20) Yang, L.; Powell, D.; Houser, R. Dalton Trans. 2007, 955–964.
(21) (a) Lam, B. M. T.; Halfen, J. A.; Young, V. G., Jr.; Hagadorn,
J. R.; Holland, P. L.; Lledꢀos, A.; Cucurull-Sꢀanchez, L.; Novoa, J. J.;
Alvarez, S.; Tolman, W. B. Inorg. Chem. 2000, 39, 4059–4072. (b)
Halfen, J. A.; Young, V. G., Jr.; Tolman, W. B. Inorg. Chem. 1998,
37, 2102–2103. (c) Halfen, J. A.; Mahapatra, S.; Wilkinson, E. C.;
Gengenbach, A. J.; Young, V. G., Jr.; Que, L., Jr.; Tolman, W. B. J. Am.
Chem. Soc. 1996, 118, 763–776. (d) Halfen, J. A.; Mahapatra, S.;
Olmstead, M. M.; Tolman, W. B. J. Am. Chem. Soc. 1994,
116, 2173–2174.
(22) Ohrenberg, C.; Liable-Sands, L. M.; Rheingold, A. L.; Riordan,
C. G. Inorg. Chem. 2001, 40, 4276–4283, and references cited therein..
(23) Melzer, M. M.; Li, E.; Warren, T. H. Chem. Commun.
2009, 5847–5849, and references cited therein..
(24) Reigle, R.; Casadonte, D.; Bott, S. J. Chem. Crystallogr. 1994,
24, 769–773.
(25) (a) Weller, F.; Dehnicke, K. Naturwissenschaften 1981,
68, 523–524. (b) Pebler, J.; Weller, F.; Dehnicke, K. Z. Anorg. Allg.
Chem. 1982, 492, 139–147.
(26) Brown, E. C.; Bar-Nahum, I.; York, J. T.; Aboelella, N. W.;
Tolman, W. B. Inorg. Chem. 2007, 46, 486–496.
(27) Addison, A. W.; Rao, T. N.; Reedijk, J.; Rijn J. van; Verschoor,
G. C. J. Chem. Soc., Dalton Trans. 1984, 1349–1356.
2612
dx.doi.org/10.1021/ic102449m |Inorg. Chem. 2011, 50, 2606–2612