Tripodal bis(imidazole) thioether copper(I) complexes: Mimics of the CuM site of copper hydroxylase enzymes

Article abstract of DOI:10.1021/ic700447k

Tripodal bis(imidazole) thioether ligands, (N-methyl-4,5-diphenyl-2- imidazolyl)₂C(OR)C(CH₃)₂SR′.(BIT ^OR,SR′; R = H, CH₃; R′ = CH₃, C(CH₃)₃, C(C₆H₅)₃), have been prepared, offering the same N₂S donor atom set as the Cu _M binding site of the hydroxylase enzymes, dopamine beta hydroxylase and peptidylglycine hydroxylating monooxygenase. Isolable copper(I) complexes of the type [(BIT^OR,SMe)Cu(CO)]PF₆ (3a and 3b) are produced in reactions of the respective tripodal ligands 1a (R = H) and 1b (R = Me) with [Cu(CH₃CN)₄]PF₆ in CH₂Cl₂ under CO (1 atm); the pyramidal structure of 3a has been determined crystallographically. The infrared (IR) ν(CO)'s of 3a and 3b (L = CO) are comparable to those of the Cu_M-carbonylated enzymes, indicating similar electronic character at the copper centers. The reaction of [(BIT ^OH,SMe)Cu(CH₃CN)]PF₆ (2a) with dioxygen produces [(BIT^O,SOMe)₂-Cu₂(DMF) ₂](PF₆)₂ (4), whose X-ray structure revealed the presence of bridging BIT-alkoxo ligands and terminal -SOMe groups. In contrast, oxygenation of 2b (R = Me) affords crystallographically defined [(BIT^OMe,SMe)₂Cu₂(μ-OH)₂](OTf) ₂ (5), in which the copper centers are oxygenated without accompanying sulfur oxidation. Complex 5 in DMF is transformed into five-coordinate, mononuclear [Cu^II(BIT^OMe,SMe)(DMF) ₂](PF₆)₂ (6). The sterically hindered BIT ^OR,SR′- ligands 9 and 10 (R′ = t-Bu; R = H, Me) and 11 and 12 (R′ = CPh₃; R = H, Me) were also prepared and examined for copper coordination/oxygenation. Oxygenation of copper(I) complex 13b derived from the BIT^OMe,SBu-t ligand is slow, relative to 2b, producing a mixture of (BIT^OMe,SBu-t)₂Cu ₂(μ-OH)₂-type complexes 14b and 15b in which the -SBu-t group is uncoordinated; one of these complexes (15b) has been ortho-oxygenated on a neighboring aryl group according to the X-ray analysis and characterization of the free ligand. Oxygenation of the copper(I) complex derived from BIT ^OMe,SCPh3 ligand 12 produces a novel dinuclear disulfide complex, [(BIT^OMe,S)₂-Cu₂(μ-OH)₂](PF ₆)₂ (17), which is structurally characterized. Reactivity studies under anaerobic conditions in the presence of t-BuNC indicate that 17 is the result of copper(I)-induced detritylation followed by oxygenation of a highly reactive copper(I)-thiolate complex.

Full text of DOI:10.1021/ic700447k

Inorg. Chem. 2007, 46, 7789−7799
Tripodal Bis(imidazole) Thioether Copper(I) Complexes: Mimics of the
CuM Site of Copper Hydroxylase Enzymes
Lei Zhou, Douglas Powell, and Kenneth M. Nicholas*
Department of Chemistry and Biochemistry, UniVersity of Oklahoma, Norman, Oklahoma 73019
Received March 7, 2007
Tripodal bis(imidazole) thioether ligands, (N-methyl-4,5-diphenyl-2-imidazolyl)2C(OR)C(CH3)2SR′ ) H,
(BITOR,SR′; R
CH3; R′ ) CH3, C(CH3)3, C(C6H5)3), have been prepared, offering the same N2S donor atom set as the CuM
binding site of the hydroxylase enzymes, dopamine beta hydroxylase and peptidylglycine hydroxylating
monooxygenase. Isolable copper(I) complexes of the type [(BITOR,SMe)Cu(CO)]PF6 (3a and 3b) are produced in
reactions of the respective tripodal ligands 1a (R
CO (1 atm); the pyramidal structure of 3a has been determined crystallographically. The infrared (IR)
3a and 3b (L CO) are comparable to those of the CuM-carbonylated enzymes, indicating similar electronic
character at the copper centers. The reaction of [(BITOH,SMe)Cu(CH3CN)]PF6 (2a) with dioxygen produces [(BITO,SOMe)2-
Cu2(DMF)2](PF6)2 (4), whose X-ray structure revealed the presence of bridging BIT alkoxo ligands and terminal
SOMe groups. In contrast, oxygenation of 2b (R
Me) affords crystallographically defined [(BITOMe,SMe)2Cu2(
)
H) and 1b (R
)
Me) with [Cu(CH3CN)4]PF6 in CH2Cl2 under
ν(CO)’s of
)
−
−
)
µ-
OH)2](OTf)2 (5), in which the copper centers are oxygenated without accompanying sulfur oxidation. Complex 5 in
DMF is transformed into five-coordinate, mononuclear [CuII(BITOMe,SMe)(DMF)2](PF6)2 (6). The sterically hindered
′
BITOR,SR ligands 9 and 10 (R′ ) t-Bu; R
) H, Me) and 11 and 12 (R′ ) CPh3; R ) H, Me) were also prepared
and examined for copper coordination/oxygenation. Oxygenation of copper(I) complex 13b derived from the
-
t
BITOMe,SBu ligand is slow, relative to 2b, producing a mixture of (BITOMe,SBu-t)2Cu2(
µ-OH)2-type complexes 14b
and 15b in which the SBu-t group is uncoordinated; one of these complexes (15b) has been ortho-oxygenated
−
on a neighboring aryl group according to the X-ray analysis and characterization of the free ligand. Oxygenation
of the copper(I) complex derived from BITOMe,SCPh ligand 12 produces a novel dinuclear disulfide complex, [(BITOMe,S)2-
3
Cu2(
presence of t-BuNC indicate that 17 is the result of copper(I)-induced detritylation followed by oxygenation of a
highly reactive copper(I) thiolate complex.
µ-OH)2](PF6)2 (17), which is structurally characterized. Reactivity studies under anaerobic conditions in the
−
Introduction
dioxygen and the substrate are activated at the CuM site and
that the CuH site serves as an electron-transfer shuttle from
ascorbate to the CuM site.3 A recent X-ray structure of the
O2-bound PHM enzyme has revealed an unusual end-on
coordination mode to the CuM center,4-6 contrasting with
The copper hydroxylase enzymes, dopamine beta hy-
droxylase (DâH) and peptidylglycine R-hydroxylating mo-
nooxygenase (PHM), catalyze the regio- and enantioselective
hydroxylation of mildly activated C-H bonds by dioxygen.1
These enzymes appear to have very similar active-site
structures, featuring two copper centers separated by ca. 12
Å, the CuH (CuA) site being ligated to three histidine-derived
imidazoles and the CuM (CuB) site by two histidines and a
methionine residue (Figure 1).2 Spectroscopic and amino acid
mutagenesis studies suggest that upon reduction both the
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10.1021/ic700447k CCC: $37.00
Published on Web 08/22/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 19, 2007 7789

Zhou et al.
biological donor set. Regarding models for the CuH site of
the hydroxylases, Sorrell and Borovik first reported copper-
(I) complexes of tripodal tris(N-methyl-2-imidazolyl)-
carbinol12a and tris(2-imidazolyl)phosphines.12d Dinuclear
[(tripod)2CuI2]Z2 complexes of the former ligand were found
to be unreactive toward dioxygen, whereas a mononuclear
copper(I) complex of the latter ligand reacts with dioxygen
at low temperature to produce a reactive peroxodicopper(II)
adduct, showing that steric and stereoelectronic effects are
important in defining CuI-O2 interactions in these systems.
We recently have investigated the chemistry of copper(I)
complexes of tripodal tris(N-methyl-4,5-diphenylimidazolyl)-
methane ligands.15 Whereas these also form dinuclear Cu2L2-
type complexes, mononuclear adducts [(imid3CR)CuL]Z are
produced with L ) acetonitrile, carbon monoxide, and
t-BuNC. The structures of the isocyanide derivatives depend
critically on the tripod methane substituent, R, showing
trigonal and tetrahedral geometries with bi- or tridentate
coordination of the tripod. Reactions of dioxygen with the
dinuclear complexes or mononuclear [(imid)3CR]CuL]Z are
sluggish, producing from the latter [(imid)3CH]CuII(acetone)-
(H2O)]Z2.
Few complexes have been reported that possess the
(amine)2thioether coordination sphere relevant to the CuM
site of the copper hydroxylases. Karlin and co-workers
recently described copper(I,II) complexes of (non-imidazole)
tridentate (pyridine, amine, and thioether) ligands (N2SR);
oxygenation of [(N2SR)CuI]+ leads to ligand sulfoxidation,
demonstrating dioxygen activation and an oxygenation
pathway not displayed by the enzymes.16 Also reported was
a N3S-thioether-CuI complex that undergoes low-temper-
ature oxygenation to form a spectroscopically characterized
end-on peroxodicopper(II) complex.17 A few copper(II)
complexes with mixed polydentate imidazole-thioether
ligands have been characterized,18 but until our recent studies,
only one copper(I) derivative had been reported.11 In our
preliminary report, we disclosed the preparation of tripodal
bis(4,5-diphenylimidazolyl) thioether (BIT) ligands, corre-
sponding copper(I) complexes, [(BITOR,SMe)Cu(L)]PF6 (L )
CH3CN, CO; R ) H, CH3), and the initial results of their
interaction with dioxygen.19 Herein, we describe new mem-
bers of this family of ligands bearing sterically hindered
thioether substituents, the novel oxygenation reactivity of
Figure 1. Copper hydroxylase active site.
the more common side-on mononuclear and bridging bime-
tallic bonding modes found in most synthetic complexes and
in the dioxygen-binding Cu-protein hemocyanin.7 The
mechanistic details of the substrate hydroxylation are unclear,
but rate-limiting C-H bond breaking with a significant
tunneling component by a Cu-oxo species is supported by
the large intrinsic primary D-isotope effect (10.6) for both
DâH and PHM.8 Recent computational studies suggest that
either a side-on Cu-superoxo9 or a Cu-oxo species10 is the
likely H-abstracting agent.
Although many synthetic model complexes for these (and
other) copper enzymes have been prepared with a variety of
polydentate amine, pyridine, and pyrazolyl-based ligands,7
remarkably few have incorporated the biologically most
relevant poly(imidazole) (from histidine) donors.11,12 Given
the significantly different basicity (pKa: imidazolium, 6.8;
pyrazolium, 2.6; pyridinium, 5.2, tertiary ammonium, 9-10)13
and donor/acceptor properties14 of imidazole vis-a`-vis the
other common nitrogen ligands, we suggest that the most
accurate structural and functional mimics for histidine-rich
metalloenzymes will be provided by incorporating the actual
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7790 Inorganic Chemistry, Vol. 46, No. 19, 2007

Tripodal Bis(imidazole) Thioether Copper(I) Complexes
Scheme 1
Figure 2. Crystal structure of 3a. Selected bond lengths (angstroms) and
angles (degrees): Cu(1)-C(43) 1.811(2), Cu(1)-N(25) 2.0005(19), Cu-
(1)-N(1) 2.0259(17), Cu(1)-S(1) 2.4392(7), C(43)-O(44) 1.123(2);
C(43)-Cu(1)-N(25) 122.90(9), C(43)-Cu(1)-N(1) 132.84(10), C(43)-
Cu(1)-S(1) 118.81(9), N(25)-Cu(1)-N(1) 90.67(7), N(25)-Cu(1)-S(1)
93.37(6), N(1)-Cu(1)-S(1) 87.33(5), Cu(1)-C(43)-O(44) 177.4(2).
the corresponding copper complexes, and provide a full
account of the chemistry of the initially reported BIT-Cu
complexes.
CuI-CO structures.21 An electronic similarity of the copper
centers in carbonyl complexes 3a and 3b with the CuM (N2S)
site of the carbonylated copper hydroxylase enzymes is
indicated by their comparable CO vibrational frequencies:
PHM (2093 cm-1),22 DâH (2089 cm-1),23 3a (2102 cm-1),
and 3b (2095 cm-1). These bands lie in the range reported
for N3-type cationic [tris(pyrazolyl)methane]- and [tris-
(imidazolyl)methane]-CuI-CO complexes12,21 but at a higher
frequency than most neutral [tris(pyrazolyl)borate]-Cu-CO
derivatives.21 Although these data suggest that relatively little
Cu-CO back-bonding is involved in the cationic N2S- and
N3-Cu-carbonyl complexes vis-a`-vis those of anionic N3-
type ligands, the IR data do not differentiate the electronic
character of the copper centers in the N2S complexes versus
the N3 complexes.
Results and Discussion
To provide the desired bis(imidazole) thioether donor set
of the CuM center in a sterically encumbered environment
that could limit the formation of bimetallic species, we first
targeted the phenylated tripodal ligands BITOH,SMe (1a) and
BITOMe,SMe (1b), which are conveniently prepared from
readily available 4,5-diphenylimidazole (Scheme 1) via the
addition of their 2-lithio derivatives to thiomethyl esters.
These were characterized spectroscopically. Compounds 1a
and 1b react readily with [Cu(CH3CN)4]PF6 (CH2Cl2, room
temperature) to produce [(BITOR,SMe)Cu(CH3CN)]PF6 (2a, R
) H; 2b, R ) Me). The former, somewhat-air-sensitive solid,
was isolated and characterized spectroscopically and is
presumed to have the pyramidal geometry depicted in
Scheme 1.
Complexes 2a and 2b (generated in situ) are convenient
precursors for investigating reactions with biorelevant sub-
strates such as carbon monoxide and dioxygen. Bubbling CO
into a CH2Cl2 solution of 2a and 2b (3 h, room temperature)
resulted in the appearance of a strong infrared (IR) absorption
near 2100 cm-1. The addition of ether-petroleum ether
induced crystallization of the colorless, air-sensitive carbonyl
complexes [(BITOR, SMe)Cu(CO)]PF6 (3a and 3b) (Scheme
1). These compounds are reasonably stable in solution with
respect to CO dissociation under N2, displaying appropriate
NMR spectra and detectable molecular ions in their elec-
trospray ionization mass spectrometry (ESIMS). The mo-
lecular structure of 3a was established by X-ray diffraction
(XRD) (Figure 2) and consists of a slightly distorted
pyramidal arrangement about the copper(I) center formed
by the tridentate BIT and CO ligands.20 Notwithstanding the
unique (imidazole)2(thioether)CuI donor atom set and a
bifurcated O-H-FPF5 hydrogen bond, the observed bond
lengths and angles are unexceptional relative to reported N3-
The reactions of hydroxy- and methoxy-tripod com-
plexes 2a and 2b with dioxygen produced remarkably
different results. Adventitious oxidation of 2a occurred
during its recrystallization from a DMF-ether solution over
1 week, affording green crystals of a new compound 4 whose
ESIMS suggested the formation of a Cu2(BIT)2-type species.
The X-ray structure of 4 (Scheme 2 and Figure 3) showed it
to be a centrosymmetric dication with two five-coordinate
copper(II) centers, each terminally bonded to one imidazole
nitrogen, a DMF oxygen, and a sulfoxide oxygen and bridged
by two alkoxo oxygens from the deprotonated tripod ligands.
The bond metrics observed in the Cu2(OR)2 core of 4 (Figure
3) are comparable to those found in several Cu2(OH)2 and
Cu2(OR)2 structures established crystallographically,24 for
example, Cu-(µ-OR), 1.92 ((0.02) Å; Cu-Cu 3.00 ((0.02)
Å; Cu-O-Cu, 102 ((2)°. Related tris(imidazole)carbinol
ligands also have been found to form polynuclear, alkoxo-
(21) (a) Fujisawa, K.; Ono, T.; Ishikawa, Y.; Amir, N.; Miyashita, Y.;
Okamoto, K.; Lehnert, N. Inorg. Chem. 2006, 45, 1698. (b) Imai, S.;
Fujisawa, K.; Kobayashi, T.; Shirasawa, N.; Fujii, H.; Yoshimura, T.;
Kitajima, N.; Moro-oka, Y. Inorg. Chem. 1998, 37, 3066-3070. (c)
Reger, D. L.; Collins, J. E.; Rheingold, A. L.; Liable-Sands, L. M.
Organometallics 1996, 15, 2029-2032. (d) Dias, H. V. R.; Lu, H. L.
Inorg. Chem. 1995, 34, 5380-5382.
(22) Jaron, S.; Blackburn, N. J. Biochemistry 1999, 38, 15086-15096.
(23) Blackburn, N. J.; Pettingill, T. M.; Seagraves, K. S.; Shigeta, R. T. J.
Biol. Chem. 1990, 265, 15383-15386.
(20) (a) The geometry index τ4, defined by Houser et al. (ref 20b), is equal
to 0.74 for 3a, a value between that of a trigonal pyramid (0.85) and
a seesaw (0.64), with the former description being closer to the
observed angles of 3a. (b) Yang, L.; Powell, D. R.; Houser, R. P.
Dalton Trans. 2007, 955-964.
Inorganic Chemistry, Vol. 46, No. 19, 2007 7791

Zhou et al.
Figure 4. Crystal structure for the cation of 5b. Selected bond lengths
(angstroms) and angles (degrees): Cu(1)-O(1)′ 1.917(3), Cu(1)-O(1)
1.931(3), Cu(1)-N(1) 1.950(3), Cu(1)-N(11) 1.959(3), Cu(1)-S(1) 2.8683-
(13), Cu(1)-Cu(1)′ 3.021(3), O(1)-O(1)′ 2.383(3); O(1)′-Cu(1)-O(1)
76.55(18), O(1)′-Cu(1)-N(1) 174.66(13), O(1)-Cu(1)-N(1) 98.34(14),
O(1)′-Cu(1)-N(11) 97.23(15), O(1)-Cu(1)-N(11) 173.74(13), N(1)-Cu-
(1)-N(11) 87.90(15), O(1)′-Cu(1)-S(1) 99.45(11), O(1)-Cu(1)-S(1)
92.73(11), N(1)-Cu(1)-S(1) 82.20(9), N(11)-Cu(1)-S(1) 87.48(9), Cu-
(1)′-O(1)-Cu(1) 103.44(18).
Figure 3. Crystal structure for the cation of 4. Selected bond lengths
(angstroms) and angles (degrees): Cu(1)-O(42′) 1.909(9), Cu(1)-O(44)
1.9369(18), Cu(1)-O(38) 1.9436(16), Cu(1)-N(1) 1.958(2), Cu(1)-O(38′)
1.9599(16), Cu(1)-Cu(1A) 3.0302(7); O(42′)-Cu(1)-O(38) 110.7(4),
O(42′)-Cu(1)-O(44) 92.0(4), O(44)-Cu(1)-N(1) 102.53(8), O(44)-Cu-
(1)-O(38) 157.02(8), O(38)-Cu(1)-N(1) 81.48(8), Cu(1)-O(38)-Cu(1)′
101.85.
Scheme 3
Scheme 2
In contrast, room-temperature oxygenation of the
BITOMe,SMe complex 2b or 3b (1 atm of O2, CH2Cl2, 12 h),
for which alkoxide formation is blocked, was accompanied
by a gradual darkening of the reaction mixture. Crystalliza-
tion by ether diffusion produced an amber compound 5,
whose ESIMS showed ions of [(BITOMe,SMe)CuOH]+ and
[(BITOMe,SMe)2Cu2(OH)2]2+ composition. Despite crystal dis-
order problems, the X-ray structure of the triflate salt 5b
was determined (Scheme 3 and Figure 4) to consist of a
dinuclear dication having a Cu2O2 core with each copper
coordinated terminally to the N2S donor set of the BITOMe,SMe
ligand with separated triflate ions. Although the O-hydrogens
bridged copper(II) complexes.25 Both the copper and the
sulfur atoms are therefore oxidized during the conversion
of 2a to 4. This facile sulfur oxidation can be compared to
known copper-mediated oxidations of thioethers,26 the sto-
ichiometric sulfur ligand oxidation of a recently reported N2S
complex by the Karlin group,17 and the copper-mediated
methionine residue oxidation of the amyloid beta peptide
(Aâ),27 but it contrasts with the apparent oxidative inertness
of the CuM active-site methionine in DâH and PHM.
were not located by XRD, 5 is formulated as a L2CuII (OH)2
2
complex based on (a) a comparison of its core metrical
parameters with other Cu2(µ-OH)223, Cu2(µ-O)2, and Cu2(µ-
O2) structures28 [Cu-O, 1.92 (av) Å; Cu-Cu, 3.02 Å; Cu-
O-Cu, 103°; O-O 2.38 Å]; (b) its mass spectrum, which
showed two additional hydrogens; and (c) its thermal stability
relative to typically unstable LnCu2(µ-O)2 complexes.28 The
hydroxo ligands of 5 are derived from dioxygen, as shown
by the reaction of 2b with 18O2/16O2; the mass spectrum of
the resulting product 5 showed appropriate higher m/z peaks.
Thus, by capping of the hydroxyl group, as in the BITOMe
(24) (a) Kaim, W.; Titze, C.; Schurr, T.; Sieger, M.; Lawson, M.; Jordanov,
J.; Rojas, D.; Garcia, A. M.; Manzur, J. Z. Anorg. Allg. Chem. 2005,
631, 2568-2574. (b) Rojas, D.; Garcia, A. M.; Manzur, J.; Vega, A.
Acta Crystallogr. 2005, C61, m115-m116. (c) Herres, S.; Flo¨rke, U.;
Henkel, G. Acta Crystallogr. 2004, C60, m659-m660. (d) Jiang, Y.-
M.; Zenga, J.-L.; Yub, K.-B. Acta. Crystallogr. 2004, C60, m543-
m545.
(25) Higgs, T. C.; Helliwell, M.; McInnes, E. J. L.; Mabbs, F. E.; Harding,
C. J.; Garner, C. D. J. Chem. Soc., Dalton Trans. 1997, 927-933.
(26) Firouzabadi, H.; Iranpoor, N.; Zolfigol, M. A. Synth. Commun. 1998,
28, 1179-1187.
(27) (a) Huang, X. D.; Atwood, C. S.; Hartshorn, M. A.; Multhaup, G.;
Goldstein, L. E.; Scarpa, R. C.; Cuajungco, M. P.; Gray, D. N.; Lim,
J.; Moir, R. D.; Tanzi, R. E.; Bush, A. I. Biochemistry 1999, 38, 7609-
7616. (b) Ciccotosto, G. D.; Barnham, K. J.; Cherny, R. A.; Masters,
C. L.; Bush, A. I.; Curtain, C. C.; Cappai, R.; Tew, D. Lett. Pept. Sci.
2004, 10, 413-417. (c) Hong, J. Y.; Schoneich, C. Free Radical Biol.
Med. 2001, 31, 1432-1441.
(28) (a) Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. ReV. 2004,
104, 1013-1045. (b) Que, L., Jr.; Tolman, W. B. Angew. Chem., Int.
Ed. 2002, 41, 1114-1137.
7792 Inorganic Chemistry, Vol. 46, No. 19, 2007

Tripodal Bis(imidazole) Thioether Copper(I) Complexes
crystals of a new compound 6 could be manually isolated.
The ESIMS of 6 suggested a [(BITOMe,SMe)CuII(DMF)n]2+
formulation. This was confirmed by its X-ray structure
(Figure 5), which showed a mononuclear copper(II) complex
cation of square-pyramidal geometry with its basal plane
defined by the two imidazole nitrogens and two DMF
oxygens and an apical -SMe group; two noninteracting
octahedral PF6 anions were located as well. It is noteworthy
that 6 retains the N2S coordination mode of the oxidized
CuM site of the hydroxylase enzymes.2,3 The CuII-S distance
of 6 (2.69 Å) is considerably longer than the CuI-S length
of 3a (2.44 Å), suggestive of tighter binding of the sulfur
atom to the reduced copper center. The same effect has been
seen with the oxidized (2.68 Å) and reduced (2.27 Å) forms
of PHM.29 As for the formation of 6 from 5, we speculate
that DMF could assist dimer dissociation to form a mono-
nuclear Cu-OH species, which is protonated by adventitious
moisture; DMF displacement of coordinated water from such
an aquo complex would provide 6.
Figure 5. Crystal structure of the dication of 6. Selected bond lengths
(angstroms) and angles (degrees): Cu(1)-O(3) 1.9474(18), Cu(1)-N(2)
1.952(2), Cu(1)-O(2) 1.9527(17), Cu(1)-N(1) 1.968(2), Cu(1)-S(1)
2.6892(8); O(3)-Cu(1)-N(2) 98.75(8), O(3)-Cu(1)-O(2) 84.85(7), N(2)-
Cu(1)-N(1) 84.70(9), O(2)-Cu(1)-N(1), 91.45(8).
Scheme 4
Because hydroxo-bridged dimers were obtained from
oxygenation of the complexes derived from the -SMe
ligands, we sought to inhibit dinuclear complex formation
and potentially kinetically stabilize mono-Cu-O2 species by
increasing the steric demand of the tripodal ligands with
bulkier S-alkyl groups. The requisite sulfur-containing esters
7 and 8 were prepared by the surprisingly effective reaction
of ethyl 2-methyl-2-bromopropanoate with tert-butyl thiolate
and trityl thiolate, respectively (Scheme 4).30 The corre-
sponding BITOMe,SR′ ligands 9 and 10 (R′ ) t-Bu; R ) H,
Me) and 11 and 12 (R′ ) CPh3; R ) H, Me) were then
obtained by double addition of the lithiated imidazole to the
esters, followed by reaction with methyl iodide. The overall
conversion to the methoxy-capped tripodal ligands could be
carried out sequentially in one pot or in separate steps after
isolation of the intermediate alcohol. All of the new
compounds had the expected compositions (by MS) and
exhibited appropriate NMR and mass spectroscopic features.
complexes, copper-centered oxygenation proceeds to give
5, avoiding not only the alkoxo bridging tripod (as in 4) but
also sulfur oxidation as well. The source of the -OH
hydrogens in 5 is uncertain but could derive from the reaction
solvent, CH2Cl2, suggesting the intervention of a reactive
mono- or dinuclear Cu-dioxygen complex. The relatively
unhindered sulfur center of the BITOMe, SMe ligand apparently
is insufficient (at least at room temperature), however, to
prevent the formation of a bridged dinuclear complex.
The Cu2(OH)2 linkage of 5 was found to be labile during
its recrystallization from DMF-ether, from which blue
The copper coordination of these two bulky ligands and
the oxygenation chemistry of the copper(I) complexes proved
to be strikingly different. The BITOMe,SBu-t ligand 10 reacts
Scheme 5
Inorganic Chemistry, Vol. 46, No. 19, 2007 7793

Zhou et al.
Figure 6. Crystal structures of the dications of 14b and 15b. Selected bond lengths (angstroms) and angles (degrees): Cu(1)-O(1) 1.898(5), Cu(1)-N(7)
1.921(5), Cu(1)-N(1) 1.927(5), Cu(1)-O(1)′ 1.952(5), Cu(1)-Cu(1)′ 3.0098(14); O(1)-Cu(1)-N(7) 95.3(2), O(1)-Cu(1)-N(1) 176.0(2), N(7)-Cu(1)-
N(1) 88.6(2), O(1)-Cu(1)-O(1)′ 77.2(3), N(7)-Cu(1)-O(1)′ 167.2(2), N(1)-Cu(1)-O(1)′ 99.2(2), O(1)-Cu(1)-Cu(1)′ 39.23(17), N(7)-Cu(1)-Cu(1)′
133.67(15), N(1)-Cu(1)-Cu(1)′ 137.07(16), O(1)′-Cu(1)-Cu(1)′ 37.93(14), Cu(1)-O(1)-Cu(1)′ 102.88(10).
Scheme 6
with [Cu(CH3CN)4]PF6 in CH2Cl2 at -20 °C to give four-
coordinate [(BITOMe,SBu-t)Cu(CH3CN)]PF6 (13a), which was
characterized spectroscopically (Scheme 5). This compound
was quite unreactive toward dioxygen, being largely recov-
ered after 5 days of exposure at 20 °C (1 atm of O2, CH2-
Cl2). To circumvent the possible inhibiting coordination
competition between dioxygen and acetonitrile, the corre-
sponding triflate salt of 13a (13b) was generated in a CH2-
concentrated ammonium hydroxide in CH2Cl2 produced two
organic compounds separable by thin-layer chromatography
(TLC). One was found to be the original BITOMe,SBu-t ligand
(10) and the other (16) was found to have an extra oxygen
atom from its high-resolution mass spectrum (C41H45N4O2S).
The additional oxygen atom was incorporated at an ortho
aromatic carbon, rather than at the thioether sulfur, as was
apparent from the 1H NMR spectrum of 16, which exhibited
three multiplets (4H) in the 6-7 ppm region typical of
phenols, whereas the -SMe resonance was virtually un-
changed relative to that of the original (unoxygenated) ligand
10. Together, these data indicate that oxidation of the ligand
occurs in part during the oxygenation of complex 13b.
Although ligand-centered C-H bond oxygenation has been
observed in a number of previous dioxygen reactions of
copper(I) polyamine complexes,31 this is the first instance
of such a process involving the (imidazolyl)2(thioether)(BIT)
or (imidazolyl)3(TIC) copper complexes. The intervention
of this pathway could be enabled by the sterically weakened
Cu-thioether linkage that influences the reactivity of the
presumed intermediate Cu-O2 complex.
Cl2 solution via the reaction of 10 with (CuOTf)2‚(toluene).
Complex 13b reacted slowly with dioxygen at -20 °C over
72 h to give an amber solution, from which crystals were
obtained upon ether diffusion. X-ray analysis of this material
showed a dinuclear Cu2(BIT)2 species that was significantly
disordered with respect to the orientation of the 5-aryl groups
near to copper, with approximately 60% of one orientation
(A) and ca. 40% of another orientation (B). This suggested
the presence of two structurally similar compounds in the
crystal. One, corresponding to orientation A, was determined
to be hydroxo-CuII dimer 14b, in which each copper is in
a square-planar arrangement defined by the two hydroxo
groups and the two imidazole nitrogens, with the SBu-t group
being uncoordinated (Figure 6). The µ-hydroxo formulation
was again supported by the atomic distances and angles
within the Cu2O2 core24 [Cu(1)-O(1) 1.898(5) Å; Cu(1)-
O(1)′ 1.952(5) Å; Cu(1)-Cu(1)′, 3.0098(14) Å; Cu(1)-
O(1)-Cu(1)′, 102.88(10)°], and the ESIMS features of the
mixture.
The other molecule present in the crystal, 15b, had the
5-position phenyl groups roughly coplanar with Cu(1), N(7),
C(8), and O(1) and the o-phenyl carbon, C(26′), within a
bonding distance (1.64 Å) of the bridging O(1), suggesting
that ortho oxygenation of the ligand aryl group had occurred
(Figure 6). This conclusion was confirmed by demetalation
and characterization of the free ligands (Scheme 6). Thus,
treatment of the mixture of complexes 14b/15b with
In contrast, when the tritylated BITOMe,SCPh ligand 12 was
3
combined with [Cu(CH3CN)4]PF6 in CH2Cl2 at room tem-
perature, a bright-yellow (rather than the usual colorless)
solution formed immediately. The solution rapidly became
purple when stirred under 1 atm of O2 (balloon) at -50 °C.
Ether diffusion into the reaction mixture produced purple
crystals whose MS data suggested the formation of a
(29) (a) Eipper, B. A.; Quon, A. S. W.; Mains, R. E.; Boswell, J. S.;
Blackburn, N. J. Biochemistry 1995, 34, 2857-2865. (b) Reddy, B.
J.; Blackburn, N. J. J. Am. Chem. Soc. 1994, 116, 1924-1931.
(30) Ammazzalorso, A.; Amoroso, R.; Baraldi, M.; Bettoni, G.; Braghiroli,
D.; De Filippis, B.; Giampietro, L.; Tricca, M. L.; Vezzalini, F. Eur.
J. Med. Chem. 2005, 40, 918-921.
(31) (a) Lewis, E. A.; Tolman, W. B. Chem. ReV. 2004, 104, 1047-1076.
(b) Itoh, S.; Taki, M.; Nakao, H.; Holland, P. L.; Tolman, W. B.;
Que, L., Jr.; Fukuzumi, S. Angew. Chem., Int. Ed. 2000, 39, 398-
400. (c) Holland, P. L.; Rodgers, K. R.; Tolman, W. B. Angew. Chem.,
Int. Ed. 1999, 38, 1139-1142.
7794 Inorganic Chemistry, Vol. 46, No. 19, 2007

Tripodal Bis(imidazole) Thioether Copper(I) Complexes
Scheme 7
dinuclear species 17, lacking the trityl unit (Scheme 7).
Indeed, XRD analysis of 17 revealed a dimeric structure
(Figure 7) featuring both a CuII(µ-OH)2 core (with supporting
Cu-O and Cu-O-Cu bond metrics) and, most surprisingly,
a bridging (noncoordinated) disulfide unit formed between
the two tripodal ligands. Each copper(II) center is essentially
square planar in its coordination to the two bridging hydroxo
groups and the two imidazole nitrogens. Trityl alcohol was
identified by TLC/NMR as the major organic product of the
reaction.
CH2Cl2 was conducted under Ar followed by the addition
of tert-butyl isocyanide (1.0 equiv) at room temperature.
ESIMS analysis of the relatively air-stable reaction mixture
indicated the formation of [(BITOMe,SH)2CuI2(t-BuNC)2]2+ and
[BITOMe,SCuI2(t-BuNC)2]+ species, both lacking the Tr group.
In addition, chromatography of this material afforded a
1
yellow complex whose H NMR and mass spectra support
the formulation as a CuI2(thiolate) complex 18. It is apparent,
therefore, that copper-promoted detritylation occurs before
oxygenation, with the trityl unit lost as the (yellow) trityl
cation, which is eventually converted by aerobic moisture
to the alcohol. A highly oxygen-sensitive neutral N2S-
(thiolate)CuI complex, that can be stabilized by coordinating
t-BuNC is likely produced. Lacking isocyanide, the initially
formed species undergoes facile oxidation, both at sulfur to
give the disulfide unit and at copper to produce a µ-oxo/
peroxo species that abstracts hydrogen (presumably from
solvent) to give the structurally characterized complex 17.
Although the transformations observed in the conversion of
12 to disulfide complex 17 are unusual, they find precedent
in the recently reported copper(I)-catalyzed cleavage of trityl
thioethers to disulfides31 and the air-induced conversion of
a related tripodal zinc thiolate complex to a dinuclear
disulfide complex.32
Figure 7. Crystal structure of the dication of 17. Selected bond lengths
(angstroms) and angles (degrees): Cu(1A)-O(1A) 1.918(10), Cu(1A)-
O(1B) 1.962(9), Cu(1A)-N(2A) 1.971(13), Cu(1A)-N(8A) 2.007(15), Cu-
(1A)-Cu(1B) 2.807(3), S(1A)-S(1B) 2.051(6), Cu(1A)-Cu(1B) 2.807(3);
O(1A)-Cu(1A)-O(1B) 77.7(4), O(1A)-Cu(1A)-N(2A) 99.3(5), O(1B)-
Cu(1A)-N(2A) 173.5(5), O(1A)-Cu(1A)-N(8A) 157.5(5), O(1B)-Cu-
(1A)-N(8A) 96.2(5), N(2A)-Cu(1A)-N(8A) 88.7(6), O(1A)-Cu(1A)-
Cu(1B) 43.5(3), O(1B)-Cu(1A)-Cu(1B) 44.4(3), N(2A)-Cu(1A)-Cu(1B)
136.0(4), N(8A)-Cu(1A)-Cu(1B) 118.4(4), C(40A)-S(1A)-S(1B) 108.2-
(6), Cu(1A)-O(1A)-Cu(1B) 93.5(4).
The reactivity of these bis(imidazolyl)-thioether-Cu
complexes is comparable in a number of respects to that of
the CuM site of the copper hydroxylase enzymes. They form
fairly stable CO adducts whose vibrational energies indicate
a electronic character similar to that of the enzymes.
Moreover, the CO complexes are qualitatively more stable
than the corresponding tris(imidazolyl) complexes that we
have recently prepared,19 also consistent with the relative
CO-adduct forming tendency of the CuH and CuM sites. The
bis(imidazolyl)-thioether-CuI complexes are reactive to-
Hence, loss of the trityl group from the BITOMe,STr ligand
and oxidation of both the copper and sulfur atoms had
occurred during the reaction of ligand 12 with [Cu(CH3CN)4]-
PF6 and subsequent oxygenation. We wondered, how and
when is the trityl group cleaved, during or prior to the
oxygenation? To address these questions, we sought to
identify the species produced initially, before oxygen expo-
sure. The reaction of BITOMe,STr with [Cu(CH3CN)4]PF6 in
(32) Ma, M.; Zhang, X.; Peng, L.; Wang, J. Tetrahedron Lett. 2007, 48,
1095-1097.
(33) Karambelkar, V. V.; Krishnamurthy, D.; Stern, C. L.; Zakharov, L.
N.; Rheingold, A. L.; Goldberg, D. P. Chem. Commun. 2002, 2772-
2773.
(34) Tang, C. C.; Davalian, D.; Huang, P.; Breslow, R. J. Am. Chem. Soc.
1978, 100, 3918-3922.
Inorganic Chemistry, Vol. 46, No. 19, 2007 7795

Zhou et al.
ward dioxygen, undergoing oxygenation at the copper center
without sulfur oxidation, as found with the CuM site. Here
again, the N3-tris(imidazolyl) analogues and the correspond-
ing CuM centers are slower to react with dioxygen. To date,
we have no visual evidence for long-lived dioxygen complex
intermediates in the oxygenation reactions of the N2S
complexes. The sterically hindered, arylated imidazolyl
ligands employed here do not prevent the formation of
bimetallic µ-peroxo/oxo complexes that are inhibited in the
enzymes, presumably because of conformational constraints
by the protein ligand that limit the association of the copper
centers. On the other hand, the protein probably provides a
more flexible coordination geometry of the copper centers
vis-a`-vis these synthetic tripodal ligands that could, in turn,
affect the copper center’s dioxygen coordination ability and
reactivity. Another structural difference between the present
synthetic ligands and the donor set provided by the protein
lies in the substitution pattern of the coordinating imidazole
units. The 2-substituted imidazoles of the synthetic ligands
are likely less basic than the 4(5)-substituted imidazole
groups of the histidines of the protein,33 again potentially
affecting the reactivity of the copper-coordinated dioxygen.
Experimental Section
Materials and Methods. All operations were carried out under
N2 or Ar by means of standard Schlenk and vacuum-line techniques.
Organic solvents were dried by standard procedures and distilled
under N2 before use. CH2Cl2 was dried over CaH2 and distilled
under N2 before use; tetrahydrofuran (THF) was dried over Na using
benzophenone as an indicator and distilled under N2 before use.
Glassware was oven-dried at 150 °C overnight. A Vacuum
Atmospheres glovebox was used in the handling of air-sensitive
solids. Infrared (IR) spectra were recorded either in a CH2Cl2
solution or in KBr pellets with a Perkin-Elmer 283-B infrared
spectrophotometer (resolution 4 cm-1). The 1H and 13C NMR
spectra were recorded on a Varian Mercury-300 spectrometer. Mass
spectra were acquired on a Finnigan TSQ 700 spectrometer in
methanol or acetonitrile solution by electrospray ionization (ESI).
Elemental analyses were performed by Midwest Microlab, LLC.
4,5-Diphenylimidazole35 and N-methyl-4,5-diphenylimidazole36
were prepared according to reported procedures. [Cu(CH3CN)4]PF6
and (CuOTf)2(C7H8) were obtained commercially. Fifty atom %
18O2 (50% 16O18O, 25% 16O2, and 25% 18O2) was obtained from
Icon Isotopes.
1,1-Bis(N-methyl-4,5-diphenyl-2-imidazolyl)-2-methyl-2-me-
thylthiopropanol (1a, BITOH,SMe). To a solution of N-methyl-4,5-
diphenylimidazole (2.34 g, 10.0 mmol) in 40 mL of dry THF under
N2 cooled to -78 °C was added n-BuLi dropwise (4.6 mL in
hexane, 10 mol). The mixture was stirred at -78 °C for 3 h to
give a red-brown solution. After dropwise addition of dry ethyl
2-methyl-2-methylthiopropinate (0.74 mL, 5.0 mmol), the mixture
was allowed to warm to room temperature overnight to give a pale-
yellow solution. The reaction was quenched with 80 mL of water
and extracted with diethyl ether, and the organic phase was dried
over MgSO4. Flash column chromatography (silica) using petroleum
ether-ethyl acetate (8:1) as the eluant gave 2.0 g (68%) of the
alcohol 1a as a white solid (recrystallization from CH2Cl2-
petroleum ether, mp 157-159 °C). 1H NMR (300 MHz, CD2Cl2):
δ 1.88(s, 3H), 1.97(s, 6H), 3.07(s, 6H), 5.67(s, 1H), 7.18-7.52(m,
20H). 13C NMR (75 MHz, CD2Cl2): δ 146.68, 135.49, 135.18,
131.62, 131.43, 129.61, 129.41, 128.57, 127.03, 126.82, 77.13,
55.22, 32.46, 26.20, 13.50. HRMS (ESI): m/z 585.2621 (M + 1);
calcd for C37H36N4OS, m/z 585.2688. IR (KBr; cm-1): 3410, 3100,
2927, 1603, 1442, 1027, 968, 780, 698. Anal. Calcd for C37H36N4-
OS: C, 75.99; H, 6.21; N, 9.58; S, 5.48. Found: C, 75.73; H, 6.29;
N, 9.56; S, 5.48.
Conclusions
Tripodal bis(imidazole) thioether ligands (N-methyl-4,5-
diphenyl-2-imidazolyl)2C(OR)C(CH3)2SR′ have been pre-
pared that provide the same (imidazole)2(thioether) ligand
set as that of the CuM binding site of the copper hydroxylase
enzymes. These complexes exhibit several close parallels
with the CuM site of the enzymes, including the formation
of electronically similar carbonyl adducts and copper-
centered oxidation and oxygenation (when R ) Me).
[(BITOR,SR′)Cu(CO)]PF6 have been prepared in which the IR
ν(CO)’s are comparable to those of the carbonylated
enzymes, indicating similar electronic character at the copper
centers. The oxygenation reactivity of these complexes
depends markedly on the carbinol and thioether substituents.
The reaction of 2a (R ) H) with dioxygen gives dinuclear
4, featuring bridging BIT-alkoxo ligands and terminal
-SOMe groups. In contrast, oxygenation of 2b (R ) Me)
affords dinuclear [(BITOMe,SMe)2Cu2(µ-OH)2](PF6)2 (5) in
which the copper centers are oxygenated without ac-
companying sulfur oxidation. Oxygenation of the copper(I)
complex derived from the sterically hindered BITOMe,SBu-t
ligand is slow, relative to 2b, producing a mixture of Cu2-
(µ-OH)2-type complexes 14b and 15b, the latter of which
has been ortho-oxygenated on a neighboring aryl group.
Oxygenation of the copper(I) complex derived from the
1-Methoxy-2-methyl-1,1-bis(N-methyl-4,5-diphenyl-2-imida-
zolyl)-2-methylthiopropane (1b, BITOMe,SMe). Sodium hydride
(0.024 g, 60% mineral oil suspension, 0.60 mmol, 1.2 equiv) was
suspended in dry THF (10 mL) under N2. Compound 1a (0.292 g,
0.50 mmol) was dissolved in dry THF (10 mL) to give a light-
yellow solution, which was added dropwise to the NaH suspension
over 2 min, and the resulting mixture was stirred for 2 h at room
temperature until no bubbles appeared. Methyl iodide (37.4 µL,
0.6 mmol, 1.2 equiv) was added dropwise, and the reaction mixture
was stirred for 20 h. Then the reaction was quenched with 40 mL
of deionized water and extracted twice with 25 mL portions of ether,
and the organic phase was dried over MgSO4. Rotary evaporation
of the ether gave 0.24 g of 1b as a light-yellow solid (80%)
(recrystallization from CH2Cl2-petroleum ether, mp 139-141 °C).
1H NMR (300 MHz, CD2Cl2, room temperature): δ 1.81(s, 3H),
2.11(br, 6H), 2.80(br, 3H), 3.29(br, 3H), 3.80(br, 3H), 7.13-7.46-
BITOMe,SCPh ligand produces a novel dinuclear bridged
3
disulfide complex 17, apparently the result of copper-induced
detritylation, followed by oxygenation of a highly reactive
thiolate complex.
Our ongoing efforts are focused on developing even more
accurate structural and functional mimics of the copper
hydroxylase enzymes and exploring the stoichiometric and
catalytic oxidation reactions promoted by these and related
poly(imidazole) complexes.
(35) Kang, P.; Foote, C. S. J. Am. Chem. Soc. 2002, 124, 9629-9638.
(36) LaRonde, F. J.; Brook, M. A. Inorg. Chim. Acta. 1999, 296, 208-
221.
7796 Inorganic Chemistry, Vol. 46, No. 19, 2007

Tripodal Bis(imidazole) Thioether Copper(I) Complexes
1
(m, 20H). H NMR (300 MHz, CD2Cl2, -80 °C): δ 1.58(s, 3H),
[(BITOMe,SMe)2Cu2(OH)2](PF6)2 (5a,b). To a yellow, dry CH2-
Cl2 solution (5 mL) of tripod ether 1b (0.060 g, 0.10 mmol) under
Ar was added dropwise a colorless CH2Cl2 solution (5 mL) of [Cu-
(CH3CN)4]PF6 (0.038 g, 0.10 mmol) or [CuOTf]2(toluene) (0.10
mmol) and the mixture was stirred at room temperature for 1 h to
give a light-yellow solution. Then a balloon filled with dioxygen
was attached to the flask and stirring was continued overnight,
during which the solution turned to yellowish brown. Amber crystals
of 5a (Z ) PF6) or 5b (Z ) OTf) were obtained after several hours
by vapor diffusion with CH2Cl2-ether (used for XRD). Yield: 60%.
LRMS (ESI): m/z 677.2 (48%, C38H3863CuN4O2S+), 677.7 (22%,
C76H7663Cu2N8O4S22+), 678.2 (100%), 678.7 (17%), 679.2 (23%,
C38H3865CuN4O2S+). IR (KBr; cm-1): 3100, 2927, 1444, 1388,
1084, 839, 701, 558. UV-vis (CH3CN): 210 nm (ꢀ ) 7.5 × 104
M-1 cm-1), 250 nm (ꢀ ) 4.2 × 104 M-1 cm-1), 330 nm (ꢀ ) 4.6
× 103 M-1 cm-1). See the X-ray experimental section for
crystallographic data on 5b. An 18O labeling experiment to prepare
5a was carried out using the same protocol with 50 atom % 18O2
(50% 16O18O, 25% 16O2, and 25% 18O2). Green crystals were
obtained whose LRMS (ESI) showed peaks at m/z 677.2 (19%),
677.7 (9.8%), 678.2 (100%), 678.7 (29%), 679.2 (47%), 679.7
(16%), 680.2 (14%), and 680.7 (2.4%).
2.05(d, 6H), 2.80(s, 3H), 3.20(s, 3H), 3.80(s, 3H), 7.13-7.46(m,
20H). 13C NMR (75 MHz, CD2Cl2, room temperature): δ 135.05,
131.29, 130.50, 128.94, 128.65, 127.96, 126.54, 126.00, 86.99,
56.94, 54.93, 26.62, 12.40. HRMS (ESI): m/z 599.2680 (M + 1);
calcd for C38H38N4OS, m/z 599.2844. IR (KBr; cm-1): 3100, 2927,
1603, 1442, 1368, 1081, 776, 697.
[(BITOH,SMe)Cu(NCCH3)]PF6 (2a). To a yellow CH2Cl2 solution
(8 mL) of N2S-tripod 1a (0.467 g, 0.80 mmol) was added dropwise
a CH2Cl2 solution (10 mL) of [Cu(CH3CN)4]PF6 (0.29 g, 0.80
mmol) under N2, and the resultant solution was stirred at room
temperature for 2 h to give a yellow solution. The solvent was
pumped off, and a pale-yellow solid was left. The solid was
dissolved in 10 mL of CH2Cl2, and 30 mL of diethyl ether was
added to precipitate 0.46 g of pale-yellow solid 2a (84%). 1H NMR
(300 MHz, CD3CN): δ 1.54(s, 6H), 1.88(s, 3H), 3.73(s, 6H), 5.20-
(s, 1H), 7.30-7.56(m, 20H). 13C NMR (75 MHz, CD2Cl2): δ
145.40, 137.42, 133.89, 133.11, 132.53, 130.48, 130.29, 129.95,
129.72, 128.95, 128.57, 82.32, 55.25, 36.13, 25.01, 16.41. LRMS
(ESI): m/z 688.2, 690.2 (100%, 52%) for C39H3963CuN5OS+ and
C39H3965CuN5OS+, respectively. HRMS (ESI): m/z 688.2281 (M+);
calcd for C39H3963CuN5OS+, m/z 688.2271. IR (KBr; cm-1): 3100,
2927, 1657, 1444, 1389, 1105, 855, 700, 558.
[(BITOH,SMe)CuCO]PF6 (3a). To a CH2Cl2 solution (5 mL) of
1a (0.234 g, 0.40 mmol) under N2 was added dropwise a CH2Cl2
solution (5 mL) of [Cu(CH3CN)4]PF6 (0.15 g, 0.40 mmol), and the
mixture was stirred at room temperature for 1 h to give a light-
yellow solution. CO was bubbled into the solution at room
temperature for 3 h, during which the solution turned to light green.
Colorless needle-shaped X-ray-quality crystals of 3a were obtained
overnight in a Schlenk tube under N2 by liquid layering with CH2-
[(BITOMe,SMe)Cu(II)(DMF)2](PF6)2 (6). Crystals of 5a (above)
were dissolved in DMF. Vapor diffusion with DMF-ether over 1
week produced green crystals of 5a and a small quantity of blue
crystals of 6. LRMS (ESI): m/z 367.1 (2.6%, [BITOMe,SMe]63Cu-
[DMF]2+), 368.1 (1.3%, [BITOMe,SMe]65Cu[DMF]2+). See the X-ray
experimental section for crystallographic data on 6.
Ethyl 2-Methyl-2-tert-butylthiopropanoate (7). Sodium hydride
(2.0 g, 60% mineral oil suspension, 50 mmol) was suspended in
dry THF (40 mL) under N2. tert-Butylthiol (5.6 mL, 50 mmol)
was added dropwise to the NaH suspension over 5 min, and the
resulting mixture was stirred for 10 min at room temperature. Ethyl
2-bromoisobutyrate (6.6 mL, 45 mmol) was added dropwise, and
then the reaction mixture was stirred at reflux under N2 for 20 h.
After cooling, the reaction was quenched with 80 mL of deionized
water and extracted twice with 50 mL portions of diethyl ether,
and the organic phase was dried over MgSO4. Flash column
chromatography (silica gel) using petroleum ether as the eluant gave
4.1 g (45%) of 7 as a colorless oil. 1H NMR (300 MHz, CD2Cl2):
δ 1.27(t, 3H), 1.32(s, 9H), 1.52(s, 6H), 4.12(q, 2H). 13C NMR (75
MHz, CD2Cl2): δ 176.0, 61.6, 49.2, 46.7, 32.4, 28.6, 14.4. HRMS
(ESI): m/z 227.1075 (M + Na); calcd for C10H20NaO2S, m/z
227.1082. IR (KBr; cm-1): 2970, 2931, 2869, 1724, 1469, 1385,
1365, 1264, 1154, 1123, 1029.
1
Cl2-petroleum ether. Yield: 10%. H NMR (300 MHz, CD2Cl2):
δ 1.37(s, 6H), 1.77(s, 3H), 3.82(s, 6H), 4.60(s, 1H), 7.20-7.37(m,
20H). 13C NMR (75 MHz, CD2Cl2): δ 178.03, 143.33, 137.14,
132.72, 132.62, 131.33, 129.30, 129.22, 128.88, 128.36, 128.14,
81.78, 55.04, 35.47, 23.82, 16.38. LRMS (ESI) m/z 647.1, 649.2
(1.5%, 0.6%) for C37H3663CuN4OS+ and C37H3665CuN4OS+, re-
spectively. IR (KBr; cm-1): 3510, 2927, 2102, 1656, 1478, 1444,
1399, 1107, 855, 700, 558. See the X-ray experimental section for
crystallographic data on 3a.
[(BITOMe,SMe)CuCO]PF6 (3b). To a CH2Cl2 solution (5 mL) of
methoxy-tripod 1b (0.060 g, 0.10 mmol) under N2 was added
dropwise a CH2Cl2 solution (5 mL) of [Cu(CH3CN)4]PF6 (0.038
g, 0.10 mmol), and the resultant mixture was stirred at room
temperature for 1 h to give a light-yellow solution. CO was bubbled
into the solution at room temperature for 3 h. White solid 3b was
obtained by vapor diffusion with CH2Cl2-diethyl ether under N2.
Yield: 44%. 1H NMR (400 MHz, CD2Cl2): δ 1.52(s, 6H), 1.69(s,
3H), 3.70(s, 3H), 3.91(s, 6H), 7.35-7.53(m, 20H). 13C NMR (100
MHz, CD2Cl2): δ 141.45, 138.09, 132.98, 132.03, 131.05, 129.85,
129.22, 128.82, 128.49, 128.38, 127.93, 89.21, 57.01, 56.82, 35.69,
24.61, 15.12. LRMS (ESI): m/z 689.2, 691.2 (100%, 38%) for
C39H3863CuN4O2S+ and C39H3865CuN4O2S+, respectively. IR (KBr;
cm-1): 3100, 2927, 2094, 1604, 1445, 1389, 1091, 839, 700, 558.
Ethyl 2-Methyl-2-triphenylmethanethiopropanoate (8). So-
dium hydride (0.60 g, 60% mineral oil suspension, 15 mmol) was
suspended in dry THF (30 mL) under N2. Triphenylmethanethiol
(3.0 g, 11 mmol) was dissolved in dry THF (30 mL) and was added
dropwise to the NaH suspension over 5 min, and the resulting
mixture was stirred for 10 min at room temperature until no bubbles
(H2) appeared. Ethyl 2-bromoisobutyrate (1.6 mL, 11 mmol) was
added dropwise, and then the reaction mixture was stirred at reflux
under N2 for 8 h. Then the reaction was quenched with 80 mL of
deionized water and extracted twice with 50 mL portions of
dichloromethane, and the organic phase was dried over MgSO4.
Rotary evaporation of the dichloromethane gave 1.8 g of 8 as a
white solid (42%) (recrystallization from CH2Cl2-petroleum ether).
1H NMR (300 MHz, CD2Cl2): δ 1.08(t, 3H), 1.21(s, 6H), 3.74(q,
2H), 7.19-7.54(m, 15H). 13C NMR (75 MHz, CD2Cl2): δ 174.1,
145.1, 130.5, 128.1, 127.1, 68.6, 61.5, 51.9, 27.6, 14.1. HRMS
(ESI): m/z 413.1504 (M + Na); calcd for C25H26NaO2S, m/z
413.1551. IR (KBr; cm-1): 3057, 2979, 1730, 1489, 1446, 1254,
Oxidation of [(BITOH,SMe)CuI(NCCH3)]PF6 (4). Crystals of 4
were obtained by slow vapor diffusion of ether into a concentrated
solution of [(BITOH,SMe)Cu(NCCH3)]PF6 (2a) in DMF. Green
needles of 4 were obtained over 1 week. Yield: 20%. LRMS
(ESI): m/z 662.1 (70%, C37H3463CuN4O2S+), 662.6 (51%, C74H68
-
-
63
Cu2N8O4S22+), 663.1 (100%), 663.6 (42%), 664.1 (26%, C37H35
65
CuN4O2S+). See the X-ray experimental section for crystallographic
data on 4.
Inorganic Chemistry, Vol. 46, No. 19, 2007 7797

Zhou et al.
1138, 1118, 1022, 747, 704. Anal. Calcd for C25H26O2S: C, 76.89;
H, 6.71. Found: C, 76.92; H, 6.68.
(ESI): m/z 744.2692 (M+); calcd for C43H4763CuN5OS+, m/z
744.2797. IR (KBr; cm-1): 2970, 2937, 1655, 1544, 1370, 1080,
842, 700, 558.
1-Methoxy-2-methyl-1,1-bis(N-methyl-4,5-diphenyl-2-imida-
zolyl)-2-tert-butylthiopropane (10, BITOMe,SBu-t). To a solution
of N-methyl-4,5-diphenylimidazole (1.4 g, 6.0 mmol) in 30 mL of
dry THF under N2 cooled to -78 °C was added t-BuLi dropwise
(4.1 mL in hexane, 6.0 mmol). The mixture was stirred at -78 °C
for 3 h to give a red-brown solution. After the addition of a dry
THF solution (10 mL) of 7 (0.41 g, 2.0 mmol), the mixture was
allowed to warm to room temperature overnight to give a yellow
solution. Sodium hydride (0.20 g, 60% mineral oil suspension, 5.0
mmol) was added to the solution under N2, and the resulting mixture
was stirred for 1 h at room temperature. Methyl iodide (312 µL,
5.0 mmol) was added dropwise, and then the reaction mixture was
stirred at reflux under N2 for 22 h. The mixture was quenched with
80 mL of water and extracted with CH2Cl2, and the organic phase
was dried over MgSO4. Flash column chromatography (silica gel)
using petroleum ether-ethyl acetate (8:1) as the eluant gave 0.30
Reaction of 13a with Dioxygen. Compound 13a (37 mg, 0.050
mmol) was dissolved in 5 mL of dry CH2Cl2, and the solution was
stirred at room temperature under O2 (1 atm) for 2 days to give a
yellowish solution. A white solid precipitated overnight during
diffusion of the ether vapor into the reaction solution and was found
1
to be 13a from its H NMR spectrum in CD2Cl2.
[(BITOMe,SBu-t)2Cu2(OH)2](OTf)2 (14b and 15b). To 10
(52 mg, 0.080 mmol) and (CuOTf)2(C7H8) (21 mg, 0.080 mmol)
was added dry CH2Cl2 (5 mL), and the resulting light-
yellow solution was stirred at room temperature for 1 h under
Ar. Then the solution was stirred under an O2 atmosphere (balloon)
at -50 °C for 72 h, producing an amber solution. Amber crystals
of 14b and 15b were obtained by slow diffusion of ether into
the solution at -20 °C (36%). LRMS (ESI): m/z 718.2 (47%)
for C82H8663Cu2N8O4S22+, 719.2, 720.2, and 721.2 (12, 22, and
1
4%) for C82H8863Cu2N8O4S22+, C82H8863Cu65CuN8O4S22+, and
g (24%) of 10 as a white solid. H NMR (300 MHz, CD2Cl2): δ
2+
C82H8865Cu2N8O4S2
,
respectively. IR (KBr; cm-1): 1080,
1.32(s, 9H), 2.30(s, br, 6H), 2.80(s, br, 3H), 3.30(s, 3H), 3.80(s,
br, 3H), 7.08-7.54(m, 20H). 13C NMR (75 MHz, CD2Cl2): δ 135.7,
132.0, 131.8, 131.6, 130.8, 129.5, 129.1, 128.6, 127.1, 126.5, 88.1,
63.8, 55.5, 47.8, 33.8, 22.9, 14.4. HRMS (ESI): m/z 641.3352 (M
+ 1); calcd for C41H45N4OS, m/z 641.3314. IR (KBr; cm-1): 3060,
2959, 1604, 1507, 1444, 1365, 1128, 1070, 1056, 1026, 964, 783,
774, 723, 697. Anal. Calcd for C41H44N4OS: C, 76.84; H, 6.92;
N, 8.74. Found: C, 76.06; H, 6.93; N, 8.77.
840, 701, 559. Anal. Calcd for C84H88Cu2F6N8O14S4: C, 58.02;
H, 5.10; N, 6.44. Found: C, 57.50; H, 5.48; N, 6.22. See the
X-ray experimental section for crystallographic data on 14b and
15b.
Reaction of 14b/15b with NH4OH (16). To a CH2Cl2 (10 mL)
solution of complexes 14b/15b (20 mg) was added concentrated
ammonium hydroxide (20 mL). The resulting bright-blue solution
was stirred at room temperature for 20 min. Then the mixture was
extracted with dichloromethane, and the organic phase was dried
over MgSO4. Preparative TLC (silica gel) using petroleum ether-
ethyl acetate (8:1) as the eluant gave two compounds. One was
identified as 10 on the basis of its TLC Rf. The other was a white
solid, determined to be 16. 1H NMR (300 MHz, CD2Cl2): δ 1.39-
(s, 9H), 2.30(s, br, 6H), 2.80(s, br, 3H), 3.41(s, 3H), 3.70(s, br,
3H), 6.43(apparent t, J ) 8.1 Hz, 1H), 6.81(apparent t, J ) 8.1
Hz, 2H), 6.98(apparent t, J ) 8.1 Hz, 1H), 7.11-7.52(m, 15H).
HRMS (ESI): m/z 657.3226 (M + 1); calcd for C41H45N4O2S, m/z
657.3263.
[(BITOMe,S)2Cu2(OH)2](PF6)2 (17). To 12 (40 mg, 0.048 mmol)
and [Cu(CH3CN)4]PF6 (19 mg, 0.048 mmol) was added dry
CH2Cl2 (5 mL), and the resulting dark-yellow solution was stirred
at room temperature for 1 h under Ar. Then the solution was
stirred under an O2 atmosphere (balloon) at -50 °C for 20 h to
give a purple solution. Purple crystals of 17 were obtained by slow
1-Methoxy-2-methyl-1,1-bis(N-methyl-4,5-diphenyl-2-imida-
zolyl)-2-tritylthiopropane (12, BITOMe,STr). To a solution of
N-methyl-4,5-diphenylimidazole (1.2 g, 5.0 mmol) in 30 mL of
dry THF under N2 cooled to -78 °C was added t-BuLi dropwise
(3.4 mL in hexane, 5.0 mmol). The mixture was stirred at -78 °C
for 3 h to give a red-brown solution. After the addition of a dry
THF solution (15 mL) of 8 (0.78 g, 2.0 mmol), the mixture was
allowed to warm to room temperature overnight to give a yellow
solution. Sodium hydride (0.20 g, 60% mineral oil suspension, 5.0
mmol) was added to the solution under N2, and the resulting mixture
was stirred for 1 h at room temperature. Methyl iodide (312 µL,
5.0 mmol) was added dropwise, and then the reaction mixture was
stirred at reflux under N2 for 20 h. The mixture was quenched with
80 mL of water and extracted with CH2Cl2, and the organic phase
was dried over MgSO4. Flash column chromatography (silica gel)
using petroleum ether-ethyl acetate (8:1) as the eluant gave 0.30
1
g (18%) of 12 as a light-yellow solid. H NMR (300 MHz, CD2-
Cl2): δ 1.75 (br s, 3H), 1.80 (br, s, 3H), 2.81(s, br, 3H), 3.34(s,
3H), 3.95(s, br, 3H), 7.15-7.47(m, 35H). 13C NMR (75 MHz, CD2-
Cl2): δ 145.9, 135.6, 132.1, 131.8, 131.0, 129.6, 129.3, 128.4,
128.0, 127.8, 127.4, 126.9, 126.6, 88.9, 68.9, 67.0, 55.7, 35.4,
31.4, 29.7, 26.0. HRMS (ESI): m/z 827.3783 (M + 1); calcd
for C56H51N4OS, m/z 827.3793. IR (KBr; cm-1):
3061, 2947, 1602, 1491, 1444, 1077, 773, 743, 699.
diffusion of ether into the solution at -20 °C (40%). LRMS (ESI):
63
m/z 662.2, 663.2, and 664.2 (66.8, 100, and 26.4%) for C74H70
-
,
2+
Cu2N8O4S22+, C74H7063Cu65CuN8O4S22+, and C74H7065Cu2N8O4S2
respectively. IR (KBr; cm-1): 1082, 839, 701, 558. See the X-ray
experimental section for crystallographic data on 17.
[(BITOMe,S)Cu2(CNBut)2]PF6 (18). To 12 (83 mg, 0.10 mmol)
and [Cu(CH3CN)4]PF6 (38 mg, 0.10 mmol) was added dry
CH2Cl2 (5 mL), and the resulting dark-yellow solution was stirred
at room temperature for 5 min under Ar. tert-Butyl isocyanide
(12 µL, 0.10 mmol) was added dropwise, and the resulting yellow-
ish solution was stirred at room temperature for 1 h. After solvent
evaporation, preparative TLC (silica gel) using ethyl acetate as
[(BITOMe,SBu-t)Cu(NCCH3)]PF6 (13a). To 10 (64 mg, 0.10
mmol) and [Cu(CH3CN)4]PF6 (38 mg, 0.10 mmol) was added dry
CH2Cl2 (5 mL), and the resulting light-yellow solution was stirred
at room temperature for 2 h under Ar. Compound 13a was obtained
as a white solid by slow diffusion of ether into the yellow solution
1
1
at room temperature under N2 (80%). H NMR (300 MHz, CD2-
the eluant gave 40 mg (50%) of 18 as a light-yellow solid. H
Cl2): δ 1.29(s, 9H), 1.67(s, br, 6H), 1.76(s, 3H), 3.55(s, 3H), 3.82-
(s, br, 6H), 7.27-7.52(m, 20H). 13C NMR (75 MHz, CD2Cl2): δ
142.4, 137.2, 133.2, 133.0, 131.6, 131.0, 130.0, 129.3, 128.4, 128.1,
90.2, 61.4, 56.7, 52.6, 36.2, 34.0, 32.8, 27.6. LRMS (ESI): m/z
703.3 (2.5%) for C41H4463CuN4OS+, 744.3, 746.3 (100.0%, 30.6%)
for C43H4763CuN5OS+ and C43H4765CuN5OS+, respectively. HRMS
NMR (300 MHz, CD2Cl2): δ 1.22(s, 18H), 1.26(s, 6H), 3.10(s,
br, 3H), 3.26(s, 3H), 3.80(s, br, 3H), 7.26-7.48(m, 20H). 13C NMR
(75 MHz, CD2Cl2): δ 147.29, 137.77, 137.08, 136.08, 135.72,
132.99, 132.39, 131.55, 130.06, 129.91, 129.61, 128.80, 128.59,
128.45, 88.89, 57.07, 56.22, 51.68, 30.28, 21.34. LRMS (ESI):
63
m/z 875.3, 877.3, and 879.3 (100, 93.8, and 7.30%) for C47H53
-
7798 Inorganic Chemistry, Vol. 46, No. 19, 2007

Tripodal Bis(imidazole) Thioether Copper(I) Complexes
Cu2N6OS+, C47H5363Cu65CuN6-OS+, and C47H5365Cu2N6OS+,
respectively. IR (KBr; cm-1): 3061, 2960, 2179, 1445, 1376, 1076,
842, 701, 558.
Chemical Society and for helpful discussions with Drs. G.
Richter-Addo and R. Houser.
Supporting Information Available: X-ray crystallographic data
for 3a, 4, 5, 6, 14b/15b, and 17. This material is available free of
charge via the Internet at http://pubs.acs.org.
Acknowledgment. We are grateful for partial support
provided by the Petroleum Research Fund of the American
IC700447K
Inorganic Chemistry, Vol. 46, No. 19, 2007 7799