Coordination Chemistry of a Tripodal S2ON Ligand: Syntheses, Structures, and Reactivity of the Molybdenum(VI) and Nickel(II) Complexes of Bis(2-mercaptoethyl)-2-amino-4-methylphenoI (H3btap) and Comparison to VvO(btap)

Article abstract of DOI:10.1021/ic9802956

The tripodal tetradentate ligand H₃btap coordinates to V^v, Mo^VI. and Ni^II via three different bonding modes to yield three complexes with unique ligand-based oxidation chemistry. For V^v and Mo^VI (1). all four of the heteroatom donors are coordinated to the metal ion forming a trigonal bipyramidal complex with the oxovanadium-(V) ion, V^VO³⁺, and an octahedral complex with the cis-dioxomolybdenum(VI) ion, [MoO₂]²⁺. Only three of the heteroatom donors of H₃btap are used to coordinate to Ni^II (2). two thiolate sulfurs and the amine nitrogen, yielding a dimeric structure in which each nickel(II) ion has NS₃ coordination. The ability of V^VO(btap) to form η²-sulfenates, while [MoO₂(btap)]^- does not form stable η²-sulfenates, has been ascribed to the electron-deficient, π-accepting nature of V^VO³⁺ relative to [Mo^VIO₂]²⁺. Crystal data for 1 (C₁₁H₁₆NO₄S₂KMo): space group Pbcn, a = 6.6596(9) A?, b = 13.7446(9) A?, c = 32.992(2) A?, α = β= γ = 90°, Z = 8. Crystal data for 2 (C₂₄H₃₈N₂O₄S₄-Ni): space group Pbcn, a = 12.0841(3) A?, b = 14.4948(4) A?, c = 16.7751(4) A?, α = β= γ = 90°, 2 = 4.

Full text of DOI:10.1021/ic9802956

Inorg. Chem. 1998, 37, 5851-5855
5851
Coordination Chemistry of a Tripodal S2ON Ligand: Syntheses, Structures, and Reactivity
of the Molybdenum(VI) and Nickel(II) Complexes of
Bis(2-mercaptoethyl)-2-amino-4-methylphenol (H3btap) and Comparison to VVO(btap)
Charles R. Cornman,*,† Kevin L. Jantzi,† Joseph I. Wirgau,† Thad C. Stauffer,†
Jeff W. Kampf,‡ and Paul D. Boyle†
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, and
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055
ReceiVed March 16, 1998
The tripodal tetradentate ligand H3btap coordinates to VV, MoVI, and NiII Via three different bonding modes to
yield three complexes with unique ligand-based oxidation chemistry. For VV and MoVI (1), all four of the
heteroatom donors are coordinated to the metal ion forming a trigonal bipyramidal complex with the oxovanadium-
(V) ion, VVO3+, and an octahedral complex with the cis-dioxomolybdenum(VI) ion, [MoO2]2+. Only three of
the heteroatom donors of H3btap are used to coordinate to NiII (2), two thiolate sulfurs and the amine nitrogen,
yielding a dimeric structure in which each nickel(II) ion has NS3 coordination. The ability of VVO(btap) to form
η2-sulfenates, while [MoO2(btap)]- does not form stable η2-sulfenates, has been ascribed to the electron-deficient,
π-accepting nature of VVO3+ relative to [MoVIO2]2+. Crystal data for 1 (C11H16NO4S2KMo): space group Pbcn,
a ) 6.6596(9) Å, b ) 13.7446(9) Å, c ) 32.992(2) Å, R ) â ) γ ) 90°, Z ) 8. Crystal data for 2 (C24H38N2O4S4-
Ni): space group Pbcn, a ) 12.0841(3) Å, b ) 14.4948(4) Å, c ) 16.7751(4) Å, R ) â ) γ ) 90°, Z ) 4.
Introduction
demonstrated for early transition metalssthis despite the
extensive study of molybdenum thiolates in relation to the
molybdenum oxotransferases.14-19 As recently reported, the
vanadium(V) complex of bis(2-mercaptoethyl)-2-amino-4-me-
thylphenol, VVO(btap), which contains two VV-SR interactions,
can be oxidized sequentially to a mono- and disulfenate complex
in which the sulfenate ligands coordinate to the metal in an
unique η2 fashion as shown in eq 1.20,21 To understand the
formation of sulfur oxygenates and their coordination modes,
we report herein the preparation and characterization of the NiII
and MoVI complexes of bis(2-mercaptoethyl)-2-amino-4-meth-
ylphenol.
The oxidation chemistry of nickel(II)-thiolate complexes has
received much attention over the past decade due to the
importance of Ni-SR coordination in nickel hydrogenases1,2
and carbon monoxide dehydrogenase.1,3 Darensbourg and co-
workers and Maroney and co-workers have demonstrated the
formation of nickel-sulfenates (monooxygenate of coordinated
sulfur, Ni-S(O)R) and nickel-sulfinate (dioxygenate of coor-
dinated sulfur, Ni-S(O)2R) analogues.4-11 These ligands are
coordinated to the nickel(II) center in an η1 fashion through
the sulfenate (or sulfinate) sulfur atom. Sulfenate and sulfinate
formation has been demonstrated for other transition ele-
ments;12,13 however, this chemistry has only recently been
† North Carolina State University.
‡ University of Michigan.
(1) The Bioinorganic Chemistry of Nickel; Lancaster, J. R., Ed.; VCH
Publishers: New York, 1988.
(2) Maroney, M. J.; Pressler, M. A.; Mirza, S. A.; Whitehead, J. P.;
Gurbiel, R. J.; Hoffman, B. M. Insights into the Role of Nickel in
Hydrogenase; Thorp, H. H., Pecoraro, V. L., Eds.; American Chemical
Society: Washington, DC, 1995; Vol. 246, pp 21-60.
(3) Ragsdale, S. W. Crit. ReV. Biochem. Mol. Biol. 1991, 26, 261-300.
(4) Maroney, M. J.; Choudhury, S. B.; Sherrod, M. J. Inorg. Chem. 1996,
35, 1073-1076.
(13) Sloan, C. P.; Kreuger, J. H. Inorg. Chem. 1975, 14, 1481.
(14) Dhawan, I. K.; Enemark, J. H. Inorg. Chem. 1996, 35, 4873-4882.
(15) Enemark, J. H.; Xiao, Z.; Bruck, M. A. Inorg. Chem. 1995, 34, 5950-
5962.
(5) Colpas, G.; Kumar, M.; Day, R. O.; Maroney, M. J. Inorg. Chem.
1990, 29, 4779-4788.
(6) Mirza, S. A.; Day, R. O.; Maroney, M. J. Inorg. Chem. 1996, 35,
1992-1995.
(16) Schultz, B. E.; Gheller, S. F.; Muetterties, M. C.; Scott, M. J.; Holm,
R. H. J. Am. Chem. Soc. 1993, 115, 2714-2722.
(7) Mirza, S. A.; Pressler, M. A.; Kumar, M.; Day, R. O.; Maroney, M.
J. Inorg. Chem. 1993, 32, 977-987.
(17) Buchanan, I.; Minelli, M.; Ashby, M. T.; King, T. J.; Enemark, J. H.;
Garner, C. D. Inorg. Chem. 1984, 23, 495-500.
(8) Kumar, M.; Colpas, G. J.; Day, R. O.; Maroney, M. J. J. Am. Chem.
Soc. 1989, 111, 8323-8325.
(18) Burgmayer, S. J. N.; Stiefel, E. I. Inorg. Chem. 1988, 27, 2518-
2521.
(9) Buonomo, R. M.; Font, I.; Darensbourg, M. Y. J. Am. Chem. Soc.
1995, 117, 963-973.
(19) Topich, J.; Bachert, J. O. I. Inorg. Chem. 1992, 31, 511-515.
(20) Cornman, C. R.; Stauffer, T. C.; Boyle, P. D. J. Am. Chem. Soc. 1997,
119, 5986-5987.
(21) Cornman, C. R.; Stauffer, T. C.; Boyle, P. D. Synthesis, Structure,
and ReactiVity of VV-Thiolate and VV-η2-Sulfenate Complexes; Crans,
D. C., Tracey, A. S., Eds.; American Chemical Society: Washington,
DC, 1998; in press.
(10) Darensbourg, M. Y.; Grapperhaus, C. A.; Maguire, M. J. Inorg. Chem.
1997, 36, 1860.
(11) Grapperhaus, C. A.; Darensbourg, M. Y.; Russell, D. H. J. Am. Chem.
Soc. 1996, 118, 1791-1792.
(12) Nicholson, T.; Zubieta, J. Inorg. Chem. 1987, 26, 2094-2101.
10.1021/ic9802956 CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/07/1998

5852 Inorganic Chemistry, Vol. 37, No. 22, 1998
Cornman et al.
Table 1. Crystallographic Data for K[MoVIO2(btap)], 1, and
Experimental Procedures
[NiII(Hbtap)]2, 2
Materials were purchased from commercial sources and used as
received unless otherwise stated. Dry solvents were prepared by
distillation from CaH2 (acetonitrile and hexanes), diphenylketyl radical
(THF), or Mg(OMe)2 (methanol) under dinitrogen.22,23 Acetone was
dried via passage through Al2O3.
1
2
empirical formula
fw
temperature
space group
C11H16NO4S2KMo
425.41
C24H38N2O4S4Ni
664.22
148 K
Pbcn
133 K
Pbcn
Bis(2-mercaptoethyl)-2-amino-4-methylphenol (H3btap). Ethyl-
ene sulfide (14 mL, 0.12 mol) was added to a refluxing toluene solution
(50 mL) of 2-amino-4-methylphenol (5.36 g, 43.5 mmol). The reaction
was heated for 40 h at 90 °C. The resulting brownish solution was
concentrated to 3 mL in Vacuo, transferred to a silica gel column, and
eluted with a 1:9:27 MeOH/CH2Cl2/hexanes solution. The clear, faintly
orange fraction was stripped of solvent via rotary evaporation, checked
for purity by 1H NMR and used without further purification. 1H NMR
(DMSO-d6) δ (ppm): 2.19 (s, 3 H), 2.49 (under solvent, m, 4 H), 3.16
(m, 4 H), 6.68 (s, 2 H), 6.83 (s, 1 H).
K[MoVIO2(btap)], 1. Mo(acac)2 (0.32 g, 0.98 mmol) was added to
a solution of KOH (0.055 g, 0.98 mmol) and H3btap (0.24 g, 0.98 mmol)
in 100 mL of methanol. This solution was stirred for 40 min and
filtered, and the solvent was removed under vacuum. The resulting
solid was dissolved in acetone and the residual particulate matter was
removed by filtration. Addition of hexanes and evaporation under
vacuum yielded 1 as an orange solid in 65% yield. X-ray-quality
crystals of 1 were obtained by slow evaporation of an acetone/hexanes
solution of 1. 1H NMR (see Figure S3, acetone-d6) δ (ppm) for 1:
2.93 (s, 3H), 3.09 (m, 4H), 3.65 (d of d, 2H), 4.25 (m, 2H), 6.34 (d,
1H), 6.58 (d, 1H), 6.76 (s, 1H). The resonance at δ ) 2.93 ppm is
assigned to the p-methyl protons. The resonances between δ ) 3.09
and 4.25 ppm are assigned to the diastereotopic methylene protons of
the thioethyl arms and the resonances at δ ) 6.34, 6.58, and 6.76 ppm
are assigned to the aromatic ring protons.
[NiII(Hbtap)]2, 2. Under an dinitrogen atmosphere, Ni(OAc)2‚4H2O
(215 mg, 0.862 mmol) was added to a 25 mL solution of acetonitrile
containing excess H3btap (236 mg, 0.951 mmol). The light brown
solution was stirred for 3 h to produce a fine brown precipitate of 2 in
79% yield. Red/brown X-ray-quality crystals were grown by slow
evaporation of a MeOH solution. 1H NMR (see Figure 3; 9:1 CH2-
Cl2/DMSO-d6) δ (ppm) for 2: 1.13 (t of d, 2H), 1.68 (d of d, 2H),
2.02 (d of d, 2H), 2.28 (t, 2H), 2.39 (s, 6H), 3.67 (t of d, 2H), 3.87 (d,
2H), 4.39 (d of d, 2H), 5.16 (t of d, 2H), 6.65 (d, 2H), 6.91 (d, 2H),
9.40 (s, 2H), 10.57 (s, 2H). The multiplets between δ ) 1.13 and 516
ppm are assigned to the methylene protons of the thioethyl groups.
The resonance at δ ) 2.39 ppm is assigned to the para-methyl protons.
The resonances at δ ) 6.65, 6.91, and 9.40 ppm are assigned the
phenolic ring protons, and the resonance at δ ) 10.57 ppm is assigned
to phenolic proton.
a (Å)
6.6596(9)
13.7446(9)
32.992(2)
90, 90, 90
3019.9(5)
1.871
12.0841(3)
14.4948(4)
16.7751(4)
90, 90, 90
2938.27(13)
1.502
b (Å)
c (Å)
R, â, γ (deg)
V (Å3)
F
Z
calc (g cm-3
)
8
4
λ (Mo KR) (Å)
0.710 73
1.40
0.710 73
1.598
0.0493
0.0697
µ (mm-1
)
R1a (all data)
wR2b (all data)
Rc [I > 1σ(I)]
Rwd [I > 1σ(I)]
0.041
0.049
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b (∑[w(Fo - Fc2)2]/∑[wFo ])1/2
.
c R )
2
4
2
∑(Fo - Fc)/∑(Fo). d Rw ) [∑(w(Fo - Fc)2)/∑(wFo )]1/2
.
set of programs.24 The structure was solved using SIR92.25 All non-H
atom positions were recovered from the initial E map. Subsequent
difference Fourier maps yielded peaks suggestive of hydrogen atom
positions, however, hydrogen atoms were added at their idealized
positions (C-H ) 0.96 Å). The phenyl and methylene hydrogens were
allowed to ride the parent carbon atom, while the methyl hydrogens
and the hydrogens on the water molecule were included as rigid groups.
Hydrogen isotropic displacement parameters rode on the parent carbon
atom’s Ueq value according to the expression U(H) ) U(C) + 0.01 Å2.
All non-H atoms were allowed to refine with anisotropic displacement
parameters. The calculated structure factors included corrections for
anomalous dispersion from the usual tabulation.26 The final full-matrix
least-squares refinement (based on F) converged at RF ) 0.041, Rw
)
0.049 (for data Inet > 1σ(Inet)). The largest peak in the final difference
map was 0.720 e/Å3. Selected distances and angles for 1 are given in
Table 2.
Structural determination for 2 was performed using a Siemens
SMART CCD-based X-ray diffractometer equipped with a normal focus
Mo-target X-ray tube (λ ) 0.710 73 Å) operated at 2000 W power (50
kV, 40 mA). X-ray intensities were measured at 133 K and the frames
integrated with the Siemens SAINT software package with a narrow
frame algorithm. The integration of the data using a primitive
orthorhombic unit cell yielded a total of 30307 reflections (between
2.19 e 2θ e 58.82°) of which 4312 were independent and 3269 were
greater than 2σ(I). The final cell constants (Table 1) were based on
the xyz centroids of 8192 reflections above 10σ(I). Analysis of the
data showed negligible decay during data collection; the data were
corrected for absorption using an empirical method (SADABS) with
transmission coefficients ranging from 0.825 to 0.962. The structure
of 2 was solved and refined with the Siemens SHELXTL (version 5.03)
software package, using the space group Pbcn with Z ) 4 for the
formula C24H38N2O4S4Ni which includes one molecule of methanol
solvate per asymmetric unit (2 per Ni dimer). All non-hydrogen atoms
were refined anisotropically while the hydrogen atoms were allowed
to refine isotropically. The final full-matrix least-squares refinement
based on F2 converged at R1 ) 0.0493 and wR2 ) 0.0697 (for all data);
the largest peak in the final difference map was 0.0434 e/Å3. Selected
distances and angles for 2 are given in Table 3.
Physical Measurements. 1H NMR spectra were obtained using a
General Electric GN 300 spectrometer using residual solvent protons
as an internal reference. UV/vis data were collected on a Hewlett-
Packard 8452A diode array spectrophotometer. Infrared spectra were
obtained on a Mattson Polaris spectrometer.
X-ray Crystal Structure Analysis. Crystallographic data for 1 and
2 are shown in Table 1. Structural determination for K[MoVIO2(btap)],
1, was performed using an Enraf-Nonius CAD4-MACH diffractometer.
The sample was maintained at a temperature of 148 K using a nitrogen
cold stream. The unit cell dimensions were determined by a fit of 24
well-centered reflections and their Friedel pairs with 28 < 2θ < 38°.
A unique octant (+h, +k, +l) of data was collected using the omega
scan mode in a nonbisecting geometry (3.0 e 2θ e 50.0°). Three
standard reflections were measured every 4800 s of X-ray exposure
time. Scaling the data was accomplished using a five-point smoothed-
curve routine fit to the intensity check reflections. The intensity data
was corrected for Lorentz and polarization effects. An empirical
absorption correction based on psi scans was applied during data
reduction. The data were reduced using routines from the NRCVAX
Results and Discussion
Structures. In the vanadium(V) complex of H3btap, the
ligand adopts a tetradentate, tripodal coordination mode to give
(24) Gabe, E. J.; Le Page, Y.; Charland, J.-P.; Lee, F. L.; White, P. S. J.
Appl. Crystallogr. 1989, 22, 384-387.
(25) Altomare, A.; Burla, M. C.; Camalli, G.; Cascarano, G.; Giacovazzo,
C.; Gualiardi, A.; Polidori, G. J. Appl. Crystallogr. 1994, 27, 435-
436.
(22) Gordon, A. J.; Ford, R. A. The Chemist’s Companion; John Wiley &
Sons: New York, 1972.
(23) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals, 4th ed.; Butterworth-Heinemann: Oxford, 1996.
(26) International Tables for X-ray Crystallography; Ibers, J., Hamilton,
W., Ed.; Kynoch Press: Birmingham, England, 1974; Vol. IV.

Coordination Chemistry of a Tripodal S2ON Ligand
Inorganic Chemistry, Vol. 37, No. 22, 1998 5853
Table 2. Bond Distances (Å) and Bond Angles (deg) for 1
Mo-S1
Mo-S2
Mo-O1
Mo-O2
Mo-O3
Mo-N
2.4406(16)
2.4729(16)
1.715(4)
2.079(4)
1.742(4)
2.387(5)
S1-C9
S2-C11
O2-C1
N-C2
1.835(6)
1.834(7)
1.343(7)
1.466(8)
1.498(8)
1.487(8)
N-C8
N-C10
S1-Mo-S2
S1-Mo-O1
S1-Mo-O2
S1-Mo-O3
S1-Mo-N
S1-Mo-K1
S1-Mo-K2
S2-Mo-O1
S2-Mo-O2
S2-Mo-O3
S2-Mo-N
O1-Mo-O2
O1-Mo-O3
O1-Mo-N
O2-Mo-O3
O2-Mo-N
O3-Mo-N
155.17(6)
98.38(15)
83.71(12)
94.58(14)
76.13(13)
129.46(5)
92.67(4)
103.21(15)
82.86(12)
91.47(14)
80.14(13)
93.54(19)
105.3(2)
C2-N-C10
C8-N-C10
O2-C1-C2
O2-C1-C6
N-C2-C1
110.9(5)
109.7(5)
119.7(5)
121.8(5)
115.9(5)
122.9(5)
108.8(5)
112.3(4)
110.8(5)
111.8(4)
100.4(2)
120.4(4)
107.6(3)
105.8(3)
109.4(3)
113.2(5)
N-C2-C3
Figure 1. Structural diagram of the anion of 1, [MoVIO2(btap)]-, with
thermal elipsoids at the 50% probability level.
N-C8-H8b
S1-C9-C8
N-C10-C11
S2-C11-C10
Mo-S2-C11
Mo-O2-C1
Mo-N-C2
Mo-N-C8
Mo-N-C10
C2-N-C8
166.86(19)
161.16(18)
74.13(16)
87.23(18)
Table 3. Important Bond Distances (Å) and Bond Angles (deg)
for 2a
Figure 2. Structural diagram of 2, [NiII(Hbtap)]2, with thermal elipsoids
at the 50% probability level.
Ni(1)-N(1)
Ni(1)-S(1)
Ni(1)-S(2)A
Ni(1)-S(2)
Ni(1)-Ni(1)A
S(1)-C(1)
1.997(2)
2.1619(6)
2.1824(5)
2.2006(5)
2.6733(5)
1.828(2)
1.832(2)
2.1823(5)
1.474(2)
1.510(3)
1.516(3)
O(1)-C(6)
O(2)-C(12)
C(1)-C(2)
C(3)-C(4)
C(5)-C(10)
C(5)-C(6)
C(6)-C(7)
C(7)-C(8)
C(8)-C(9)
C(9)-C(10)
C(9)-C(11)
1.367(2)
1.409(3)
1.509(3)
1.508(3)
1.396(3)
1.405(3)
1.394(3)
1.386(3)
1.389(3)
1.396(3)
1.506(3)
In the neutral nickel dimer 2, each H3btap ligand uses only
three of the four donor atoms to coordinate to the nearly square
planar nickel centers as shown in Figure 2. The largest deviation
from the best plane defined by the nickel atom and its four
ligands is -0.089 Å observed for the nickel atom. The dimer
consists of two such square planar units sharing an edge of their
coordination polyhedra. The phenolic oxygen of the ligand is
S(2)-C(4)
S(2)-Ni(1)A
N(1)-C(5)
N(1)-C(2)
N(1)-C(3)
1
not coordinated and remains protonated as evidenced by H
N(1)-Ni(1)-S(1)
N(1)-Ni(1)-S(2)A
S(1)-Ni(1)-S(2)A
N(1)-Ni(1)-S(2)
S(1)-Ni(1)-S(2)
S(2)A-Ni(1)-S(2)
N(1)-Ni(1)-Ni(1)A
S(1)-Ni(1)-Ni(1)A
S(2)A-Ni(1)-Ni(1)A 52.73(2)
S(2)-Ni(1)-Ni(1)A
C(1)-S(1)-Ni(1)
C(4)-S(2)-Ni(1)A
C(4)-S(2)-Ni(1)
Ni(1)A-S(2)-Ni(1)
92.07(5)
167.91(5)
95.98(2)
90.08(5)
175.86(2)
81.36(2)
115.19(5)
123.75(2)
C(5)-N(1)-C(2) 110.5(2)
C(5)-N(1)-C(3) 110.6(2)
C(2)-N(1)-C(3) 110.0(2)
C(5)-N(1)-Ni(1) 114.79(12)
C(2)-N(1)-Ni(1) 107.79(12)
C(3)-N(1)-Ni(1) 102.83(12)
NMR spectroscopy (singlet at δ ) 10.55 ppm in 9:1 CD2Cl2/
DMSO-d6, see Figure 3A) and X-ray crystallography. The
nickel atoms in 2 are coordinated by the amine (Ni-N ) 1.997-
(2) Å), a terminal thiolate (Ni-Sterminal ) 2.1619(6) Å), and
two bridging thiolate sulfur atoms (Ni-Sbridge ) 2.1824(5) and
2.2006(5) Å). The shorter Ni-Sbridge distance is to the thiolate
sulfur of the second [Hbtap]2- ligand. Similar coordination
modes have been observed for other NiII complexes of N-
substituted bis(2-mercaptoethyl)amine.5
C(2)-C(1)-S(1)
108.4(2)
C(1)-C(2)-N(1) 109.6(2)
C(4)-C(3)-N(1) 110.3(2)
52.106(14) C(3)-C(4)-S(2)
109.44(14)
C(10)-C(5)-N(1) 118.9(2)
C(6)-C(5)-N(1) 122.1(2)
O(1)-C(6)-C(7) 121.7(2)
O(1)-C(6)-C(5) 119.3(2)
96.82(8)
110.47(7)
97.26(7)
75.17(2)
The Ni2S2 core of 2 adopts a butterfly geometry. The Ni-
Ni distance is 2.6733(5) Å while the S-S distance for the
bridging sulfur atoms is 2.86 Å. The fold angle, defined as the
angle between the best NiS2 planes, is 80.5°, which is similar
to other Ni2S2 core angles. The core S-Ni-S angle is 95.98-
(2)°, near the ideal 90° expected for a square planar geometry.
However, the Ni-S-Ni angle of 75.17(2)° is smaller than
expected for sulfur coordination. The alkyl groups of the
bridging thiolate ligands adopt a syn-endo geometry, while the
phenol substituents on the amine adopt a syn-exo geometry.
Reactivity. The anion of 1, [MoVIO2(btap)]-, is stable under
an aerobic atmosphere in many solvents including acetone,
acetonitrile, DMSO and water. Complex 1 does react with
m-chloroperoxybenzoic acid or tert-butylhydroperoxide; how-
ever, this oxidation results in decomposition of the complex
yielding a pale yellow precipitate, and an intractable mixture
of ligand-based products. The solid is assigned to the formation
of a molybdate oligomer due to its simple infrared spectrum
a Symmetry transformations used to generate equivalent atoms: #1,
-x + 1, y, -z + /2.
3
the trigonal bipyramidal complex VVO(btap).20 In this geom-
etry, VVO(btap) can be oxidized to form the six-coordinate η2-
monosulfenate and subsequently the seven-coordinate η2,η2-
disulfenate (eq 1). In the anionic complex 1, the [btap]3- ligand
coordinates in a tetradentate fashion to the cis-dioxomolybde-
num(VI), [(MoVIO2)]2+, unit as shown in Figure 1. The phenolic
oxygen, two trans thiolate sulfurs, and an oxo donor define the
equatorial plane in this structure. A second oxo donor and the
amine nitrogen of the [btap]3- ligand complete the polyhedron.
The Mo-heteroatom distances (Table 2) are as expected for a
cis-dioxomolybdenum(VI) complex.27
(27) Average distances are based on a search of the Cambridge Structural
Database: Mo-SR, 2.47(9) Å; Mo-Ooxo, 1.71(4) Å; Mo-OAr )
2.04(12) Å; Mo-N (tertiary amine) ) 2.41(6) Å; Allen, F. H. Kennard,
O. Chem. Des. Autom. News 1993, 8, 31-37.
1
(Supporting Information Figure S4) and the absence of a H
NMR spectrum in D2O. Furthermore, upon standing, an NMR
sample from the oxidation reaction (in acetone and containing

5854 Inorganic Chemistry, Vol. 37, No. 22, 1998
Cornman et al.
sulfinate ligands, [RSO2]-, as shown in eq 2.6 Addition of
dioxygen to a solution of 2 (in CD2Cl2/DMSO-d6; 9:1), either
as air or pure O2, results in the rapid conversion (<1 min) of 2
to the disulfide of [Hbtap]2- and a nickel-containing product
of unknown composition. The 1H NMR of the resulting solution
is shown as Figure 3B. Resonances at 3.0 and 3.7 ppm in Figure
3B are due to the methylene protons of the disulfide (confirmed
by independent synthesis). Using the residual solvent protons
(CHDCl2) as an internal concentration standard, one finds that
free disulfide accounts for only 20-30% of the [Hbtap]2- in
the sample (four experiments). (Each of the multiplets in Figure
3A′ corresponds to two protons of the dimer, while the triplet
in Figure 3B corresponds to 4 protons of the disulfide derived
from [Hbtap]2-.) Expanding the NMR sweepwidth (Figure 3C)
shows many broad, overlapping resonances between -5 and
50 ppm, strongly suggesting the presence of an S ) 1 NiII
system. EPR spectroscopy shows a very weak signal at g ) 2
1
(singlet) indicating that an uncoupled S ) /2 system is not
present in any significant quantity. Attempts to purify the
paramagnetic species have been unsuccessful. However, we
hypothesize that these paramagnetic complexes contain a
disulfide ligand and two additional ligands, perhaps H2O, as
shown in eq 3. The free disulfide observed by 1H NMR (Figure
3B) would then result from dissociation of the disulfide ligand
from the complex formed through eq 3.
Figure 3. 1H NMR of (A) [NiII(Hbtap)]2, (A′) expansion of 1-5.5
ppm region of spectrum in A, (B) the air-oxidized solution of [NiII-
(Hbtap)]2, and (C) the paramagnetic region of the spectrum in B. Spectra
acquired on 9:1 CD2Cl2/DMSO-d6 solutions at ambient temperature
using the residual solvent proton from CD2Cl2 as an internal standard
for chemical shift and intensity. Asterisks if spectrum A′ denote residual
solvent protons (5.32 and 2.45 ppm), solvent impurities, and small
amounts of the oxidation products (spectrum B).
the yellow precipitate) yielded crystals that were shown by X-ray
crystallography to be an acetone solvate of [â-Mo8O26]4-, a
known polyoxomolybdate.28 This structure will be reported
separately. The ligand-based products are tentatively assigned
to various inter- and intramolecular disulfides.
Formation of a similar complex has been previously dem-
onstrated for the I2 oxidation of a nickel-thiolate dimer in which
the R-group of the complex in eq 2 contains a thioether that
coordinates in the final complex.29 This would suggest that
nickel-thiolate dimers of ligands with a surplus donor group
are oxidized to disulfide, while those dimers without the surplus
donor group are oxidized to sulfinates. The reaction of 2 with
I2 results in the immediate formation of disulfide (analogous to
the reaction with dioxygen) and subsequent appearance (over
24 h) of paramagnetically shifted 1H NMR resonances (similar
to Figure 3C) between -60 and 50 ppm with multiple sharp
resonances in the 10-30 ppm region. The product(s) of the
reaction of 2 with iodine are currently under further investiga-
tion.
The oxidation chemistry of the ligand is strongly dependent
on the electronic structure of the metal complex; late transition
metal complexes such as [Ni(Hbtap)]2 react with dioxygen while
early transition metal complexes ([MoO2(btap)]- and VO(btap))
are stable toward dioxygen but are reactive toward peroxides
and peracids. The work of Darensbourg and co-workers9,10 and
The neutral nickel dimer, 2, is stable under dinitrogen and is
soluble in THF and DMSO but is only sparingly soluble in most
1
other organic solvents. The H NMR of 2 in CD2Cl2/DMSO-
d6 (9:1), shown as Figure 3A, is entirely consistent with the
solid-state structure. The presence of eight resonances in the
1-5 ppm region of the spectrum (Figure 3A′, assigned to the
protons of the thioethyl arms) indicates that the methylene
protons are diastereotopic. Therefore, the coordinated tertiary
amine is not rapidly inverting on the NMR time scale. The
resonance at δ ) 9.40 ppm is assigned to H10 (see Figure 2)
of the aromatic ring. This downfield shift, relative to the other
aromatic protons, suggests that the Ni2S2 core has a deshielding
effect on H10. Maroney and co-workers have demonstrated
that similar nickel dimers react slowly with dioxygen (weeks)
to form ligand-oxidized complexes in which the two terminal
thiolate ligands, RS-, are converted to the corresponding
(28) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer-Verlag:
(29) Kumar, M.; Day, R. O.; Colpas, G. J.; Maroney, M. J. J. Am. Chem.
Soc. 1989, 111, 5974-5976.
Berlin, 1983; p 44.

Coordination Chemistry of a Tripodal S2ON Ligand
Inorganic Chemistry, Vol. 37, No. 22, 1998 5855
Scheme 1
[MoO2(btap)]- the strong π-donation of the second oxo ligand
in [MoO2(btap)]- may preclude this stabilizing effect leading
to reactive sulfenates that proceed to form disulfides. We are
currently preparing monooxomolybdenum(VI), [MoVIO]4+, com-
plexes to test this hypothesis.
Conclusions
The tripodal tetradentate ligand H3btap coordinates to VV,
MoVI, and NiII Via three different bonding modes to yield three
complexes with unique ligand-based oxidation chemistry. For
VV and MoVI, all four of the heteroatom donors are coordinated
to the metal ion forming a trigonal bipyramidal complex with
the oxovanadium(V) ion, [VVO]3+, and an octahedral complex
with the cis-dioxomolybdenum(VI) ion, [MoO2]2+. Only three
of the heteroatom donors of H3btap are used to coordinate to
NiII, two thiolate sulfurs and the amine nitrogen, yielding a
dimeric structure in which each nickel(II) ion has NS3 coordina-
tion. The ability of VVO(btap) to form η2-sulfenates, while
[MoO2(btap)]-, 1, does not form stable η2-sulfenates, has been
ascribed to the electron-deficient, π-accepting nature of [VVO]3+
relative to [MoVIO2]2+. This is the likely explanation for the
absence of the sulfenate structural motif in MoVI-thiol biomi-
metic chemistry. With [Ni(Hbtap)]2, 2, we have observed the
oxidative formation of paramagnetic (S ) 1) NiII complexes
that probably have a disulfide ligand (as opposed to NiII-
sulfenate/sulfinate ligation). We have attributed this reactivity
to the availability of the phenolic ligand which can substitute
for one of the sulfur donors in the first coordination sphere of
the nickel(II) ion. In the biological realm where many potential
ligands are present, Ni-sulfenate/sulfinate formation/observation
may be precluded due to disulfide formation and subsequent
electron-transfer reactions.
Maroney and co-workers6,7 has amply demonstrated the dioxy-
gen reactivity of nickel-thiolates, which has been attributed to
the nucleophilicity of the sulfur lone pairs of the coordinated
thiolate ligand.4 In the case of early transition elements, which
have a d0 electron configuration, the sulfur lone pairs are
involved in π-bonding to the electron deficient metal center,
thereby diminishing their nucleophilic character. This being
the case, we hypothesize that the oxidant (peroxide or peracid)
must be activated, via metal coordination, to react with the
coordinated thiol. This hypothesis is currently being explored
in this laboratory.
Our inability to convert the octahedral complex [MoO2(btap)]-
into a stable sulfenate in a manner analogous to VO(btap) is
probably due to the higher coordination number in [MoO2(btap)]-
(6 vs 5 for VO(btap)) and the presence of the second oxo ligand
in [MoO2(btap)]-. The HOMO and LUMO orbitals for meth-
ylsulfenic acid (as a peroxide-like tautomer) are presented in
Scheme 1.30 The π* nature of the HOMO is analogous to that
found for peroxide as indicated in most undergraduate inorganic
chemistry texts. Side-on η2-coordination of peroxide (RO-
O-) or sulfenate (RS-O-) to an electron deficient metal center
such as [VVO]3+ (Scheme 1) removes electron density from
these π* orbitals, thereby stabilizing the ligand. In the case of
Acknowledgment. Funding for this work was provided by
the National Science Foundation (CHE-9702873). K.L.J. was
supported by an NSF-REU fellowship (CHE-9610196). The
X-ray diffractometer at NCSU was purchased using funds from
the NSF (CHE-9509532).
Supporting Information Available: Figures S1 and S2 showing
1
full numbering schemes for 1 and 2, Figure S3 showing the H NMR
spectrum of 1, Figure S4 showing the IR spectra of H3btap, complex
1, and the oxidation product of 1, and tables listing details of
crystallographic structures of 1 and 2 (14 pages). X-ray crystallographic
files for 1 and 2, in CIF format, are available on the Internet only.
Ordering and access information is given on any current masthead page.
(30) (a) The electronic structure for MeSOH was calculated using density
functional theory (BP86/D**) as implemented in the computational
program Spartan. (b) Spartan SGI, 5.0.2; Wavefunction, Inc.: Irvine,
CA, 1997.
IC9802956