Synthesis and Characterization of Cu2(I,I), Cu 2(I,II), and Cu2(II,II) Compounds Supported by Two Phthalazine-Based Ligands: Influence of a Hydrophobic Pocket

Article abstract of DOI:10.1021/ic035009r

The copper coordination chemistry of two phthalazine-based ligands of differing steric bulk was investigated. A family of dinuclear complexes were prepared from reactions of [Cu₂(bdptz)(MeCN)₂](OTf) ₂, 1(OTf)₂, where bdptz = 1,4-bis(2,2′ -dipyridylmethyl)phthalazine. Treatment of 1(OTf)₂ with NaO ₂CCH₃ afforded the class I mixed-valent compound [Cu ₂(bdptz)₂](OTf)₃, 2(OTf)₃, by disproportionation of Cu(I). Compound 2(OTf)₃ displays an electron paramagnetic resonance spectrum, with g_∥ = 2.25 (A _∥ = 169 G) and g_⊥ = 2.06, and exhibits a reversible redox wave at -452 mV versus Cp₂Fe⁺/Cp ₂Fe. The complex [Cu₂(bdptz)(μ-OH)(MeCN) ₂](OTf)₃, 3(OTf)₃, was prepared by chemical oxidation of 1 with AgOTf, and exposure of 1 to dioxygen afforded [Cu ₂(bdptz)(μ-OH)₂]₂(OTs)₄, 4(OTs)₄, which can also be obtained directly from [Cu(H ₂O)₆](OTs)₂. In compound [Cu ₂(bdptz)(μ-vpy)](OTf)₂, 5(OTf)₂, where vpy = 2-vinylpyridine, the vpy ligand bridges the two Cu(I) centers by using both its pyridine nitrogen and the olefin as donor functionalities. The sterically hindered compounds [Cu₂(Ph₄bdptz)(MeCN) ₂](OTf)₂, 6(OTf)₂, and [Cu₂(Ph ₄-bdptz)(μ-O₂CCH₃)](OTf), 7(OTf), were also synthesized, where Ph₄bdptz = 1,4-bis[bis(6-phenyl-2-pyridyl)methyl]-phthalazine. Complexes 1-7 were characterized structurally by X-ray crystallography. In 6 and 7, the four phenyl rings form a hydrophobic pocket that houses the acetonitrile and acetate ligands. Complex 6 displays two reversible redox waves with E_1/2 values of +41 and +516 mV versus Cp₂Fe⁺/Cp₂Fe. Analysis of oxygenated solutions of 6 by electrospray ionization mass spectrometry reveals probable aromatic hydroxylation of the Ph₄bdptz ligand. The different chemical and electrochemical behavior of 1 versus 6 highlights the influence of a hydrophobic binding pocket on the stability and reactivity of the dicopper(I) centers.

Full text of DOI:10.1021/ic035009r

Inorg. Chem. 2004, 43, 1751−1761
Synthesis and Characterization of Cu2(I,I), Cu2(I,II), and Cu2(II,II)
Compounds Supported by Two Phthalazine-Based Ligands: Influence of
a Hydrophobic Pocket
,†
Jane Kuzelka,† Sumitra Mukhopadhyay,† Bernhard Spingler,‡ and Stephen J. Lippard*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts
02139, and Institute of Inorganic Chemistry, UniVersity of Zu¨rich, CH-8057 Zu¨rich, Switzerland
Received August 27, 2003
The copper coordination chemistry of two phthalazine-based ligands of differing steric bulk was investigated. A
family of dinuclear complexes were prepared from reactions of [Cu2(bdptz)(MeCN)2](OTf)2, 1(OTf)2, where bdptz )
1,4-bis(2,2′-dipyridylmethyl)phthalazine. Treatment of 1(OTf)2 with NaO CCH afforded the class I mixed-valent
2
3
compound [Cu2(bdptz)2](OTf)3, 2(OTf)3, by disproportionation of Cu(I). Compound 2(OTf)3 displays an electron
paramagnetic resonance spectrum, with g ) 2.25 (A ) 169 G) and g ) 2.06, and exhibits a reversible redox
|
|
wave at −452 mV versus Cp2Fe+/Cp2Fe. The complex [Cu2(bdptz)(µ-OH)(MeCN)2](OTf)3, 3(OTf)3, was prepared
by chemical oxidation of 1 with AgOTf, and exposure of 1 to dioxygen afforded [Cu2(bdptz)(µ-OH)2]2(OTs)4, 4(OTs)4,
which can also be obtained directly from [Cu(H O)6](OTs)2. In compound [Cu2(bdptz)(µ-vpy)](OTf)2, 5(OTf)2, where
2
vpy ) 2-vinylpyridine, the vpy ligand bridges the two Cu(I) centers by using both its pyridine nitrogen and the
olefin as donor functionalities. The sterically hindered compounds [Cu (Ph bdptz)(MeCN)2](OTf)2, 6(OTf)2, and [Cu (Ph -
2
4
2
4
bdptz)(µ-O CCH )](OTf), 7(OTf), were also synthesized, where Ph4bdptz ) 1,4-bis[bis(6-phenyl-2-pyridyl)methyl]-
2
3
phthalazine. Complexes 1−7 were characterized structurally by X-ray crystallography. In 6 and 7, the four phenyl
rings form a hydrophobic pocket that houses the acetonitrile and acetate ligands. Complex 6 displays two reversible
redox waves with E1/2 values of +41 and +516 mV versus Cp2Fe+/Cp2Fe. Analysis of oxygenated solutions of 6
by electrospray ionization mass spectrometry reveals probable aromatic hydroxylation of the Ph4bdptz ligand. The
different chemical and electrochemical behavior of 1 versus 6 highlights the influence of a hydrophobic binding
pocket on the stability and reactivity of the dicopper(I) centers.
Introduction
zymes, notably hemocyanin,4,6 tyrosinase,4 and catechol
oxidase,3 feature active sites with coupled Cu(I) ions in a
histidine-rich coordination environment. The copper centers
interact with dioxygen to afford dicopper(II) peroxo species.
This reaction is reversible for hemocyanin, whereas further
activation of the peroxo unit occurs in both tyrosinase and
catechol oxidase, ultimately resulting in aromatic ring
oxidation.
The reductive activation of dioxygen by low-valent copper
ions is accomplished in a variety of biological contexts.
Numerous proteins employ copper to oxidize organic
substrates,1-5 bind dioxygen reversibly,4,6 or mediate electron
transfer.4,7,8 The reduced forms of several copper metalloen-
* Author to whom correspondence should be addressed. E-mail:
lippard@lippard.mit.edu.
Inspired by the natural systems, many laboratories have
devoted significant effort to prepare small-molecule model
compounds that mimic both the active site geometry and
† Massachusetts Institute of Technology.
‡ University of Zu¨rich.
(1) Rogers, M. S.; Dooley, D. M. Curr. Opin. Chem. Biol. 2003, 7, 189-
196.
(2) Lieberman, R. L.; Shrestha, D. B.; Doan, P. E.; Hoffman, B. M.;
Stemmler, T. L.; Rosenzweig, A. C. Proc. Natl. Acad. Sci. U.S.A.
2003, 100, 3820-3825.
(6) Magnus, K. A.; Ton-That, H.; Carpenter, J. E. Chem. ReV. 1994, 94,
727-735.
(3) Eicken, C.; Krebs, B.; Sacchettini, J. C. Curr. Opin. Struct. Biol. 1999,
9, 677-683.
(7) Messerschmidt, A. In Multi-Copper Oxidases; Messerschmidt, A., Ed.;
World Scientific Publishing Co. Pte. Ltd.: Singapore, 1997; pp 23-
79.
(4) Solomon, E. I.; Sundaram, U. M.; Mackonkin, T. E. Chem. ReV. 1996,
96, 2563-2605.
(5) Klinman, J. P. Chem. ReV. 1996, 96, 2541-2561.
(8) Solomon, E. I.; Baldwin, M. J.; Lowery, M. D. Chem. ReV. 1992, 92,
521-542.
10.1021/ic035009r CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/04/2004
Inorganic Chemistry, Vol. 43, No. 5, 2004 1751

Kuzelka et al.
Chart 1
of such perturbations in multidentate dinucleating ligands
on the dioxygen reactivity of dicopper(I) complexes have
not been as extensively investigated. We report here how
such changes influence the chemistry of the phthalazine-
supported dicopper compounds. Previously, we employed
the nitrogen-rich hexadentate ligands bdptz and Ph4bdptz
(Chart 1), where bdptz is 1,4-bis(2,2′-dipyridylmethyl)-
phthalazine and Ph4bdptz is 1,4-bis[bis(6-phenyl-2-pyridyl)-
methyl]phthalazine, to prepare dinuclear compounds of
several divalent first-row transition-metal ions.20-24 These
two dinucleating ligands feature a phthalazine bridge linking
two clusters of pyridine groups and support dimetallic
structures with a variety of ancillary ligands. In this paper,
we describe the copper(I) and copper(II) chemistry of these
two phthalazine-based ligands. The readily assembled com-
plex [Cu2(bdptz)(MeCN)2](OTf)2, 1(OTf)2, serves as a
convenient starting material for preparing the compounds
[Cu2(bdptz)2](OTf)3, 2(OTf)3, [Cu2(bdptz)(µ-OH)(MeCN)2]-
(OTf)3, 3(OTf)3, [Cu2(bdptz)(µ-OH)2]2(OTs)4, 4(OTs)4, and
[Cu2(bdptz)(µ-vpy)](OTf)2, 5(OTf)2.25 The use of Ph4bdptz
permitted the isolation of [Cu2(Ph4bdptz)(MeCN)2](OTf)2,
6(OTf)2, and [Cu2(Ph4bdptz)(µ-O2CCH3)](OTf), 7(OTf). All
of the compounds were studied by X-ray crystallography,
and the electrochemical behavior of 1, 2, and 6 was
investigated. Pronounced differences in the reactivity and
stability between the two series of complexes derived from
bdptz and Ph4bdptz were encountered. Our results show that
the attachment of a hydrophobic binding pocket to a
multidentate ligand platform, a feature highly desirable for
facilitating the activation of small-molecule substrates,
dramatically influences the properties of the resulting
Figure 1. Examples of {Cu2O2} adducts supported by multidentate
dinucleating ligands.
dioxygen reactivity of these copper proteins.9-14 Typically,
mononuclear copper(I) complexes supported by various
N-donor ligands or dicopper(I) compounds bearing ligands
in which two clusters of nitrogen-rich moieties are linked
by a hydrocarbon spacer react with dioxygen to afford Cu2-
O2 adducts. There has been less work utilizing dinucleating
ligands in which the linker moiety directly coordinates to
and bridges two copper centers for the reductive activation
of dioxygen. Several noteworthy examples are depicted in
Figure 1, including a terminally bound peroxo species
supported by a phenoxide bridge15,16 and a relatively rigid
1,8-naphthyridine spacer that facilitates the formation of a
(µ-1,2-peroxo) intermediate with a short copper‚‚‚copper
separation of ∼2.84 Å.17 As a final example, a multidentate
functionalized pyrazolate ligand allowed for the crystal-
lographic characterization of an unusual tetranuclear (µ4-
peroxo)Cu4(II) complex.18 Clearly, dinucleating ligands
enable the preparation of unique {Cu2O2} species distinct
from those generated by the reaction of dioxygen with
mononuclear copper(I) complexes.
Although dramatic differences in the reactivity of mono-
nuclear copper(I) compounds with dioxygen can result from
small changes in the substituents of the ligand,19 the effects
(9) Zhang, C. X.; Liang, H.-C.; Humphreys, K. J.; Karlin, K. D. In
Catalysis by Metal Complexes; James, B. R., van Leeuwen, P. W. N.
M., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands,
2003; Vol. 26, pp 79-121.
(20) Kuzelka, J.; Spingler, B.; Lippard, S. J. Inorg. Chim. Acta 2002, 337,
212-222.
(10) Stack, T. D. P. Dalton Trans. 2003, 1881-1889.
(11) Itoh, S.; Fukuzumi, S. Bull. Chem. Soc. Jpn. 2002, 75, 2081-2095.
(12) Schindler, S. Eur. J. Inorg. Chem. 2000, 2311-2326.
(13) Tolman, W. B. Acc. Chem. Res. 1997, 30, 227-237.
(14) Kitajima, N.; Moro-oka, Y. Chem. ReV. 1994, 94, 737-757.
(15) Pate, J. E.; Cruse, R. W.; Karlin, K. D.; Solomon, E. I. J. Am. Chem.
Soc. 1987, 109, 2624-2630.
(21) Barrios, A. M.; Lippard, S. J. Inorg. Chem. 2001, 40, 1060-1064.
(22) Barrios, A. M.; Lippard, S. J. Inorg. Chem. 2001, 40, 1250-1255.
(23) Barrios, A. M.; Lippard, S. J. J. Am. Chem. Soc. 2000, 122, 9172-
9177.
(24) Barrios, A. M.; Lippard, S. J. J. Am. Chem. Soc. 1999, 121, 11751-
11757.
(25) Abbreviations used: OTf- ) triflate; OTs- ) tosylate; vpy )
2-vinylpyridine; bdpdz ) 3,6-bis(di-2-pyridylmethyl)pyridazine; TMPA
) tris(2-pyridylmethyl)amine; H-MePY2 ) bis[2-(2-pyridyl)ethyl]-
methylamine; bTMPA ) 6,6′-bis[[bis(2-pyridylmethyl)amino]methyl]-
2,2′-bipyridine; L0 ) tris(2-pyridyl)methoxymethane; L2 ) 5,5,16,16-
tetramethyl-23,24-dioxa-3,7,14,18-tetraazatricyclo[18.2.1.19,12]tetracosa-
1(22),2,7,9,11,13,18,20-octaene; D1 ) [5-(2-{6-[(bis(2-pyridylmeth-
ylamino))methyl]-3-pyridyl}ethyl)-(2-pyridylmethyl)]bis(2-pyridyl-
methyl)amine; BBAN ) 2,7-bis(N,N-dibenzylaminomethyl)-1,8-naph-
thyridine; [BAr′4]- ) [(3,5-(CF3)2C6H3)4B]-.
(16) Karlin, K. D.; Cruse, R. W.; Gultneh, Y.; Hayes, J. C.; Zubieta, J. J.
Am. Chem. Soc. 1984, 106, 3372-3374.
(17) He, C.; DuBois, J. L.; Hedman, B.; Hodgson, K. O.; Lippard, S. J.
Angew. Chem., Int. Ed. 2001, 40, 1484-1487.
(18) Meyer, F.; Pritzkow, H. Angew. Chem., Int. Ed. 2000, 39, 2112-
2115.
(19) Liang, H.-C.; Zhang, C. X.; Henson, M. J.; Sommer, R. D.; Hatwell,
K. R.; Kaderli, S.; Zuberbu¨hler, A. D.; Rheingold, A. L.; Solomon,
E. I.; Karlin, K. D. J. Am. Chem. Soc. 2002, 124, 4170-4171.
1752 Inorganic Chemistry, Vol. 43, No. 5, 2004

Phthalazine-Based Dicopper Compounds
[Cu2(bdptz)(µ-OH)2]2(OTs)4, 4(OTs)4. Method A. A solution
of [Cu(H2O)6](OTs)2 (110 mg, 0.215 mmol) in MeCN/H2O (10:1,
4 mL) was treated with solid bdptz (50 mg, 0.11 mmol) to generate
a dark-blue solution to which an aqueous solution of NaOH (1 M,
215 µL, 0.215 mmol) was added. The resulting blue-green mixture
was stirred for 30 min and filtered through Celite, and Et2O vapor
was allowed to diffuse into the solution. Green crystals of 4(OTs)4‚
MeCN‚12H2O, suitable for X-ray diffraction study, were isolated
(30 mg, 29%). FTIR (cm-1, KBr): 3447 (br, m), 3076 (w), 3048
(w), 3008 (w), 2919 (w), 2868 (w), 1607 (m), 1592 (m), 1570 (w),
1473 (w), 1444 (m), 1364 (w), 1233 (m), 1212 (s), 1191 (s), 1165
(s), 1118 (s), 1031 (s), 1008 (s), 918 (w), 875 (w), 819 (m), 769
(m), 680 (s), 650 (w), 636 (w), 625 (w), 581 (w), 563 (s), 549 (m),
526 (w). Anal. Calcd for C88H78N12O17S4Cu4, 4(OTs)4‚H2O: C,
53.98; H, 4.02; N, 8.58. Found: C, 53.76; H, 3.81; N, 8.61.
Method B. An orange solution of 1(OTf)2 (65 mg, 0.067 mmol)
in MeCN (4 mL) was gently purged with O2 for 45 min to yield a
green solution. An aqueous solution of NaOTs (2.7 M, 0.5 mL,
1.3 mmol) was added dropwise to afford a green precipitate, which
was collected on filter paper and washed with H2O. Slow evapora-
tion of the filtrate gave green crystals of 4(OTs)4 (30 mg, 46%).
[Cu2(bdptz)(µ-vpy)](OTf)2, 5(OTf)2. Treatment of 1(OTf)2 (150
mg, 0.154 mmol) with vpy (18.3 µL, 0.170 mmol) in CH2Cl2 (6
mL) afforded a yellow mixture, which was stirred for 30 min.
Filtration through Celite was followed by evaporation of the solvent
under reduced pressure. The resulting residue was dissolved in
MeOH, and Et2O vapor was allowed to diffuse into the solution.
Orange crystals of 5(OTf)2‚Et2O, suitable for X-ray crystallography,
coordination compounds relative to the complexes lacking
such a binding cavity.
Experimental Section
General Procedures. Dichloromethane, acetonitrile, and diethyl
ether were saturated with argon and purified by passage over a
column of activated alumina under argon.26 Methanol was distilled
from Mg and I2 under nitrogen. The compounds [Cu(MeCN)4]X
(X ) OTf27 or BF428), bdptz,24 and Ph4bdptz20 were prepared
according to published literature procedures. Prior to use, vpy was
freshly distilled. All other reagents were purchased from commercial
sources and used as received. Air-sensitive manipulations, including
the synthesis of the triflate salts of 1-3 and 5-7, were performed
under nitrogen in an MBraun glovebox or by using standard Schlenk
techniques.
[Cu2(bdptz)(MeCN)2](OTf)2, 1(OTf)2. To a colorless solution
of [Cu(MeCN)4](OTf) (404 mg, 1.07 mmol) in MeCN (5 mL) was
added solid bdptz (253 mg, 0.543 mmol) to generate an orange
suspension. After further addition of MeCN (∼4 mL), the solution
became homogeneous and was stirred for 30 min. The orange
solution was saturated with Et2O and left at room temperature
overnight to give 1(OTf)2 as an orange microcrystalline solid (436
mg, 83%). FTIR (cm-1, KBr): 3126 (w), 3097 (w), 3073 (w), 3051
(w), 3000 (w), 2932 (w), 1596 (s), 1569 (m), 1536 (w), 1473 (m),
1441 (s), 1414 (w), 1364 (m), 1328 (w), 1300 (s), 1284 (s), 1261
(s), 1238 (s), 1165 (m), 1147 (m), 1029 (s), 861 (w), 822 (w), 771
(s), 759 (s), 683 (w), 637 (s), 573 (s), 517 (m). 1H NMR (300 MHz,
CD2Cl2) δ: 9.16 m (2H), 8.69 d (J ) 13 Hz, 4H), 8.27 m (2H),
8.11 d (J ) 26 Hz, 4H), 7.81 m (4H), 7.36 m (4H), 7.08 s (2H),
2.47 s (6H). Anal. Calcd for C36H28N8O6F6S2Cu2: C, 44.40; H,
2.90; N, 11.51. Found: C, 44.16; H, 2.86; N, 11.38.
were obtained by this procedure (128 mg, 86%). FTIR (cm-1
,
KBr): 3131 (w), 3073 (w), 2976 (w), 2933 (w), 2855 (w), 1598
(s), 1570 (m), 1549 (w), 1531 (w), 1473 (m), 1442 (m), 1419 (w),
1365 (w), 1351 (w), 1260 (s), 1223 (m), 1161 (m), 1115 (w), 1029
(s), 937 (w), 861 (w), 774 (m), 756 (m), 679 (w), 637 (s), 585 (m),
[Cu2(bdptz)2](OTf)3, 2(OTf)3. To an orange solution of 1(OTf)2
(50 mg, 0.051 mmol) in MeOH (2 mL) was added dropwise a
solution of NaO2CCH3 (8.4 mg, 0.10 mmol) in MeOH (2 mL). A
red-brown mixture was generated and stirred for 2 h. Filtration
through a plug of Celite followed by exposure to Et2O vapor
diffusion yielded 2(OTf)3 as a red crystalline solid (20 mg, 53%).
Single crystals of 2(OTf)3‚2MeCN‚0.5Et2O suitable for X-ray
diffraction study were grown from MeCN by Et2O vapor diffusion.
FTIR (cm-1, KBr): 3062 (w), 3022 (w), 2929 (w), 1596 (m), 1596
(m), 1570 (m), 1546 (w), 1472 (m), 1442 (m), 1405 (m), 1364
(m), 1276 (s), 1223 (m), 1150 (m), 1054 (w), 1030 (s), 854 (w),
832 (w), 817 (w), 759 (m), 685 (w), 680 (w), 655 (w), 636 (s),
585 (m), 572 (m), 517 (m). Anal. Calcd for C63H44N12O9F9S3Cu2:
C, 50.20; H, 2.94; N, 11.15. Found: C, 49.91; H, 2.77; N, 11.19.
[Cu2(bdptz)(µ-OH)(MeCN)2](OTf)3, 3(OTf)3. To an orange
solution of 1(OTf)2 (100 mg, 0.103 mmol) in MeOH (4 mL) was
added AgOTf (53 mg, 0.21 mmol) in MeOH (2 mL), and a cloudy,
yellow-green mixture was formed. The suspension was stirred for
30 min, followed by filtration through Celite and evaporation of
the solvent under reduced pressure. Blue X-ray quality crystals of
3(OTf)3‚1.75MeCN were obtained by vapor diffusion of Et2O into
1
572 (m), 517 (m). H NMR (300 MHz, CD2Cl2) δ: 9.39 m (1H),
9.27 m (1H), 8.89 d (J ) 18 Hz, 1H), 8.72 dd (J ) 14 Hz, 2H),
8.56 d (J ) 14 Hz, 1H), 8.37 d (J ) 18 Hz, 2H), 8.29-8.10 m
(7H), 7.92-7.82 m (3H), 7.71 m (1H), 7.57 m (1H), 7.48-7.34 m
(5H), 7.24-7.13 m (2H), 5.60 d (J ) 53 Hz, 1H), 4.85 d (J ) 31
Hz, 1H). Anal. Calcd for C39H29N7O6F6S2Cu2: C, 46.99; H, 2.93;
N, 9.84. Found: C, 47.19; H, 3.17; N, 9.99.
[Cu2(Ph4bdptz)(MeCN)2](OTf)2, 6(OTf)2. To a solution of [Cu-
(MeCN)4](OTf) (67 mg, 0.18 mmol) in MeCN (10 mL) was added
solid Ph4bdptz (69 mg, 0.090 mmol), and the resulting orange
mixture was stirred for 30 min. The slightly cloudy solution was
filtered through Celite and evaporated to dryness. The residue was
dissolved in CH2Cl2, and Et2O vapor diffusion into this solution
afforded 6(OTf)2 as a yellow flocculent solid (83 mg, 73%). X-ray
-
quality crystals of the BF4 salt, 6(BF4)2, were obtained from the
corresponding reaction using [Cu(MeCN)4](BF4), followed by
recrystallization from MeCN with Et2O vapor diffusion. FTIR
(cm-1, KBr): 3062 (w), 3033 (w), 2993 (w), 2932 (w), 1591 (s),
1579 (m), 1561 (s), 1542 (m), 1497 (w), 1448 (s), 1421 (m), 1357
(m), 1263 (s), 1223 (s), 1148 (s), 1030 (s), 919 (w), 813 (m), 764
a solution of MeCN and CH2Cl2 (47 mg, 40%). FTIR (cm-1
,
1
(s), 699 (s), 676 (m), 637 (s), 572 (m), 516 (m). H NMR (300
KBr): 3219 (br, m), 1670 (w), 1611 (m), 1575 (w), 1479 (w), 1450
(m), 1373 (w), 1289 (s), 1237 (s), 1165 (s), 1029 (s), 823 (w), 772
(m), 685 (w), 637 (s), 578 (m), 517 (m). Difficulty in separating
3(OTf)3 from an oil that formed upon crystallization prevented
satisfactory elemental combustion analysis.
MHz, CH2Cl2) δ: 9.36 m (2H), 8.37 m (2H), 8.20 d (J ) 25 Hz,
4H), 7.92 t (J ) 25 Hz, 4H), 7.55-7.45 m (16H), 7.40-7.32 m
(10H), 1.21 s (6H). Anal. Calcd for C60H44N8O6F6S2Cu2: C, 56.38;
H, 3.47; N, 8.77. Found: C, 56.09; H, 3.38; N, 8.67.
[Cu2(Ph4bdptz)(µ-O2CCH3)](OTf), 7(OTf). To a mixture of
6(OTf)2 (52 mg, 0.041 mmol) in CH2Cl2 (4 mL) was added
dropwise a solution of NaO2CCH3 (6.7 mg, 0.081 mmol) in MeOH
(1 mL). The resulting dark-red solution was stirred for 30 min and
evaporated to dryness under reduced pressure. The residue was
(26) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518-1520.
(27) Kubas, G. J. In Inorganic Synthesis; Shriver, D. F., Ed.; John Wiley
& Sons: New York, 1979; Vol. 19, pp 90-92.
(28) Heckel, E. Chem. Abstr. 1967, 66, 46487e.
Inorganic Chemistry, Vol. 43, No. 5, 2004 1753

Kuzelka et al.
dissolved in CH2Cl2 and filtered through Celite, and diffusion of
by the detector and pixels residing outside the active area of the
detector or behind the beam stop were masked during the integra-
tion.
Et2O vapor into this solution afforded a mixture of 6(OTf)2 and
7(OTf), as revealed by an X-ray diffraction study. FTIR (cm-1
,
KBr): 3062 (w), 3032 (w), 2923 (w), 1589 (s), 1579 (s), 1559 (s),
1448 (s), 1420 (m), 1358 (w), 1280 (s), 1255 (s), 1223 (w), 1156
(m), 1029 (s), 821 (w), 808 (m), 782 (w), 760 (s), 742 (w), 697
(s), 693 (s), 653 (w), 638 (s), 573 (w), 517 (w).
The structures were solved by direct methods using SHELXS-
97 software31 and refined on F2 by using the SHELXL-97 program32
incorporated in the SHELXTL33 software package. Empirical
absorption corrections were applied by using the SADABS
program,34 and PLATON software35 was used to check the
possibility of higher symmetry. All non-hydrogen atoms were
located, and their positions were refined with anisotropic thermal
parameters by least-squares cycles and Fourier syntheses. Hydrogen
atoms were assigned idealized positions and given a thermal
parameter 1.2 times the thermal parameter of the carbon atom to
which each was attached.
1
Physical Measurements. H NMR spectra were obtained on a
300 MHz Varian Mercury spectrometer. FTIR spectra were recorded
on a Thermo Nicolet Avatar 360 spectrometer. Optical spectra were
measured on a Hewlett-Packard 8453 diode-array spectrophotom-
eter, and low-temperature experiments were carried out by using a
custom-made quartz cuvette fused onto a vacuum-jacketed Dewar.
Frozen solution electron paramagnetic resonance (EPR) spectra of
2(OTf)3 were recorded on a Bruker model 300 electron spin
polarization (ESP) X-band spectrometer operating at 9.37 GHz and
running WinEPR software. A temperature of 77 K was maintained
by using a specially designed coldfinger filled with liquid N2.
Electrospray ionization mass spectrometry (ESI-MS) spectra of
oxidized solutions of 6(OTf)2 were obtained on a Bruker Daltonics
APEXII Tesla Fourier transform mass spectrometer in the MIT
Department of Chemistry Instrumentation Facility (Figures S1-
S3 in the Supporting Information).
The Et2O molecule in the structure of 2 is located on a special
position and has disordered methylene carbon atoms, each of which
was modeled at 50% occupancy. In the structure of 3, one triflate
is disordered over two positions and was refined with occupancies
of 58 and 42%. Four acetonitrile solvent molecules are present in
the lattice. One of these molecules is located on a special position
and was refined with 50% occupancy; a second molecule was also
modeled with 50% occupancy, and the remaining two solvent
molecules share a common methyl group and were refined with
occupancies of 50 and 25%, with the occupancy of the shared
carbon atom being 75%. The structure of 4 includes 2 acetonitrile
solvent molecules that were refined with 50% occupancy and 18
water molecules, 8 of which were modeled with full occupancy, 6
with 50% occupancy, and 4 with 25% occupancy. In the structure
of 5, the oxygen atom of the Et2O solvent molecule was refined
over two positions with occupancies of 75 and 25%. One fluorine
atom of a tetrafluoroborate counterion in the structure of 6(BF4)2
is disordered over two positions and was modeled with occupancies
of 50%. Crystallographic data for compounds 2-6 are listed in
Table 1, and selected bond lengths and angles of the compounds
are listed in Tables 2-4. Complete atom-labeling schemes are
provided in Figures S4-S8 in the Supporting Information.
Electrochemistry. Cyclic voltammograms were recorded in an
MBraun glovebox under a nitrogen atmosphere using an EG&G
model 263 potentiostat. The three-component cell consisted of a
platinum working electrode, Ag/AgNO3 (0.1 M in MeCN) reference
electrode, and platinum wire auxiliary electrode. The supporting
electrolyte was a 0.5 M Bu4N(PF6) solution. All of the measure-
ments were referenced externally to Cp2Fe. Linear plots of current
versus (scan rate)1/2 for 2(OTf)3 and 6(OTf)2 are displayed in Figures
S9 and S10 in the Supporting Information, respectively.
Collection and Reduction of X-ray Data. Single crystals were
coated in Infineum V8512 oil (formerly called Paratone N oil) on
the ends of glass capillaries (0.1 mm diameter) and rapidly cooled
under a low-temperature nitrogen cold stream maintained by a
Bruker KRYOFLEX BVT-AXS nitrogen cryostat. Crystals of
6(BF4)2 were found to be thermally sensitive, and room temperature
data for this structure were subsequently acquired. Intensity data
were collected on a Bruker SMART (1-3) or APEX (4-7) CCD
(charge-coupled device) X-ray diffractometer with graphite-mono-
chromatized Mo KR radiation (λ ) 0.710 73 Å), controlled by a
Pentium-based PC running the SMART software package.29
Crystals were mounted on a three-circle goniometer with ø fixed
at 54.73°, and the diffracted radiation was detected on a phosphor
screen that was maintained at 0 °C and held at a fixed distance of
6.0 cm from the crystal. Least-squares analysis of the unit-cell
parameters of a well-centered ylide test crystal was used to
determine the detector center and the crystal-to-detector distance.
A preliminary unit cell of a carefully optically centered crystal was
calculated by a series of 20 or 30 data frames, measured at 0.3°
increments of ω, with three different 2θ and φ values. A total of
16 frames were collected with exposure times of 10 or 30 s in the
absence of the X-ray beam to correct for the background detector
current. The detector was positioned at a 2θ value of 29.10°, and
data were measured using ω scans of 0.3° per frame with either 10
or 30 s exposure, such that a hemisphere of 25-28.29° was
collected. A series of 600 frames were collected in each of the
first three shells, and the initial 50 frames of the first shell were
re-collected at the end of the data collection in order to check for
any crystal decay. The starting positions of the crystal for the first
three shells were φ ) 0° and ω ) -28°, φ ) 90° and ω ) -28°,
and φ ) 180° and ω ) -28°, respectively. The raw data frames
were integrated using the SAINT software,30 and the orientation
matrix, initial background, and spot shape measurements were
continuously updated during the integration. Background frame
information was updated as indicated in eq 1, where B′ is the update
Results and Discussion
Synthesis of a Dicopper(I) Starting Material. Compound
1(OTf)2 was obtained in good yield (>80%) by the reaction
of [Cu(MeCN)4](OTf) and bdptz in a 2:1 ratio. Although
(29) SMART: Software for the CCD Detector System, version 5.626; Bruker
AXS: Madison, WI, 2000.
(30) SAINT: Software for the CCD Detector System, version 5.01; Bruker
AXS: Madison, WI, 1998.
(31) Sheldrick, G. M. SHELXS-97: Program for the Solution of Crystal
Structure; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(32) Sheldrick, G. M. SHELXL-97: Program for the Solution of Crystal
Structure; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(33) SHELXTL: Program Library for Structure Solution and Molecular
Graphics, version 5.10; Bruker AXS: Madison, WI, 1998.
(34) Sheldrick, G. M. SADABS: Area-Detector Absorption Correction;
University of Go¨ttingen: Go¨ttingen, Germany, 1996.
B′ ) (7B + C)/8
(1)
pixel value, B is the background pixel value prior to updating, and
C is the pixel value of the current frame. Spatial distortions induced
(35) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 2000.
1754 Inorganic Chemistry, Vol. 43, No. 5, 2004

Phthalazine-Based Dicopper Compounds
Table 1. Summary of X-ray Crystallographic Data
2(OTf)3‚2MeCN‚0.5Et2O
3(OTf)3‚1.75MeCN
4(OTs)4‚MeCN‚12H2O
5(OTf)2‚Et2O
6(BF4)2
formula
fw
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
C69H44N14Cu2O9.5S3F9
1615.44
P1h
C
40.5H23N9.75Cu2O10S3F9
C90H79N13Cu4O28S4
2173.06
P21/c
14.128(2)
25.037(4)
14.146(2)
C43H29N7Cu2O7S2F6
1060.93
P21/n
15.005(8)
18.358(10)
16.459(9)
C58H38N8Cu2B2F8
1147.66
P1h
1200.45
C2/c
21.197(4)
23.198(4)
22.497(4)
15.408(5)
15.733(5)
15.804(5)
67.990(5)
77.430(5)
83.600(5)
3464.9(19)
2
11.355(2)
14.373(3)
17.592(3)
85.391(4)
80.904(3)
66.756(3)
2604.4(9)
2
111.357(3)
95.695(3)
91.162(8)
10 303(3)
8
4979.3(13)
2
4533(4)
4
T (°C)
F
-85
-85
-70
-100
22
1.463
0.893
1.17-25.00
13 599
8491
7127
712
0.0602
0.1594
1.187, -0.407
calcd (g cm-3
)
1.548
1.548
1.449
1.555
µ(Mo KR) (mm-1
θ range (deg)
total no. of data
no. of unique data
observed dataa
no. of parameters
R1a,b
)
0.798
1.42-25.00
18 335
12 069
9471
970
0.0413
1.042
1.008
1.113
1.35-25.00
26 967
9099
5664
683
0.0761
0.2162
1.054, -0.887
1.63-28.26
30 356
11 380
8774
1.66-28.29
27 923
10 330
8497
658
613
0.0492
0.1281
0.788, -0.410
0.0500
0.1226
0.749, -0.337
wR2a,c
0.1011
0.653, -0.457
max, min peaks
(e Å-3
)
a Observation criterion: I > 2σ(I). b R1 ) ∑|Fo| - |Fc|/∑|Fo|. c wR2 ) {∑[w(Fo - Fc )2]/∑[w(Fo )2]}1/2
.
2
2
2
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 2 and 3a
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 4 and 5a
bond lengths bond angles
bond lengths bond angles
2
Cu1‚‚‚Cu2
5.2037(15)
2.036(3)
2.035(3)
2.287(3)
2.030(2)
2.065(3)
2.299(3)
2.076(3)
2.035(3)
2.090(3)
2.020(2)
3.1774(12)
2.048(6)
2.299(6)
2.006(6)
1.993(8)
1.909(5)
2.046(6)
2.264(6)
2.001(6)
2.006(8)
1.905(5)
N1-Cu1-N4
85.35(10)
93.93(10)
178.43(10)
86.84(10)
94.96(10)
90.77(10)
119.09(10)
110.31(10)
142.69(10)
4
Cu1‚‚‚Cu2
3.1690(7)
3.5603(7)
2.053(2)
2.275(3)
2.021(2)
1.905(2)
1.926(2)
2.066(2)
2.327(3)
2.016(3)
1.907(2)
1.925(2)
3.7024(13)
2.144(2)
2.144(2)
2.015(2)
1.951(2)
2.018(2)
2.103(3)
2.060(2)
2.050(3)
2.008(3)
1.368(4)
Cu1-O1-Cu2
112.47(10)
135.19(12)
81.67(9)
86.83(10)
88.16(10)
98.50(9)
Cu1-N1
Cu1-N3
Cu1-N4
Cu1-N7
Cu1-N9
Cu1-N10
Cu2-N5
Cu2-N6
Cu2-N11
Cu2-N12
Cu1‚‚‚Cu2
Cu1-N1
Cu1-N3
Cu1-N4
Cu1-N7
Cu1-O1
Cu2-N2
Cu2-N5
Cu2-N6
Cu2-N8
Cu2-O1
N1-Cu1-N7
N4-Cu1-N10
N7-Cu1-N10
N3-Cu1-N9
N5-Cu2-N6
N5-Cu2-N11
N5-Cu2-N12
N6-Cu2-N12
Cu1‚‚‚Cu2A
Cu1-N1
Cu1-N3
Cu1-N4
Cu1-O1
Cu1-O2A
Cu2-N2
Cu2-N5
Cu2-N6
Cu2-O1
Cu2-O2
Cu1‚‚‚Cu2
Cu1-N1
Cu1-N3
Cu1-N4
Cu1-N7
Cu2-N2
Cu2-N5
Cu2-N6
Cu2-C36
Cu2-C37
C36-C37
Cu2-O2-Cu1A
N1-Cu1-N3
N1-Cu1-N4
N3-Cu1-N4
O1-Cu1-N3
O1-Cu1-N4
N2-Cu2-N5
N2-Cu2-N6
N5-Cu2-N6
O1-Cu2-N5
O1-Cu2-N6
N1-Cu1-N3
N1-Cu1-N4
N3-Cu1-N4
N1-Cu1-N7
N3-Cu1-N7
N4-Cu1-N7
N2-Cu2-N5
N2-Cu2-N6
N5-Cu2-N6
168.70(10)
81.53(9)
87.43(10)
84.88(10)
100.28(10)
170.08(9)
87.69(9)
3
Cu1-O1-Cu2
N1-Cu1-N3
N1-Cu1-N4
N3-Cu1-N4
O1-Cu1-N3
O1-Cu1-N4
N2-Cu2-N5
N2-Cu2-N6
N5-Cu2-N6
O1-Cu2-N5
O1-Cu2-N6
112.8(3)
86.7(2)
88.1(2)
5
88.60(9)
82.7(2)
91.64(10)
124.40(9)
115.08(10)
136.24(10)
89.54(9)
105.66(19)
168.6(2)
84.9(2)
87.8(2)
85.4(2)
90.90(9)
89.84(10)
100.7(2)
169.8(2)
a Numbers in parentheses are the estimated standard deviations of the
last significant figure. Atoms are labeled as indicated in Figure 2.
a Numbers in parentheses are the estimated standard deviations of the
last significant figure. Atoms are labeled as indicated in Figure 2.
the ability of bdptz to promote the formation of dinuclear
complexes is well established,21-24 1 is the first example of
a dicopper(I) compound with this ligand. X-ray crystal-
lographic analysis was sufficient to ascertain the structure
of 1, but poor data quality precluded a satisfactory refine-
ment, and substitution of the triflate counterions for other
units did not afford better crystals. The cationic moiety
contains two Cu(I) centers, ∼3.74 Å apart, bridged by the
phthalazine unit and further ligated by the pyridine rings of
bdptz and two MeCN solvent molecules. NMR spectroscopy
and elemental analysis support the formulation of compound
1(OTf)2. Coordination of acetonitrile rather than the triflate
counterions to the metal centers reflects its good solvation
properties, and the ability to substitute these coordinated
solvent molecules for other ligands renders 1 a convenient
starting material for preparing a variety of compounds, as
shown in Scheme 1.
Preparation and Characterization of a Mixed-Valent
Cu(I)Cu(II) Compound. Treatment of 1(OTf)2 with NaO2-
CCH3 in MeOH did not yield the anticipated acetate-bridged
complex. Rather, the mixed-valent Cu(I)Cu(II) compound
2(OTf)3 was isolated. The concomitant precipitation of Cu
metal over the course of the reaction suggests that dispro-
portionation of Cu(I) is responsible for the generation of the
divalent metal component.36 Initial coordination of the acetate
to copper(I) probably occurs, followed by disproportionation
and rearrangement to afford 2.
Inorganic Chemistry, Vol. 43, No. 5, 2004 1755

Kuzelka et al.
Scheme 1
Table 4. Selected Bond Lengths (Å) and Angles (deg) for 6a
of 5.204(2) Å suggests that the unpaired electron is probably
not delocalized between the two metal centers.
bond lengths
bond angles
Examination of 2 by EPR spectroscopy confirms that the
valencies are localized. A frozen solution of 2(OTf)3 in
MeOH or MeCN gave a four-line EPR spectrum with g| )
2.25 (A| ) 169 G) and g ) 2.06 at 77 K. This spectrum,
shown in Figure 3, is typical of an isolated Cu(II) ion,
confirming that 2 is a class I mixed-valent compound,38 with
the unpaired electron localized on the Cu(II) site. A similar
spectrum was reported for the closely related pyridazine
derivative [Cu2(bdpdz)2](ClO4)3.25,37 Complex 2 is an ex-
ample of a synthetic model in which the stereochemistry at
the metal centers dictates the location of the unpaired
electron.39 In contrast, the CuA centers of cytochrome c
oxidase and nitrous oxide reductase contain fully delocalized
mixed-valent centers in close proximity to each other and
display a seven-line EPR spectrum typical for class III mixed-
valent dicopper units.40
Synthesis and Crystallographic Analysis of Hydroxide-
Bridged Dicopper(II) Complexes. Compound 1(OTf)2 was
treated with 1 equiv of AgOTf to determine whether a mixed-
valent complex could also be accessed through chemical
oxidation. The dicopper(II) complex 3(OTf)3 was formed,
however, indicating that Ag+ is too strong an oxidant to
achieve this goal. As expected, the structure of 3, shown in
Figure 2, is similar to that of the cation in [Cu2(bdptz)(µ-
OH)(H2O)2](OTs)3, a complex that was prepared directly
from [Cu(H2O)6](OTs)2.21 Three coordination sites of each
pseudo-square-pyramidal copper(II) ion are occupied by the
nitrogen donors of bdptz. In addition to the phthalazine unit,
the two Cu(II) centers are also bridged by a single oxygen
6
Cu1‚‚‚Cu2
3.7923(9)
2.141(4)
2.132(4)
2.069(4)
1.893(4)
2.099(4)
2.103(4)
2.145(4)
1.879(5)
N1-Cu1-N3
91.64(15)
88.23(14)
120.23(15)
90.36(15)
90.32(15)
89.54(14)
129.58(16)
87.10(15)
Cu1-N1
Cu1-N3
Cu1-N4
Cu1-N7
Cu2-N2
Cu2-N5
Cu2-N6
Cu2-N8
N1-Cu1-N4
N1-Cu1-N7
N3-Cu1-N4
N2-Cu2-N5
N2-Cu2-N6
N2-Cu2-N8
N5-Cu2-N6
a Numbers in parentheses are the estimated standard deviations of the
last significant figure. Atoms are labeled as indicated in Figure 4.
The structure of 2, shown in Figure 2, features two copper
centers that are asymmetrically coordinated by two bdptz
ligands and closely resembles that of the related mixed-valent
complex, [Cu2(bdpdz)2](ClO4)3, where bdpdz is the py-
ridazine analogue of bdptz, formed by reduction of a Cu(II)
salt.25,37 The distorted tetrahedral Cu(I) site is ligated by a
pair of pyridine groups from each bdptz ligand, whereas the
axially elongated pseudo-octahedral environment of the
Cu(II) center utilizes the remaining pyridine rings and one
phthalazine N donor of each ligand. As a result, one
phthalazine nitrogen atom of each bdptz ligand remains
unmetalated. A Jahn-Teller distortion accounts for the axial
bonds of the Cu(II) site being significantly longer [Cu1-
Nax,av ) 2.293(2) Å] than the equatorial ones [Cu1-Neq,av
) 2.042(1) Å]. The latter are virtually identical to the bond
distances of the Cu(I) center [Cu2-Nav ) 2.055(1) Å]; the
anticipated elongation of the metal-ligand bonds upon
moving from a coordination number of four to six is offset
by the increase in the oxidation state of the copper center,
which causes bond contraction. The long Cu‚‚‚Cu separation
(38) Robin, M. B.; Day, P. In AdVances in Inorganic Chemistry and
Radiochemistry; Emele´us, H. J., Sharpe, A. G., Eds.; Academic
Press: New York, 1967; Vol. 10, pp 247-422.
(36) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.;
John Wiley & Sons: New York, 1988.
(37) Manzur, J.; Garc´ıa, A. M.; Vega, A.; Spodine, E. Polyhedron 1999,
18, 2399-2404.
(39) Dunaj-Jurc´o, M.; Ondrejovic´, G.; Meln´ık, M.; Garaj, J. Coord. Chem.
ReV. 1988, 83, 1-28.
(40) Neese, F.; Zumft, W. G.; Antholine, W. E.; Kroneck, P. M. H. J. Am.
Chem. Soc. 1996, 118, 8692-8699.
1756 Inorganic Chemistry, Vol. 43, No. 5, 2004

Phthalazine-Based Dicopper Compounds
Figure 2. ORTEP diagrams of [Cu2(bdptz)2]3+ (2), [Cu2(bdptz)(µ-OH)(MeCN)2]3+ (3), [Cu2(bdptz)(µ-OH)2]24+ (4), and [Cu2(bdptz)(µ-vpy)]2+ (5) showing
50% probability thermal ellipsoids for all non-hydrogen atoms (left to right, top to bottom).
of >0.5 Å relative to that in 1. The Cu-N bond lengths of
the pyridine groups bound trans to the hydroxide are
substantially shorter [Cu-Nav ) 2.004(4) Å] than those of
the other pyridine ligands [Cu-Nav ) 2.282(4) Å]. Coor-
dination of an acetonitrile solvent molecule to each metal
center completes their coordination spheres.
Reaction of 1(OTf)2 with dioxygen or treatment of
[Cu(H2O)6](OTs)2 with bdptz and NaOH in a ratio of 2:1:2
yielded the tetranuclear compound 4(OTs)4. The structure
of cation 4 is composed of two crystallographically equiva-
lent (µ-hydroxo)dicopper(II) units that are joined by two
additional hydroxide groups (Figure 2). The {Cu2(bdptz)-
(µ-OH)}3+ moieties of 4 have bond lengths and angles similar
to those in 3, including the contraction of the Cu-N bond
located trans to the bridging hydroxide ligands. The phthala-
Figure 3. X-band EPR spectrum of 2(OTf)3 as a frozen solution (4.8 mM
in MeOH) at 77 K.
zine-bridged metal ions are separated by 3.169(1) Å with a
Cu-O(H)-Cu angle of 112.47(10)°, essentially identical to
that found in the structure of 3. In contrast, the Cu‚‚‚Cu
separation between the two dinuclear units is significantly
greater [Cu1‚‚‚Cu2A ) 3.560(1) Å] presumably resulting
from the presence of one, rather than two, bridging group
and the larger Cu-O(H)-Cu bond angle of 135.19(12)°.
atom. The Cu-O bond distances of 1.909(5) and 1.905(5)
Å unequivocally show that this ligand is a hydroxide ion,
which was probably introduced as an adventitious solvent
component. The Cu-O(H)-Cu bond angle is 112.8(3)°, and
the presence of the single-atom bridge significantly dimin-
ishes the Cu‚‚‚Cu separation in 3 to 3.177(1) Å, a reduction
Inorganic Chemistry, Vol. 43, No. 5, 2004 1757

Kuzelka et al.
Tetranuclear copper(II) compounds with four hydroxide
ligands are known to adopt cubane41,42 or “stepped-
cubane”43-45 type structures. Prior to this work, the {M4(µ-
OH)4}4+ structural motif of 4 was not observed with
copper(II), although it was characterized with nickel(II),24
and similar structures of iron(III) with oxo, rather than
hydroxo, ligands have also been encountered.46 The forma-
tion of tetranuclear 4(OTs)4 rather than a bis(µ-hydroxo)-
dicopper(II) compound probably results because the geo-
metric restrictions imposed by the bridging phthalazine unit
disfavor the {Cu2(µ-OH)2}2+ core found in other com-
plexes.47-50
The structure of cation 5, shown in Figure 2, reveals that
each distorted tetrahedral Cu(I) center is ligated by bdptz in
the usual manner, with Cu-N bond distances that average
to 2.081(1) Å. The pyridine N atom of the vpy ligand
coordinates to Cu1 with a bond length of 1.951(2) Å, which
is somewhat shorter than the corresponding bonds in
the complex [Cu2(µ-vpy)2(vpy)2](ClO4)2 (1.981(9) and
2.008(10) Å).51 Coordination of the alkene moiety to Cu2 is
asymmetric, as was observed in [Cu2(µ-vpy)2(vpy)2](ClO4)2,
with longer bond lengths for the inner (2.050(3) Å) than the
outer (2.008(3) Å) olefinic carbon atoms. The olefinic
CdC bond length in 5 of 1.368(4) Å matches well with those
of the coordinated vpy ligands in [Cu2(µ-vpy)2(vpy)2](ClO4)2
(1.358(16) and 1.368(17) Å) and is slightly longer than those
of the noncoordinated vpy CdC groups (1.319(23) and
1.307(23) Å) in the latter complex.
Preparation and Crystal Structure of a Bridging
Vinylpyridine Dicopper(I) Derivative. Complex 5(OTf)2
was synthesized in high yield (86%) by the reaction of
1(OTf)2 with vpy. Both the pyridine N atom and alkene
moiety of vpy bind to the dimetallic core. The only other
structurally characterized compounds in which vpy coordi-
nates to two metal centers in this fashion are [Cu2(µ-vpy)2-
Synthesis and Structural Characterization of a Bis-
(acetonitrile) Complex with Ph4bdptz. Tetranuclear com-
pounds such as 4(OTs)4 form through undesired bimolecular
interactions, frequently resulting from oxidation reactions.
Because bdptz supports such structures with several first-
row transition-metal ions, we introduced the sterically
hindered derivative Ph4bdptz in an effort to prevent the
formation of the high-nuclearity species. Treatment of
[Cu(MeCN)4](OTf) with Ph4bdptz in a 2:1 ratio afforded
6(OTf)2 in >70% yield. As an analogue of 1, compound 6
also serves as a useful starting material. Although the triflate
salt is a flocculent solid, single crystals suitable for X-ray
51
(vpy)2](ClO4)2 and the linear polymers [(vpy)CuCl]∞ and
[(vpy)CuBr]∞.52 The η2 interaction of an alkene53-61 or
aromatic group62 with copper(I) centers supported by three
nitrogen donors is well-documented. It is also noteworthy
that copper(I) is the only metal known to bind olefins in
nature.36 In particular, plants utilize a copper(I) center to bind
ethylene, a hormone that is involved in many aspects of plant
development, including fruit ripening.63,64
-
crystallographic study were obtained when BF4 was used
(41) Sletten, J.; Sørensen, A.; Julve, M.; Journaux, Y. Inorg. Chem. 1990,
29, 5054-5058.
(42) Dedert, P. L.; Sorrell, T.; Marks, T. J.; Ibers, J. A. Inorg. Chem. 1982,
21, 3506-3517.
(43) van Albada, G. A.; Mutikainen, I.; Roubeau, O.; Turpeinen, U.;
Reedijk, J. Inorg. Chim. Acta 2002, 331, 208-215.
(44) Zheng, Y.-Q.; Lin, J.-L. Z. Anorg. Allg. Chem. 2002, 628, 203-208.
(45) Mathews, I. I.; Manohar, H. J. Chem. Soc., Dalton Trans. 1991, 2139-
2143.
(46) Barrios, A. M.; Lippard, S. J. Unpublished results.
(47) van Albada, G. A.; Mutikainen, I.; Turpeinen, U.; Reedijk, J. Inorg.
Chim. Acta 2001, 324, 273-277.
(48) Youngme, S.; Somjitsripunya, W.; Chinnakali, K.; Chantrapromma,
S.; Fun, H.-K. Polyhedron 1999, 18, 857-862.
(49) Mann, K. L. V.; Jeffery, J. C.; McCleverty, J. A.; Thornton, P.; Ward,
M. D. J. Chem. Soc., Dalton Trans. 1998, 89-97.
(50) Wu, L.-P.; Keniry, M. E.; Hathaway, B. Acta Crystallogr., Sect. C
1992, 48, 35-40.
as the counterion.
The geometry around the copper centers in cation 6, shown
in Figure 4, is similar to that in 1. The copper(I) ions are
separated by 3.792(1) Å, which is ∼0.05 Å longer than the
Cu‚‚‚Cu distance in 1. The average Cu-N(Ph4bdptz) bond
length of 2.115(2) Å is substantially longer than the
Cu-N(MeCN) bond distances of 1.893(4) and 1.879(5) Å.
This difference in bond lengths is evident in related
compounds containing {N3(aromatic)CuI(MeCN)} frag-
ments65-67 and is a result of the different nitrogen-atom
orbital hybridizations and lesser steric demands of the linear
CH3CN ligand. The Cu-N bond lengths in 6 are slightly
(51) Munakata, M.; Kitagawa, S.; Simono, H.; Emori, T.; Masuda, H. J.
Chem. Soc., Chem. Commun. 1987, 1798-1799.
(52) Engelhardt, L. M.; Healy, P. C.; Kildea, J. D.; White, A. H. Aust. J.
Chem. 1989, 42, 185-199.
(53) Dias, H. V. R.; Lu, H.-L.; Kim, H.-J.; Polach, S. A.; Goh, T. K. H.
H.; Browning, R. G.; Lovely, C. J. Organometallics 2002, 21, 1466-
1473.
longer than the corresponding distances in 1 (∆Cu-N(bdptz′)
∼
0.05, ∆Cu-N(MeCN) ∼ 0.02 Å). The increased metal-ligand
and metal-metal distances in 6 relative to the bdptz analogue
result from a combination of electronic and steric factors;
the electron-withdrawing phenyl rings of Ph4bdptz weaken
the Cu-N bonds, and the steric repulsion created by the
proximity of these bulky groups is minimized when the
ligand components are farther from the metal centers. The
N(Ph4bdptz)-Cu-N(Ph4bdptz) bond angles average out to
∼90°, and the N(Ph4bdptz)-Cu-N(MeCN) bond angles
(54) Zhang, J.; Xiong, R.-G.; Zuo, J.-L.; Che, C.-M.; You, X.-Z. J. Chem.
Soc., Dalton Trans. 2000, 2898-2900.
(55) Shimazaki, Y.; Yokoyama, H.; Yamauchi, O. Angew. Chem., Int. Ed.
1999, 38, 2401-2403.
(56) Sanyal, I.; Murthy, N. N.; Karlin, K. D. Inorg. Chem. 1993, 32, 5330-
5337.
(57) Nelson, S. M.; Lavery, A.; Drew, M. G. B. J. Chem. Soc., Dalton
Trans. 1986, 911-920.
(58) Thompson, J. S.; Harlow, R. L.; Whitney, J. F. J. Am. Chem. Soc.
1983, 105, 3522-3527.
(63) Rodr´ıguez, F. I.; Esch, J. J.; Hall, A. E.; Binder, B. M.; Schaller, G.
E.; Bleecker, A. B. Science 1999, 283, 996-998.
(64) Dey, P. M., Harborne, J. B., Eds. Plant Biochemistry; Academic
Press: London, 1997.
(65) Jonas, R. T.; Stack, T. D. P. Inorg. Chem. 1998, 37, 6615-6629.
(66) Blake, A. J.; Hill, S. J.; Hubberstey, P.; Li, W.-S. J. Chem. Soc., Dalton
Trans. 1998, 909-915.
(67) Canty, A. J.; Engelhardt, L. M.; Healy, P. C.; Kildea, J. D.; Minchin,
N. J.; White, A. H. Aust. J. Chem. 1987, 40, 1881-1891.
(59) Pasquali, M.; Floriani, C.; Gaetani-Manfredotti, A.; Chiesi-Villa, A.
Inorg. Chem. 1979, 18, 3535-3542.
(60) Pasquali, M.; Floriani, C.; Gaetani-Manfredotti, A.; Chiesi-Villa, A.
J. Am. Chem. Soc. 1978, 100, 4918-4919.
(61) Dai, X.; Warren, T. H. Chem. Commun. 2001, 1998-1999.
(62) Osako, T.; Tachi, Y.; Taki, M.; Fukuzumi, S.; Itoh, S. Inorg. Chem.
2001, 40, 6604-6609.
1758 Inorganic Chemistry, Vol. 43, No. 5, 2004

Phthalazine-Based Dicopper Compounds
reaction does not go to completion even in the presence of
a large excess of NaO2CCH3 (>20 equiv), as evidenced by
the isolation of a mixture of 6(OTf)2 and 7(OTf). This result
is probably due to MeCN being a better ligand than acetate
for copper(I) and is supported by the complete regeneration
of 6(OTf)2 from the mixture upon addition of MeCN.
A full crystallographic refinement of 7(OTf) was hampered
by the severe disorder of the triflate counterion, and the
-
-
-
-
substitution of triflate by BF4 , PF6 , ClO4 , or BPh4 did
not afford single crystals of suitable quality to permit analysis
by X-ray diffraction. The structure of cation 7 is shown in
Figure 4. Briefly, Ph4bdptz coordinates two copper(I) ions
in the usual manner, and the acetate ligand bridges the metal
centers with Cu-O bond distances of ∼1.93 Å. The
Cu‚‚‚Cu separation of ∼3.53 Å is shorter than that in 1 and
6, which lack a second bridging ligand, but longer than in
the single-atom-bridged cations 3 and 4. Whereas (µ-
carboxylato)dicopper(II) compounds are common, the analo-
gous dicopper(I) complexes are rare, typically requiring
π-acceptor ancillary ligands,70-73 sterically demanding and
preorganized bis(carboxylate) platforms,74,75 or a copper-
copper interaction.76
Electrochemical Studies of Compounds 1(OTf)2 and
2(OTf)3. Cyclic voltammograms (CVs) of compounds 1(OTf)2,
2(OTf)3, and 6(OTf)2 were recorded in MeCN, and the E1/2
values of 2(OTf)3, 6(OTf)2, and selected mononuclear and
dinuclear compounds are compiled in Table 5. Compound
1 showed highly irreversible electrochemical behavior,
indicating that the dinuclear core is not stable under these
conditions. Compound 2, on the other hand, displayed a
reversible wave with an E1/2 value of -452 mV (∆Ep ) 74
mV) versus Cp2Fe+/Cp2Fe when potentials lower than -100
mV were applied. Application of higher potentials resulted
in irreversible redox waves, as shown in Figure 5. When
converted to the same scale, the CuIICuII/CuIICuI couple of
2 is more than 150 mV below that of [Cu2(bdpdz)2]-
(ClO4)3.25,37 Although the two compounds are structurally
very similar, phthalazine (pKa ) 3.47)77 is a stronger base
than pyridazine (pKa ) 2.24),77 which may explain why the
copper(I) center in 2 is easier to oxidize. The redox potential
of 2 is comparable to that of the copper(I) complexes [Cu-
(TMPA)(MeCN)](ClO4),25,78,79 [Cu(H-MePY2)](B(C6H5)4),25,80
and [Cu2(bTMPA)](ClO4)2,25,81 all of which bear ligands
featuring a combination of pyridine and amine substituents.
Figure 4. ORTEP diagram of [Cu2(Ph4bdptz)(MeCN)2]2+ (6) showing
50% probability thermal ellipsoids for all non-hydrogen atoms and a ball-
and-stick representation of [Cu2(Ph4bdptz)(µ-O2CCH3)]+ (7) (top to bottom).
range from 113.63(16) to 140.39(17)°, similar to the angles
found in the Cs- and C3ν-symmetric mononuclear copper(I)
compounds with facial-capping N-donor ligands.65,68,69 These
bond angles are essentially unchanged relative to those in 1.
Preparation of a Carboxylate-Bridged Dicopper(I)
Complex. In contrast to the mixed-valent compound 2(OTf)3,
which was isolated using the bdptz analogue, reaction of
6(OTf)2 with NaO2CCH3 yields the acetate-bridged
dicopper(I) complex 7(OTf). The steric bulk provided by the
phenyl substituents of Ph4bdptz precludes the formation of
a compound with a structure similar to that of 2, and the
high redox potentials of 6 (vide infra) stabilize the +1
oxidation state and disfavor disproportionation. The synthesis
of 7(OTf) proceeds in a mixture of CH2Cl2 and MeOH. The
latter solvent is necessary to dissolve NaO2CCH3, and no
reaction occurs in its absence, even after prolonged reaction
times (>12 h) did not improve matters. Furthermore, the
(70) Kitagawa, S.; Kondo, M.; Kawata, S.; Wada, S.; Maekawa, M.;
Munakata, M. Inorg. Chem. 1995, 34, 1455-1465.
(71) Toth, A.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. Inorg. Chem. 1987,
26, 236-241.
(72) Pasquali, M.; Floriani, C.; Venturi, G.; Gaetani-Manfredotti, A.; Chiesi-
Villa, A. J. Am. Chem. Soc. 1982, 104, 4092-4099.
(73) Pasquali, M.; Leoni, P.; Floriani, C.; Gaetani-Manfredotti, A. Inorg.
Chem. 1982, 21, 4324-4328.
(74) Hagadorn, J. R.; Zahn, T. I.; Que, L., Jr.; Tolman, W. B. Dalton Trans.
2003, 1790-1794.
(75) LeCloux, D. D.; Lippard, S. J. Inorg. Chem. 1997, 36, 4035-4046.
(76) He, C.; Lippard, S. J. Inorg. Chem. 2000, 39, 5225-5231.
(77) Gilchrist, T. L. Heterocyclic Chemistry, 3rd ed.; Addison-Wesley
Longman Ltd.: Essex, U.K., 1997.
(78) Zhang, C. X.; Kaderli, S.; Costas, M.; Kim, E.; Neuhold, Y.-M.; Karlin,
K. D.; Zuberbu¨hler, A. D. Inorg. Chem. 2003, 42, 1807-1824.
(79) Lee, D.-H.; Wei, N.; Murthy, N. N.; Tyekla´r, Z.; Karlin, K. D.; Kaderli,
S.; Jung, B.; Zuberhu¨ler, A. D. J. Am. Chem. Soc. 1995, 117, 12498-
12513.
(68) Lam, B. M. T.; Halfen, J. A.; Young, V. G., Jr.; Hagadorn, J. R.;
Holland, P. L.; Lledo´s, A.; Cucurull-Sa´nchez, L.; Novoa, J. J.; Alvarez,
S.; Tolman, W. B. Inorg. Chem. 2000, 39, 4059-4072.
(69) Conry, R. R.; Ji, G.; Tipton, A. A. Inorg. Chem. 1999, 38, 906-913.
Inorganic Chemistry, Vol. 43, No. 5, 2004 1759

Kuzelka et al.
Table 5. CV Data for Compounds 2(OTf)3, 6(OTf)2, and Related
Mononuclear and Dinuclear Models
Figure 6. CV of 2 mM 6(OTf)2 in MeCN with 0.5 M Bu4N(PF6) as the
supporting electrolyte and a scan rate of 100 mV/s.
of which is stabilized by the facially capping tris(pyridyl)
moiety.25,65
a
complex
E1/2
solvent
MeCN
ref
[Cu(TMPA)(MeCN)](ClO4)
-400
78
79
80
Electrochemistry of 6(OTf)2. Irreversible electrochemical
behavior was observed with 6 in CH2Cl2. In contrast, two
well-separated reversible waves with E1/2 potentials of +41
mV (∆Ep ) 122 mV) and +516 mV (∆Ep ) 124 mV) versus
Cp2Fe+/Cp2Fe corresponding to the CuIICuI/CuICuI and CuII-
CuII/CuIICuI couples, respectively, were observed when
MeCN was employed as the solvent (Figure 6). Evidently,
the species generated electrochemically from 6 are stabilized
in the coordinating solvent. Addition of MeCN often
improves the reversibility of CV traces of copper(I) com-
pounds, possibly by providing an additional ligand to
stabilize the copper(II) species.65,82 The relatively high redox
potentials displayed by 6 compared to the majority of the
compounds listed in Table 5 are probably largely due to the
electron-withdrawing phenyl substituents of Ph4bdptz, which
make oxidation to Cu(II) more difficult. Electrochemical
studies on a series of dicopper(I) complexes supported by
macrocyclic ligands, including the compound [Cu2(L2)-
(MeCN)2](ClO4)2, also showed well-resolved sequential one-
electron redox waves at comparably high potentials.25,83 By
using the E1/2 values for the two redox couples, a compro-
portionation constant, Kcom, of 1 × 108 was calculated for 6
according to the equilibrium in eq 2.84 The large value of
-628b DMF
[Cu(H-MePY2)](B(C6F5)4)
[Cu(L0)(MeCN)](OTf)
[Cu2(bdpdz)2](ClO4)3
-310
DMF
+60c MeCN/CH2Cl2 65
-290d MeCN/H2O
37
76
81
79
83
[Cu2(BBAN)(µ-O2CCPh3)](OTf)
-25
THF
[Cu2(bTMPA)](ClO4)2
-346e DMF
-639b DMF
+140f MeCN
+568f
[Cu2(D1)(MeCN)2](ClO4)2
[Cu2(L2)(MeCN)2](ClO4)2
2
6
-452
+41
+516
MeCN
MeCN
this paper
this paper
a mV(vs Cp2Fe+/Cp2Fe). Cp2Fe+/Cp2Fe ) +20 mV versus Ag/AgNO3.
c Cp2Fe+/Cp2Fe ) +450 mV versus saturated calomel electrode (SCE).
d Cp2Fe+/Cp2Fe ) +380 mV versus SCE in MeCN with (Bu4N)ClO4 as
the supporting electrolyte (see: Connelly, N. G.; Geiger, W. E. Chem. ReV.
1996, 96, 877-910). e Cp2Fe+/Cp2Fe ) +545 mV versus Ag/AgCl.
f Cp2Fe+/Cp2Fe ) +401 mV versus Ag/AgCl.
b
Kcom
[CuICuI(Ph4bdptz)]2+ + [CuIICuII(Ph4bdptz)]4+ y
z
2[CuICuII(Ph4bdptz)]3+ (2)
K
com, comparable to that determined for [Cu2(L2)(MeCN)2]-
Figure 5. CVs of 2 mM 2(OTf)3 in MeCN with 0.5 M Bu4N(PF6) as the
supporting electrolyte. The full CV was recorded with a scan rate of 200
mV/s, and the inset shows the CV recorded with a scan rate of 100 mV/s
when potentials below -0.1 V versus Cp2Fe+/Cp2Fe were applied.
(ClO4)2 (Kcom ) 2 × 107),25,83 suggests that the mixed-valent
species should have significant thermodynamic stability,
although attempts to generate it from 6 have thus far been
unsuccessful. In contrast to the two redox waves seen with
6, only one apparent wave, corresponding to the oxidation
In contrast, the E1/2 value of 2 is significantly more negative
than that of [Cu(L0)(MeCN)](OTf), the +1 oxidation state
(82) Carrier, S. M.; Ruggiero, C. E.; Houser, R. P.; Tolman, W. B. Inorg.
Chem. 1993, 32, 4889-4899.
(80) Zhang, C. X.; Liang, H.-C.; Kim, E.; Shearer, J.; Helton, M. E.; Kim,
E.; Kaderli, S.; Incarvito, C. D.; Zuberbu¨hler, A. D.; Rheingold, A.
L.; Karlin, K. D. J. Am. Chem. Soc. 2003, 125, 634-635.
(81) Lee, D.-H.; Murthy, N. N.; Karlin, K. D. Inorg. Chem. 1997, 36, 5785-
5792.
(83) Yates, P. C.; Drew, M. G. B.; Trocha-Grimshaw, J.; McKillop, K. P.;
Nelson, S. M.; Ndifon, P. T.; McAuliffe, C. A.; Nelson, J. J. Chem.
Soc., Dalton Trans. 1991, 1973-1979.
(84) Gagne´, R. R.; Koval, C. A.; Smith, T. J.; Cimolino, M. C. J. Am.
Chem. Soc. 1979, 101, 4571-4580.
1760 Inorganic Chemistry, Vol. 43, No. 5, 2004

Phthalazine-Based Dicopper Compounds
of both copper centers, is displayed by [Cu2(bTMPA)](ClO4)2
and [Cu2(D1)(MeCN)2](ClO4)2. This result indicates that the
two noninteracting metal ions in each of these latter
compounds are oxidized at approximately the same poten-
tial.25,79,81 The bridging phthalazine unit in 6, therefore,
confers greater communication between the copper ions.
Dioxygen Reactivity Studies of Dicopper Complexes.
The addition of dioxygen to an orange solution of 1(OTf)2
in MeOH at -78 °C caused an immediate color change to
pale green. Similar behavior was observed when low-
temperature oxygenations were performed in CH2Cl2 or
MeCN. The optical spectrum of the reaction products does
not indicate the formation of a copper-dioxygen adduct such
as a peroxo, bis(µ-oxo), or superoxo intermediate, all of
which have distinctive electronic transitions in the visible
region.85 Oxygenation of 1 afforded the tetranuclear com-
pound 4, possibly by dimerization of a (peroxo)dicopper(II)
species followed by rearrangement.86 If a {Cu2O2}2+ adduct
were generated in the initial stages of the reaction, it might
form and decay too rapidly to allow observation by either
conventional UV-vis spectroscopy or stopped-flow methods
at -60 °C. Reaction of dioxygen with a solution of mixed-
valent 2 in CH2Cl2 proceeded more slowly, turning the
solution green only after several hours at room temperature.
This decreased reactivity is inconsistent with the low redox
potential of the compound and may result from restricted
access of dioxygen to the copper(I) site. Although the final
product has not been characterized, oxidation of the py-
ridazine analogue afforded a mixture of [Cu2(bdpdz)2](ClO4)4
and a compound formulated as [Cu2(bdpdz)2(OH)2](ClO4)2.25,37
Oxygenation of 5 in CH2Cl2, MeOH, or MeCN proceeded
similarly to that of 1 and also afforded the tetranuclear
species. No activation of the vinyl substituent of 5 was
detected in either the presence or absence of the proton
source [H(OEt2)2](BAr′4)25,87 or when H2O2 was used as an
oxidant. Related compounds similarly oxidized to Cu(II)
without involvement of an alkene unit.56,61
rather high redox potentials of 6 observed by CV. Compound
6 is inert to dioxygen in MeCN but turns brown over several
minutes upon exposure to dioxygen in CH2Cl2. Similarly,
addition of H2O2 to a solution of 6 in MeCN afforded a
brown solution at room temperature, but no reaction occurred
at -40 °C. The decreased dioxygen reactivity of Ph4bdptz
relative to the bdptz complexes is also shared by complex
7, which is an air-stable solid that turns green in solution
only after several weeks at room temperature. Attempts to
crystallize the product of the reaction between 6 and
dioxygen have thus far been unsuccessful. Analysis of the
reaction mixture by ESI-MS (see the Supporting Information,
Figures S1-S3) shows peaks corresponding to a number of
different compounds including unreacted 6 and mononuclear
copper(I) compounds. A peak at m/z ) 1062, however,
exhibits the correct mass and isotope pattern for {[Cu2(Ph4-
bdptz)](OTf)}+ after the loss of a proton and incorporation
of an oxygen atom, possibly corresponding to the hydroxy-
lation of a phenyl ring of the Ph4bdptz ligand. Aromatic
hydroxylation by copper(I) complexes upon reaction with
dioxygen is well-precedented,9,11,14 and charge balance could
be attained by a mixed-valent Cu(I)Cu(II) species such as
that shown in Figure S3 of the Supporting Information.
Conclusions
A family of dicopper compounds were prepared and
characterized with the ligands bdptz and Ph4bdptz. The
phthalazine bridge supports a range of metal‚‚‚metal dis-
tances and a variety of ancillary ligands but does not permit
the detection of dioxygen adducts. The incorporation of the
phenyl substituents in Ph4bdptz prevents the formation of
undesired high-nuclearity species, however, and allows the
observation of two reversible redox waves by CV.
A difficult feature of metalloenzymes to model effectively
is the composition and chemistry of their secondary coor-
dination spheres. Many proteins have a hydrophobic channel
that facilitates access of small molecules to the active site,
which is buried in the protein interior. The phenyl rings of
Ph4bdptz form a binding pocket that mirrors this property
and suggests a general strategy for the introduction of
secondary sphere components into a dinucleating scaffold.
The sterically hindered Ph4bdptz ligand was expected to
prevent the formation of tetranuclear species because of the
presence of the phenyl rings on the pyridine groups that
extend well below the line joining the two copper centers.
An attractive feature of Ph4bdptz is that the phenyl groups
form a hydrophobic cavity in which the two copper centers
are positioned in such a way as to provide a binding pocket.
This property has potential value for the activation of small-
molecule substrates in future work. The steric bulk provided
by the pocket, however, appears to affect the access of O2
to the dimetallic core and significantly diminishes the
reactivity of compounds supported by this ligand. The
decreased reactivity toward dioxygen is consistent with the
Acknowledgment. This work was supported by grants
from the National Science Foundation and National Institute
of General Medical Sciences. J.K. acknowledges NSERC
for a graduate student fellowship. We thank Ms. Li Li of
the MIT Department of Chemistry Instrumentation Facility
for performing the ESI-MS measurements and Professor
Thomas J. Smith for valuable discussions.
Supporting Information Available: ESI-MS spectra of oxi-
dized solutions of 6(OTf)2, the full numbering schemes of 2-6,
scan rate plots for 2(OTf)3 and 6(OTf)2, and X-ray crystallographic
files in CIF format. This material is available free of charge via
the Internet at http://pubs.acs.org.
(85) Blackman, A. G.; Tolman, W. B. Struct. Bonding 2000, 97, 179-
211.
(86) Feig, A. L.; Becker, M.; Schindler, S.; van Eldik, R.; Lippard, S. J.
Inorg. Chem. 1996, 35, 2590-2601.
(87) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992,
11, 3920-3922.
IC035009R
Inorganic Chemistry, Vol. 43, No. 5, 2004 1761