Synthesis, Structure, and Reactivity of Model Complexes of Copper Nitrite Reductase

Article abstract of DOI:10.1021/ic950392o

The copper(I) and copper(II) complexes with the nitrogen donor ligands bis[(1-methylbenzimidazol-2-yl)methyl]-amine (1-BB), bis[2-(1-methylbenzimidazol-2-yl)ethyl]amine (2-BB), N-acetyl-2-BB (AcBB), and tris[2-(1-methylbenzimidazol-2-yl)ethyl]nitromethane (TB) have been studied as models for copper nitrite reductase. The copper(II) complexes form adducts with nitrite and azide that have been isolated and characterized. The Cu(II)-(1-BB) and Cu(II)-AcBB complexes are basically four-coordinated with weak axial interaction by solvent or counterion molecules, whereas the Cu(II)-(2-BB) and Cu(II)-TB complexes prefer to assume five-coordinate structures. A series of solid state structures of Cu(II)-(1-BB) and -(2-BB) complexes have been determined. [Cu(1-BB)(DMSO-O)₂](ClO₄)₂: tri clinic, P1? (No. 2), a = 9.400(1) A?, b = 10.494(2) A?, c = 16.760(2) A?, α = 96.67(1)°, β = 97.10(1)°, γ = 108.45(1)°, V = 1534.8(5) A?³, Z = 2, number of unique data [I ≥ 3σ(I)] = 4438, number of refined parameters = 388, R = 0.058. [Cu(1-BB)(DMSO-O)₂](BF₄)₂: triclinic, P1 (No. 2), a = 9.304(5) A?, b = 10.428(4) A?, c = 16.834(8) A?, α = 96.85(3)°, β = 97.25(3)°, γ = 108.21(2)°, V = 1517(1) A?³, Z = 2, number of unique data [I ≥ 2σ(I)] = 3388, number of refined parameters = 397, R = 0.075. [Cu(1-BB)(DMSO-O)(NO₂)](ClO₄): triclinic, P1? (No. 2), a = 7.533(2) A?, b = 8.936(1) A?, c = 19.168(2) A?, α = 97.66(1)°, β = 98.62(1)°, γ = 101.06(1)°, V = 1234.4(7) A?³, Z = 2, number of unique data [I ≥ 2σ(I)~] = 3426, number of refined parameters = 325, R = 0.081. [Cu(2-BB)(MeOH)(ClO₄)](ClO₄): triclinic, P1? (No. 2), a = 8.493(3) A?, b = 10.846(7) A?, c = 14.484(5) A?, α = 93.71(4)°, β = 103.13(3)°, γ = 100.61(4)°, V= 1270(1) A?³, Z = 2, number of unique data [I ≥ 2σ(I)] = 2612, number of refined parameters = 352, R = 0.073. [Cu(2-BB)(N₃)](ClO₄): monoclinic, P2₁/n (No. 14), a = 12.024(3) A?, b = 12.588(5) A?, c = 15.408(2) A?, β = 101,90(2)°, V = 2282(1) A?³, Z = 4, number of unique data [I ≥ 2σ(I)] = 2620, number of refined parameters = 311, R = 0.075. [Cu(2-BB)(NO₂)](ClO₄)(MeCN): triclinic, P1? (No. 2), a = 7.402(2) A?, b = 12.500(1) A?, c = 14.660(2) A?, α = 68.14(1)°, β = 88.02(2)°, γ = 78.61(1)°, V = 1233.0(4) A?³, Z = 2, number of unique data [I ≥ 2σ(I)] = 2088, number of refined parameters = 319, R = 0.070. In all the complexes the 1-BB or 2-BB ligands coordinate the Cu(II) cations through their three donor atoms. The complexes with 2-BB appear to be more flexible than those with 1-BB. The nitrito ligand is bidentate in [Cu(2-BB)(NO₂)](ClO₄)(MeCN) and essentially monodentate in [Cu(1-BB)(DMSO-O)(NO₂)](ClO₄). The copper(I) complexes exhibit nitrite reductase activity and react rapidly with NO₂^- in the presence of stoichiometric amounts of acid to give NO and the corresponding copper(II) complexes. Under the same conditions the reactions between the copper(I) complexes and NO⁺ yield the same amount of NO, indicating that protonation and dehydration of bound nitrite are faster than its reduction. The NO evolved from the solution was detected and quantitated as the [Fe(EDTA)(NO)] complex. The order of reactivity of the Cu(I) complexes in the nitrite reduction process is [Cu(2-BB)]⁺ > [Cu(1-BB)]⁺ > [Cu(TB)]⁺ > [Cu(AcBB)]⁺.

Full text of DOI:10.1021/ic950392o

Inorg. Chem. 1996, 35, 1101-1113
1101
Synthesis, Structure, and Reactivity of Model Complexes of Copper Nitrite Reductase
Luigi Casella* and Oliviero Carugo
Dipartimento di Chimica Generale, Universita` di Pavia, Via Taramelli 12, 27100 Pavia, Italy
Michele Gullotti, Sergio Doldi, and Massimiliano Frassoni
Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Universita` di Milano, Centro CNR,
Via Venezian 21, 20133 Milano, Italy
ReceiVed March 31, 1995X
The copper(I) and copper(II) complexes with the nitrogen donor ligands bis[(1-methylbenzimidazol-2-yl)methyl]-
amine (1-BB), bis[2-(1-methylbenzimidazol-2-yl)ethyl]amine (2-BB), N-acetyl-2-BB (AcBB), and tris[2-(1-
methylbenzimidazol-2-yl)ethyl]nitromethane (TB) have been studied as models for copper nitrite reductase. The
copper(II) complexes form adducts with nitrite and azide that have been isolated and characterized. The Cu(II)-
(1-BB) and Cu(II)-AcBB complexes are basically four-coordinated with weak axial interaction by solvent or
counterion molecules, whereas the Cu(II)-(2-BB) and Cu(II)-TB complexes prefer to assume five-coordinate
structures. A series of solid state structures of Cu(II)-(1-BB) and -(2-BB) complexes have been determined.
[Cu(1-BB)(DMSO-O)2](ClO4)2: triclinic, P1h (No. 2), a ) 9.400(1) Å, b ) 10.494(2) Å, c ) 16.760(2) Å, R )
96.67(1)°, â ) 97.10(1)°, γ ) 108.45(1)°, V ) 1534.8(5) Å3, Z ) 2, number of unique data [I g 3σ(I)] ) 4438,
number of refined parameters ) 388, R ) 0.058. [Cu(1-BB)(DMSO-O)2](BF4)2: triclinic, P1h (No. 2), a ) 9.304(5)
Å, b ) 10.428(4) Å, c ) 16.834(8) Å, R ) 96.85(3)°, â ) 97.25(3)°, γ ) 108.21(2)°, V ) 1517(1) Å3, Z ) 2,
number of unique data [I g 2σ(I)] ) 3388, number of refined parameters ) 397, R ) 0.075. [Cu-
(1-BB)(DMSO-O)(NO2)](ClO4): triclinic, P1h (No. 2), a ) 7.533(2) Å, b ) 8.936(1) Å, c ) 19.168(2) Å, R )
97.66(1)°, â ) 98.62(1)°, γ ) 101.06(1)°, V ) 1234.4(7) Å3, Z ) 2, number of unique data [I g 2σ(I)] ) 3426,
number of refined parameters ) 325, R ) 0.081. [Cu(2-BB)(MeOH)(ClO4)](ClO4): triclinic, P1h (No. 2), a )
8.493(3) Å, b ) 10.846(7) Å, c ) 14.484(5) Å, R ) 93.71(4)°, â ) 103.13(3)°, γ ) 100.61(4)°, V ) 1270(1)
Å3, Z ) 2, number of unique data [I g 2σ(I)] ) 2612, number of refined parameters ) 352, R ) 0.073.
[Cu(2-BB)(N3)](ClO4): monoclinic, P21/n (No. 14), a ) 12.024(3) Å, b ) 12.588(5) Å, c ) 15.408(2) Å, â )
101,90(2)°, V ) 2282(1) Å3, Z ) 4, number of unique data [I g 2σ(I)] ) 2620, number of refined parameters )
311, R ) 0.075. [Cu(2-BB)(NO2)](ClO4)(MeCN): triclinic, P1h (No. 2), a ) 7.402(2) Å, b ) 12.500(1) Å, c )
14.660(2) Å, R ) 68.14(1)°, â ) 88.02(2)°, γ ) 78.61(1)°, V ) 1233.0(4) Å3, Z ) 2, number of unique data [I
g 2σ(I)] ) 2088, number of refined parameters ) 319, R ) 0.070. In all the complexes the 1-BB or 2-BB
ligands coordinate the Cu(II) cations through their three donor atoms. The complexes with 2-BB appear to be
more flexible than those with 1-BB. The nitrito ligand is bidentate in [Cu(2-BB)(NO2)](ClO4)(MeCN) and
essentially monodentate in [Cu(1-BB)(DMSO-O)(NO2)](ClO4). The copper(I) complexes exhibit nitrite reductase
-
activity and react rapidly with NO2 in the presence of stoichiometric amounts of acid to give NO and the
corresponding copper(II) complexes. Under the same conditions the reactions between the copper(I) complexes
and NO+ yield the same amount of NO, indicating that protonation and dehydration of bound nitrite are faster
than its reduction. The NO evolved from the solution was detected and quantitated as the [Fe(EDTA)(NO)]
complex. The order of reactivity of the Cu(I) complexes in the nitrite reduction process is [Cu(2-BB)]+ > [Cu-
(1-BB)]+ > [Cu(TB)]+ > [Cu(AcBB)]+.
Introduction
family of denitrifying bacteria found in soils and sediments.3
These bacteria couple the anaerobic, stepwise reduction of
The complex pattern of reactions undergone by inorganic
nitrogen compounds in the environment is of vital importance
for the ecological equilibrium on Earth.1 In nitrogen fixation,
dinitrogen is reduced to ammonia in a reaction catalyzed by a
multimetal enzyme containing iron and molybdenum.2 Dini-
trogen is released into the atmosphere by the activity of a broad
oxidized nitrogen compounds to oxidative phosphorylation, in
a respiratory chain analogous to the dioxygen consumption
process occurring in aerobic organisms. All steps of biological
denitrification depend on metal enzymes,4 iron and copper
enzymes being of particular importance. Nitrite reductases
belonging to two different classes, containing heme cd1 or
copper as cofactor, are key enzymes in this process. The recent
X-ray crystal structure determination of the copper nitrite
X Abstract published in AdVance ACS Abstracts, January 15, 1996.
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0020-1669/96/1335-1101$12.00/0 © 1996 American Chemical Society

1102 Inorganic Chemistry, Vol. 35, No. 5, 1996
Casella et al.
reductase from Achromobacter cycloclastes has shown that the
functional unit contains one type 1 copper center and one type
2 copper center separated by 12.5 Å.5 The role of the type 1
copper is to transfer an electron from reduced pseudoazurin to
the type 2 copper,6 which is the site of nitrite binding7 and
reduction.8 Sequence identity and low-angle X-ray scattering
measurements suggest that many other copper nitrite reductases
conform to the same structural features.9
Chart 1
The economic and ecological importance of denitrification,
with the prominent role played by copper enzymes in the
transformation of NxOy species, and the number of studies in
which the reaction between these species and other copper
proteins, such as hemocyanin,10 tyrosinase,11 laccase,12 cerulo-
plasmin,13 ascorbate oxidase,11 and cytochrome c oxidase,14 is
poorly understood have stimulated recent interest in the interac-
tion of NxOy species with synthetic copper complexes. In
particular, a few copper(II)-nitrite15 and copper(I)-nitrite16
adducts of potential relevance as models for the copper nitrite
reductase-substrate complexes have been reported, as well as
the reactions between mononuclear and dinuclear copper(I)
complexes and NO17,18 and the observation that on addition of
acid the Cu(I)-NO2- adduct releases NO.16b In this paper we
describe a series of copper(I) and copper(II) complexes with
the nitrogen donor ligands reported in Chart 1 that allow some
systematic investigation of the copper nitrite reductase biomi-
metic chemistry. In particular, we report the characterization
of the adducts of the copper(II) complexes with nitrite and azide
(an efficient inhibitor of the enzyme from A. cycloclastes),20
including several X-ray structures, and the reactivity of the
copper(I) complexes in the two steps of the nitrite reductase
reaction, (1).
(5) (a) Godden, J. W.; Turley, G.; Teller, D. C.; Adman, E. T.; Liu, M.
Y.; Payne, W. J.; LeGall, J. Science 1991, 153, 438. (b) Adman, E.
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D., Tyekla´r, Z., Eds.; Chapman & Hall: New York, 1993; p 397.
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Adman, E. T.; Horinouchi, S.; Beppu, T. Biochemistry 1994, 33, 5246.
(b) Suzuki, S.; Kohzuma, T.; Deligeer; Yamauchi, K.; Nakamura, N.;
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11145.
Results and Discussion
(7) (a) Howes, B. D.; Abraham, Z. H. L.; Lowe, D. J.; Bru¨ser, T.; Eady,
R. R.; Smith, B. E. Biochemistry 1994, 33, 3171. (b) Kohzuma, T.;
Shidara, S.; Suzuki, S. Bull. Chem. Soc. Jpn. 1994, 67, 138.
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1529.
(9) (a) Fenderson, F.; Kumar, S.; Adman, E. T.; Liu, M. Y.; Payne, W.
J.; LeGall, J. Biochemistry 1991, 30, 7180. (b) Glockner, A. B.; Ju¨ngst,
A.; Zumft, W. G. Arch. Microbiol. 1993, 160, 18. (c) Nishiyama, M.;
Suzuki, J.; Kukimoto, M.; Ohnuki, T.; Horinouchi, S.; Beppu, T. J.
Gen. Microbiol. 1993, 139, 725. (d) Ye, R. W.; Fries, M. R.;
Bezborodniknov, S. G.; Averill, B. A.; Tiedje, J. M. Appl. EnViron.
Microbiol. 1993, 59, 250. (e) Grossman, J. G.; Abraham, Z. H. L.;
Adman, E. T.; Neu, M.; Eady, R. R.; Smith, B. E.; Hasnain, S. S.
Biochemistry 1993, 32, 7360.
(10) (a) Salvato, B.; Giacometti, G. M.; Beltramini, M.; Zilio, F.;
Giacometti, G.; Magliozzo, R. S.; Peisach, J. Biochemistry 1989, 28,
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W. B.; Solomon, E. I. J. Am. Chem. Soc. 1989, 111, 6106.
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1987, 916, 38.
Synthesis. The attempt to mimic the tris(imidazole) array
found in the type 2 copper center of A. cycloclastes requires
the use of tridentate ligands with nitrogen donors. Those
employed in the present investigation are a series of poly-
(benzimidazole) derivatives shown in Chart 1. The ligands
2-BB and TB were reported previously by us,21,22 while 1-BB
was obtained by reaction of iminodiacetic acid and N-methyl-
o-diaminobenzene. The acetylated ligand AcBB22 was used for
comparison purposes, as a ligand with lower denticity, since
N-acetylation prevents the central nitrogen atom of the chain
from acting as a donor. The basic difference between 1-BB
and 2-BB is the size of the chelate rings that they form upon
coordination of a metal ion, which is smaller (five- Vs six-
membered) in the former case. We23 and others24 have already
observed that this effect has an influence on the structure,
spectra, and reactivity of copper complexes.
(12) Martin, C. T.; Morse, R. H.; Kanne, R. M.; Gray, H. B.; Malmstro¨m,
B. G.; Chan, S. I. Biochemistry 1981, 20, 5147.
(13) Musci, G.; Di Marco, S.; Bonaccorsi di Patti, M. C.; Calabrese, L.
Biochemistry 1991, 30, 9866.
(14) Brudvig, G. W.; Stevens, T. H.; Chan, S. I. Biochemistry 1980, 19,
5275.
All the ligands readily form 1:1 complexes with copper(I) or
copper(II). Those of copper(I) are air sensitive, especially in
solution. The complexes [Cu(1-BB)]+ and [Cu[(2-BB)]+ are
(15) (a) Tolman, W. B. Inorg. Chem. 1991, 30, 4877. (b) Jiang, F.; Conry,
R. R.; Bubacco, L.; Tyekla´r, Z.; Jacobson, R. R.; Karlin, K. D.;
Peisach, J. J. Am. Chem. Soc. 1993, 115, 2093. (c) Komeda, N.; Nagao,
H.; Adachi, G.; Suzuki, M.; Uehara, A.; Tanaka, K. Chem. Lett. 1993,
1521.
(16) (a) Halfen, J. A.; Mahapatra, S.; Olmstead, M. M.; Tolman, W. B. J.
Am. Chem. Soc. 1994, 116, 2173. (b) Halfen, J. A.; Tolman, W. B. J.
Am. Chem. Soc. 1994, 116, 5475.
(17) (a) Carrier, S. M.; Ruggiero, C. E.; Tolman, W. B. J. Am. Chem. Soc.
1992, 114, 4407. (b) Ruggiero, C. E.; Carrier, S. M.; Antholine, W.
E.; Whittaker, J. W.; Cramer, C. J.; Tolman, W. B. J. Am. Chem.
Soc. 1993, 115, 11285. (c) Ruggiero, C. E.; Carrier, S. M.; Tolman,
W. B. Angew. Chem., Int. Ed. Engl. 1994, 33, 895.
(18) Paul, P. P.; Karlin, K. D. J. Am. Chem. Soc. 1991, 113, 6331.
(19) Paul, P. P.; Tyekla´r, Z.; Farooq, A.; Karlin, K. D.; Liu, S.; Zubieta, J.
J. Am. Chem. Soc. 1990, 112, 2430.
(20) Hulse, C. L.; Averill, B. A.; Tiedje, J. M. J. Am. Chem. Soc. 1989,
111, 2322.
(21) Casella, L.; Gullotti, M.; Radaelli, R.; Di Gennaro, P. J. Chem. Soc.,
Chem. Commun. 1991, 1611.
(22) Casella, L.; Monzani, E.; Gullotti, M.; Gliubich, F.; De Gioia, L. J.
Chem. Soc., Dalton Trans. 1994, 3203.
(23) (a) Casella, L.; Gullotti, M.; Carugo, O.; Garofani, S.; Zanello, P. Inorg.
Chem. 1993, 32, 2056. (b) Casella, L.; Gullotti, M. In Bioinorganic
Chemistry of Copper; Karlin, K. D., Tyekla´r, Z., Eds.; Chapman &
Hall: New York, 1993; p 292.
(24) (a) Zubieta, J.; Karlin, K. D.; Hayes, J. C. In Copper Coordination
Chemistry: Biochemical and Inorganic PerspectiVes; Karlin, K. D.,
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D. J. Am. Chem. Soc. 1993, 115, 2677.

Model Complexes of Copper Nitrite Reductase
Inorganic Chemistry, Vol. 35, No. 5, 1996 1103
stereochemistry of these complexes can be described with a
sulfoxide oxygen atom (O2) in the apical position and with the
four basal positions occupied by the imidazole (N1 and N3)
and amino (N5) nitrogen atoms of the 1-BB ligand and an
oxygen atom (O1), which belongs to a sulfoxide ligand in the
case of [Cu(1-BB)(DMSO-O)2](ClO4)2 and [Cu(1-BB)(DMSO-
O)2](BF4)2 and to a nitrite ligand in the case of [Cu(1-BB)-
(DMSO-O)(NO2)](ClO4).
[Cu(1-BB)(DMSO-O)2](ClO4)2 and [Cu(1-BB)(DMSO-O)2]-
(BF4)2 are isomorphous and have been submitted to crystal
structure analysis since they were obtained by different reactions
(see below). In these two complexes, a sixth coordinative
position, at the apex of a distorted octahedron, could be occupied
by a monodentate perchlorate [Cu-O1] ) 2.874(4) Å] or by a
monodentate tetrafluoroborate [Cu-F ) 2.849(5) Å]. However,
both the high values of these bond distances and the fact that
the same arrangement is shown by two quite different anions
Figure 1. View of the [Cu(1-BB)(DMSO-O)2]2+ cation in the [Cu-
(1-BB)(DMSO-O)2](ClO4)2 crystal (only S2′ is shown for clarity).
-
(ClO4 and BF4-) suggest that these metal-anion contacts
should be better considered as due to electrostatic stabilization
of the crystal packing.28
If the nitrite oxygen atom O3 is disregarded as a donor in
[Cu(1-BB)(DMSO-O)(NO2)](ClO4), the coordinative bond dis-
tances are very similar in all three complexes. The Cu-O1
one is slightly shorter when O1 belongs to the nitrite ligand
[1.962(6) Å] than when it belongs to a sulfoxide [1.980(4) Å
or 1.982(3) Å]. Other differences among the coordinative bond
distances might be a consequence of different packing forces.
Again disregarding the nitrite oxygen atom O3 in [Cu(1-BB)-
(DMSO-O)(NO2)](ClO4), also the coordinative bond angles
centered on the Cu2+ ion are very similar in all three complexes.
The trans-basal ones range between 168.9(2) and 173.9(3)°
[O1-Cu-O2] and between 157.6(2) and 161.8(2)° [N3-Cu-
N5]. The cis-basal ones are near 90°, though those involving
the two chelate rings are always smaller than the others. The
apical-basal bond angles range between 83.0(2) and 101(2)°.
Bond distances and angles within 1-BB ligands are very similar
in all three complexes and fall in the expected range.29 Also
the geometries of the sulfoxide ligands are as expected.30 This
ligand frequently coordinates copper(II) through the oxygen
atom, preferring this coordination mode to the S-coordination.
The DMSO ligand in the apical position is disordered, as occurs
quite often, with the sulfur atom statistically distributed on
opposite sides of the plane defined by the O2, C21, and C22
atoms. The nitrite ligand chelates the Cu2+ ion very asym-
metrically; the Cu-O3 bond [2.678(8) Å] is much longer than
the Cu-O1 one [1.962(6) Å], and consequently also the N-O3
bond [1.20(1) Å] is much shorter than the N-O1 bond [1.25(1)
Å].
Figure 2. View of the [Cu(1-BB)(DMSO-O)(NO2)]+ cation in the [Cu-
(1-BB)(DMSO-O)(NO2)](ClO4) crystal.
likely to be three-coordinate, with a T-shaped geometry similar
to that found for bis(benzimidazole) thioether complexes.25 The
sharp ν(NH) bands occurring in their IR spectra at 3355 and
3310 cm-1, respectively, red-shifted with respect to the free
ligands, indicate that the central nitrogen donor is coordinated.
The [Cu(AcBB)]+ complex appears to be two-coordinate, since
in this case the stretching frequency of the carbonyl group, which
is the only potential donor group in the chain, shows no red
shift with respect to the free ligand. The complex [Cu(TB)]+
1
is also assumed to be three-coordinate, since in its H NMR
spectrum the signals of all methylene groups adjacent to the
benzimidazole rings are downfield shifted to δ 4.1, removing
the accidental degeneracy of the methylene signals of the free
ligand at δ 3.8; also the shape of the aromatic signals is
completely different for the complex. The structure may be
either monomeric or dimeric, as found for the copper(I)
complexes with tris(pyrazolyl)borate ligands.26 The copper(II)
complexes are coordinately unsaturated, and therefore, as shown
by the crystal structures discussed below, they coordinate solvent
molecules or counterions in the solid state and readily form
adducts with exogenous anions.
Structures. (A) Complexes with 1-BB. Three copper(II)
complexes with 1-BB have been structurally characterized: [Cu-
(1-BB)(DMSO-O)2](ClO4)2 (Figure 1), [Cu(1-BB)(DMSO-O)2]-
(BF4)2 (Supporting Information), and [Cu(1-BB)(DMSO-O)-
(NO2)](ClO4) (Figure 2). The first two complexes are five-
coordinated in a distorted square pyramidal stereochemistry.
[Cu(1-BB)(DMSO-O)(NO2)](ClO4) may also be considered
(B) Complexes with 2-BB. Three copper(II) complexes with
2-BB have been characterized: [Cu(2-BB)(MeOH)(ClO4)]ClO4
(Figure 3), [Cu(2-BB)(N3)]ClO4 (Figure 4), and [Cu(2-BB)-
(NO2)]ClO4‚MeCN (Figure 5). All of them are cationic, and
in all cases the 2-BB ligand coordinates the copper(II) cation
through its imidazole (N1 and N3) and amino (N5) nitrogen
atoms. However, both the coordination numbers and stereo-
chemistries of these three complexes are quite different.
In [Cu(2-BB)(MeOH)(ClO4)]ClO4, the metal center is five-
coordinated in a compressed trigonal bipyramidal stereochem-
istry, with the two imidazole nitrogen atoms N1 and N3 in axial
positions and the amino nitrogen atom N5, the perchlorate
-
five-coordinated, since one of the oxygen atoms of the NO2
ligand is quite far from the metal [Cu-O3 ) 2.671(8) Å].27
Thus, also [Cu(1-BB)(DMSO-O)(NO2)](ClO4) can be seen in
a distorted square pyramidal stereochemistry. Therefore, the
(25) Dagdigian, J. V.; McKee, V.; Reed, C. A. Inorg. Chem. 1982, 21,
1332.
(26) (a) Mealli, C.; Arcus, C. S.; Wilkinson, J. L.; Marks, T. J.; Ibers, J.
A. J. Am. Chem. Soc. 1976, 98, 711. (b) Carrier, S. M.; Ruggiero, C.
E.; Houser, R. P.; Tolman, W. B. Inorg. Chem. 1993, 32, 4889.
(27) Hitchman, M. A.; Rowbottom, G. L. Coord. Chem. ReV. 1982, 42,
55.
(28) Rosenthal, M. R. J. Chem. Educ. 1973, 50, 331.
(29) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A.
G. Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1.
(30) Calligaris, M.; Carugo, O. Coord. Chem. ReV., submitted for publica-
tion.

1104 Inorganic Chemistry, Vol. 35, No. 5, 1996
Casella et al.
In [Cu(2-BB)(NO2)]ClO4‚MeCN, the stereochemistry of the
metal center can be described as intermediate between those of
the other two copper(II)/2-BB complexes, since the nitrite ligand
chelates the Cu2+ ion, but with an obviously very small bite.
So, although the coordination number is 5, as in [Cu(2-BB)-
(MeOH)(ClO4)]ClO4, the overall stereochemistry is more similar
to that of the four-coordinated [Cu(2-BB)(N3)]ClO4 complex.
In fact, the bond angles N1-Cu-N3, N1-Cu-N5, and N3-
Cu-N5 have very close, although statistically different, values
in [Cu(2-BB)(NO2)]ClO4‚MeCN and [Cu(2-BB)(N3)]ClO4.
The above results indicate the quite high flexibility of the
2-BB ligand. In particular, it is worth noting that the N1-
Cu-N3 angle can widen from 150.5(3)° in [Cu(2-BB)(N3)]ClO4
or 156.8(3)° in [Cu(2-BB)(NO2)]ClO4‚MeCN to 171.6(3)° in
[Cu(2-BB)(MeOH)(ClO4)]ClO4, without deforming the N1-
Cu-N5 and N3-Cu-N5 angles, which have very similar values
in all three complexes. It is also interesting to observe that the
Cu-X (X ) N1, N3, N5) bond distances are not seriously
affected by the variation of stereochemistry at the metal center.
All these coordinative bonds have statistically equal values and
fall in the expected range.31 Also the bond distances and angles
of the 2-BB ligand do not feel the variation of stereochemistry
at the metal center. The main statistically significant differences
can be explained, at least in part, by the statistical disorder
affecting carbon atom C19 in [Cu(2-BB)(MeOH)(ClO4)]ClO4.
Anyway, all the 2-BB bond distances fall in the expected
Figure 3. View of the [Cu(2-BB)(MeOH)(ClO4)]+ cation in the [Cu-
(2-BB)(MeOH)(ClO4)](ClO4) crystal (only C19a is shown for clarity).
range.29
The geometries of the additional ligands (ClO4-, MeOH, N3
,
-
and NO2-) are as expected. Perchlorate deviates little from a
tetrahedral shape [O-Cl-O angles range between 107.8(5) and
111.2(5)°], but the Cl-O1 bond [1.453(7) Å] is significantly
longer than the other Cl-O bonds [which range between
1.392(9) and 1.409(7) Å], presumably as a consequence of the
coordination of O1 to the metal center.32 Azide is nearly linear
[N6-N7-N8 ) 176.3(9)°], but the two N-N bonds are
statistically equal [1.150(8) and 1.172(9) Å] in spite of the end-
on coordination of N3-, which often causes small differences
in the two bond lengths.33 Both the methanol and azide
molecules are bent by about 120° with respect to their ligands
on copper(II). The chelation of the nitrite ligand is very
asymmetric [Cu-O5 ) 2.439(8) Å, Cu-O6 ) 2.008(6) Å],
Figure 4. View of the [Cu(2-BB)(N3)]+ cation in the [Cu(2-BB)(N3)]-
(ClO4) crystal.
-
with the consequent expected distortion of the NO2 ion from
C2V symmetry [N6-O5 ) 1.23(1) Å, N6-O6 ) 1.29(1) Å].27
(C) Comparison between 1-BB and 2-BB Structures. The
two bis(benzimidazole) ligands 1-BB and 2-BB are closely
similar, the only difference being in the alkyl chain between
the secondary amino group and the heterocyclic moiety, which
contains two carbon atoms in 2-BB and one carbon atom in
1-BB. Such a small difference does not cause important
consequences when only intraligand bond distances and angles
are considered but has important effects on the stereochemistry
of coordination to the metal center. In the complexes with 1-BB
only five-membered chelate rings can be formed, while only
six-membered chelate rings can be formed in the case of 2-BB.
Therefore, the geometry of the interaction between Cu2+ and
the donor atoms is quite different in the two cases. The bond
distances between the Cu2+ ion and the amino nitrogen atom
are shorter in the case of 2-BB than in the case of 1-BB.
Figure 5. View of the [Cu(2-BB)(NO2)]+ cation in the [Cu(2-BB)-
(NO2)](ClO4)(MeCN) crystal.
oxygen atom O1, and the methanol oxygen atom O9 in
equatorial positions. In fact, the N1-Cu-N3 angle [171.6(3)°]
approaches 180°, the equatorial bond angles centered on the
Cu2+ ion range between 114.5(4) and 126.3(4)°, and the axial-
equatorial bond angles range between 86.6(6) and 94.5(3)°.
In [Cu(2-BB)(N3)]ClO4, the metal center is four-coordinated
in a stereochemistry intermediate between square planar and
tetrahedral. In fact, the bond angles centered on the Cu2+ ion
range between 91.1(3) and 150.5(3)°.
(31) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.
G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1.
(32) Gowda, N. M. N.; Naikar, S. B.; Reddy, G. K. N. AdV. Inorg. Chem.
Radiochem. 1984, 28, 255.
(33) (a) Agnus, Y.; Louis, R.; Gisselbrecht, J.-P.; Weiss, R. J. Am. Chem.
Soc. 1984, 106, 93. (b) Felthouse, T. R.; Hendrickson, D. N. Inorg.
Chem. 1978, 17, 444. (c) Dori, Z.; Ziolo, R. F. Chem. ReV. 1973, 73,
249.

Model Complexes of Copper Nitrite Reductase
Inorganic Chemistry, Vol. 35, No. 5, 1996 1105
Moreover, the amino nitrogen atom is more tetrahedral in the
case of 2-BB than in the case of 1-BB {the data for [Cu(2-
BB)(MeOH)(ClO4)]ClO4 being disregarded because of the
statistical disorder of atom C19}. The bond distances between
the Cu2+ ion and the imidazole nitrogen atoms are not
statistically different in 1-BB and 2-BB, but in the latter case
the nitrogen lone pair can point directly at the metal cation,
while in the case of 1-BB the Cu-N bond is bent of about 25°
with respect to the nitrogen lone pair, though in both cases the
cation lies on the imidazole plane. Moreover, the higher
flexibility of a six-membered chelate ring with respect to a five-
membered ring allows a higher relative mobility for the two
benzimidazole moieties within the same ligand. The angles
between the planes of the two benzimidazoles are much more
restricted in the case of the 1-BB complexes, where they range
between 7.1(7) and 12.6(3)°, than in the case of the 2-BB
complexes, where they range between 13.9(6) and 38.1(2)°.
in the former complex the anion is monodentate, while in the
latter it is probably bridging (and the complex binuclear), as
suggested below by the electronic spectrum and the EPR
behavior of the complex. The absence of a detectable νs(N3)
mode in the range ∼1300 cm-1 for [Cu(AcBB)(N3)]ClO4
supports a dimeric formulation for this complex.
The electronic and EPR spectra of the copper(II) complexes
and their nitrite and azide adducts show they have a tendency
to assume a tetragonal field in solution. For most of the
complexes derived from 1-BB and AcBB the presence of a
single d-d band in the optical spectrum, between 630 and 670
nm (Table 1), indicates basically four coordinated species, with
weak axial interaction. On the other hand, the d-d spectra of
the complexes derived from 2-BB and TB generally display
two bands, with a more intense high-energy component, which
suggest square pyramidal structures.35 According to this
interpretation, the coordination mode of nitrite found in the
structures of the adducts [Cu(1-BB)(NO2)]+ and [Cu(2-BB)-
(NO2)]+ would seem to be maintained in solution, and we
presume that nitrite acts as a chelating ligand also in [Cu(TB)-
(NO2)]+ and [Cu(AcBB)(NO2)]+ (where the AcBB ligand is
also bidentate). In the case of the azide adducts, according to
the established trend36 and as shown by the structure of [Cu-
(2-BB)(N3)]+, since the charge of the copper(II) complexes is
dipositive and one of the in-plane positions is accessible,
coordination of the anion occurs equatorially. A solvent
molecule completes the coordination set of five-coordinate
complexes. The general trend in the d-d transitions suggests
that the nitrite ligand lies lower in the spectrochemical series
than azide in these copper(II) complexes.
The EPR data obtained from frozen solutions are consistent
with the above conclusions, since the spectra show the
characteristic pattern for tetragonal stereochemistries (g| > g )
(Table 2). In several cases, the reduced values of the hyperfine
constant |A|| suggest some significant pseudotetrahedral distor-
tion of the tetragonal chromophore. The azide adduct [Cu-
(AcBB)(N3)]+ is completely EPR silent. This behavior was
already noted for the copper(II)-hydroxo complex with the
same ligand22 and suggests the presence of binuclear, dibridged
species in both cases, with strong antiferromagnetic coupling
between the copper(II) centers.
Very important for the description of the spectroscopic
properties of the complexes are the charge transfer bands
occurring in the near-UV region (Table 1). Besides the intense
intraligand absorptions below 300 nm, associated with benz-
imidazole π f π* transitions, the spectra of the copper(II)
complexes display a band near 350 nm and, for those derived
from 1-BB and 2-BB, a detectable shoulder near 300 nm. These
bands can be attributed to π(benzimidazole) f Cu(II)23a,25 and
σ(amino) f Cu(II)37 LMCT transitions, respectively. However,
the ∼350-nm band increases remarkably in intensity in the
series: [Cu(1-BB)]2+ ) [Cu(2-BB)]2+ < [Cu(AcBB)]2+ < [Cu-
(TB)]2+, with a change in extinction coefficient of 1 order of
magnitude between the extremes. For [Cu(AcBB)]2+ and [Cu-
(TB)]2+ this band is flanked by a shoulder at 310 nm that is
missing in the spectra of their adducts with nitrite or azide.
Addition of small amounts of hydroxide to solutions of the
complexes shows that the more intense absorptions observed
for [Cu(AcBB)]2+ and [Cu(TB)]2+ in the range 300-400 are
Another relevant consequence of the higher flexibility of the
six-membered chelated rings with respect to the five-membered
ones concerns the different steric requirements. Though the
structures reported in the present paper could represent erratic
examples, the [Cu(2-BB)]2+ fragment seems to show a higher
variability in expanding the metal coordination number: “true”
coordination numbers 4 and 5 have been observed in the case
of [Cu(2-BB)(N3)]ClO4 and [Cu(2-BB)(MeOH)(ClO4)]ClO4,
respectively, together with a coordination number “intermediate”
between 4 and 5 in the case of [Cu(2-BB)(NO2)]ClO4‚MeCN.
In contrast, the coordination numbers seem restricted to 5 in
the case of the complexes with 1-BB, the only exception being
[Cu(1-BB)(DMSO-O)(NO2)](ClO4), where a coordination num-
ber intermediate between 5 and 6 (but closer to 5, because of
the basically monodentate character of the nitrito ligand) could
be assigned. It seems also reasonable to suppose that the
different steric requirements of the [Cu(2-BB)]2+ and [Cu(1-
BB)]2+ fragments are one of the reasons that in the case of [Cu-
(2-BB)(NO2)]ClO4‚MeCN the nitrite anion is more symmetri-
cally chelated to Cu2+ than in the case of [Cu(1-BB)(DMSO-
O)(NO2)](ClO4).
Spectroscopy of Cu(II) Complexes. The difference in the
mode of binding of nitrite to the [Cu(1-BB)]2+ and [Cu(2-BB)]2+
centers is reflected by the vibration modes of the anions in the
adducts [Cu(1-BB)(NO2)]ClO4 and [Cu(2-BB)(NO2)]ClO4. In
the former case the two ν(NO2) stretchings are well separated,
ν(NdO) at 1390 cm-1 and ν(N-O) buried under the broad and
intense perchlorate band at 1100 cm-1, while for the chelating
nitro group of the second complex the two bands are closer in
energy, νa(NO2) occurring at 1334 cm-1 and νs(NO2) at 1265
cm-1, in agreement with expectation.27,34a The IR spectra of
[Cu(TB)(NO2)]ClO4 and [Cu(AcBB)(NO2)]ClO4 suggest that
a chelating nitro group is present also in these adducts. The
νa(NO2) mode occurs at 1383 cm-1 (shifted to 1360 cm-1 for
the 15NO2- adduct) in the former case and at 1360 cm-1 (1338
-
cm-1 for the 15NO2 adduct) in the second case, while the νs-
(NO2) mode occurs at 1210 cm-1 (red-shifted to ∼1200 and
almost submerged under the broad perchlorate band for the
-
-
15NO2 adduct) and 1245 cm-1 (1241 cm-1 in the 15NO2
adduct), respectively. The main IR signature of coordinated
azide is little sensitive to the mode of binding of the anion.34 In
the spectra of the present series of azide adducts, the νa(N3)
mode occurs in the range 2054-2110 cm-1. Though it is almost
coincident for [Cu(2-BB)(N3)]ClO4 and [Cu(AcBB)(N3)]ClO4,
(35) (a) Hathaway, B. J. Struct. Bonding 1984, 57, 55. (b) Hathaway, B. J.
In ComprehensiVe Coordination Chemistry; Wilkinson, G., Ed.; Perga-
mon: New York, 1987; Vol. 5, p 533.
(34) (a) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds, 4th ed.; Wiley: New York, 1986; Part III.
(b) Karlin, K. D.; Hayes, J. C.; Hutchinson, J. P.; Zubieta, J. J. Chem.
Soc., Chem. Commun. 1983, 376.
(36) Casella, L.; Gullotti, M.; Pallanza, G.; Buga, M. Inorg. Chem. 1991,
30, 221.
(37) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier:
Amsterdam, 1984; Chapter 5.

1106 Inorganic Chemistry, Vol. 35, No. 5, 1996
Casella et al.
Table 1. Electronic Spectral Data for Copper(II) Complexes in MeCN Solution
λ
max, nm (ꢀ, M-1 cm-1
)
intraligand
CT
d-d
complex
[Cu(1-BB)]2+
244
(16 900)
246
(13 600)
244
(14 100)
248
(16 300)
246
(14 300)
246
(14 800)
250
(14 800)
250
(14 500)
252
(14 000)
252
(22 100)
254
(20 000)
246
272
(14 300)
272
(15 300)
272
(14 300)
272
(13 300)
272
(15 000)
272
(13 900)
274
(13 200)
272
(13 200)
272
(16 700)
274
(24 000)
276
(23 500)
274
280
(14 200)
280
(15 100)
280
(14 400)
280
(11 700)
280
(11 500)
280
(12 450)
280
(12 100)
280
(12 600)
282
(15 100)
280
(22 000)
280
(20 000)
282
300 sh
(350)
350
(270)
364
(1400)
392
(2700)
375
(260)
372
(450)
420
(1800)
342
(800)
636
(135)
630
(120)
622
(275)
676
(190)
670
(210)
652
(500)
670
(45)
674
(80)
656
(180)
710
(85)
645
(30)
660
[Cu(1-BB)(NO2)]+
[Cu(1-BB)(N3)]+
[Cu(2-BB)]2+
300 sh
(1400)
300 sh
(1500)
300 sh
(650)
300 sh
(700)
300 sh
(800)
730 sh
(210)
790
(170)
750 sh
(170)
735 sh
(450)
[Cu(2-BB)(NO2)]+
[Cu(2-BB)(N3)]+
[Cu(AcBB)]2+
310 sh
(1000)
374
[Cu(AcBB)(NO2)]
[Cu(AcBB)(N3)]+
[Cu(TB)]2+
(430)
370 sh
(1500)
310 sh
(2200)
370
(400)
370
436
(1400)
364
500 sh
(800)
870
(65)
870
(20)
(2700)
[Cu(TB)(NO2)]+
[Cu(TB)(N3)]+
440
(22 300)
(25 000)
(22 500)
(2200)
(2130)
(280)
of complexes, we find that λmax for the π(N3-) f Cu(II) LMCT
transition increases in the order [Cu(1-BB)(N3)]+ < [Cu(2-BB)-
(N3)]+ < [Cu(AcBB)(N3)]+ ∼ [Cu(TB)(N3)]+. This suggests
that the ligand 1-BB has higher donor strength than 2-BB, in
agreement with expectation (it forms five-membered rather than
six-membered chelate rings) and the higher-energy position of
the d-d bands of its complexes. On the basis of the above
criterion, we would expect the AcBB complex to occupy the
highest position in the series. However, the azide-Cu(II)
LMCT band of this complex shows multicomponent structure,
and together with the EPR silence of the adduct [Cu(AcBB)-
(N3)]+ (Table 2) discussed above, this indicates the compound
is dimeric, with a double azide bridge.
Table 2. EPR Parameters of Copper(II) Complexes in Frozen
MeCN Solutions at -150 °C
complex
g|
g
10-4|A|| (cm -1
)
[Cu(1-BB)]2+
2.273
2.261
2.256
2.060
2.065
2.058
170
173
173
[Cu(1-BB)(NO2)]+
[Cu(1-BB)(N3)]+ a
[Cu(2-BB)]2+
b
[Cu(2-BB)(NO2)]+
2.259
2.259
2.090
2.089
143
139
[Cu(2-BB)(N3)] +
[Cu(AcBB)]2+
2.332
2.279
2.065
2.057
(EPR silent)
144
160
[Cu(AcBB)(NO2)] +
[Cu(AcBB)(N3)] +
[Cu(TB)]2+
2.335
2.296
2.298
2.078
2.064
2.066
148
170
159
[Cu(TB)(NO2)] +
[Cu(TB)(N3)] +
Of greatest interest in the present context is the analysis of
the near-UV region of the copper(II)-nitrite complexes, since
very little is known about the optical properties of this
chromophore. Free nitrite absorbs very weakly in the near-
UV (λmax ) 352 nm, ꢀ ) 26 M-1 cm-1 in methanol solution).
The nitrite complexes reported here exhibit a band near 370
nm that is systematically more intense than that due to the
benzimidazole to copper(II) LMCT (Table 1). As already noted,
[Cu(AcBB)]2+ and [Cu(TB)]2+ are apparent exceptions because
partial formation of the hydroxide complexes leads to abnormal
increase of absorption in the same region. For the adducts [Cu-
(2-BB)(NO2)]+, [Cu(AcBB)(NO2)]+, and [Cu(TB)(NO2)]+, the
intensities of the 370-nm band are very similar (ꢀ ) 400-450
M-1 cm-1) and suggest that an intensity contribution of about
200 M-1 cm-1 is due to the bound nitrite. Interestingly, for
[Cu(1-BB)(NO2)]+ the intensity of this band is much higher (ꢀ
∼ 14 00 M-1 cm-1). We therefore propose that the origin of
the 370-nm band is a copper(II)-nitrite charge transfer transition
and that the intensity difference observed here for the two groups
of nitrite adducts correlates with the mode of binding of the
anion, O,O-bidentate chelate nitrite giving rise to weak absorp-
tion and O-monodentate nitrite to moderately intense absorption
in the near-UV. Both the possibilities of n(NO2) f d(Cu)
LMCT and d(Cu) f π*(NO2) MLCT assignments apparently
a In MeCN-MeOH. b Mixture of species.
Table 3. Nitrite Reductase Activity of Copper(I) Complexes at
Room Temperature in MeCN Solution
reacn time
(min)
% NO detected as
[Fe(EDTA)(NO)]
system
[Cu(1-BB)]+ + NO2- + 2H+
5
5
5
5
5
5
5
5
5
5
5
15
15
44
47
65
[Cu(1-BB)]+ + NO+
[Cu(2-BB)]+ + NO2- + 2H+
[Cu(2-BB)]+ + NO+
66
-
[Cu(2-BB)]+ + NO2
∼0
[Cu(AcBB)]+ + NO2- + 2H+
[Cu(AcBB)]+ + NO+
25
27
31
33
15
<5
<1
<1
[Cu(TB)]+ + NO2- +2H+
[Cu(TB)]+ + NO+
[Cu(MeCN)4]+ + NO2- +2H+
NO2- + 2H+
[Cu(2-BB)]2+ + NO2- + 2H+
[Cu(2-BB)(NO2)]+ + 2H+
in fact due to copper(II)-hydroxo complexes formed by the
dissociation of bound water (∆ꢀ ∼ 2000 M-1 cm-1 at [Cu(II)]:
[OH-] ) 1:1). This behavior was already noted for the
previously reported [Cu(TB)]2+ complex,38 where the EPR-silent
2+
dimeric complex [Cu(TB)(OH)]2 is formed.
The azide complexes display the characteristic LMCT band
near 400 nm. The position of this band correlates roughly with
the effective charge on the Cu(II) center.36 In the present series
(38) Alzuet, G.; Casella, L; Perotti, A.; Borras, J. J. Chem. Soc., Dalton
Trans. 1994, 2347.

Model Complexes of Copper Nitrite Reductase
Inorganic Chemistry, Vol. 35, No. 5, 1996 1107
Table 4. Crystal Data and Experimental Details for the Crystallographic Analyses of [Cu(2-BB)(MeOH)(ClO4)](ClO4) (I),
[Cu(2-BB)(N3)](ClO4) (II), [Cu(2-BB)(NO2)](ClO4) (III), [Cu(1-BB)(DMSO-O)2](ClO4)2 (IV), [Cu(1-BB)(DMSO-O)2](BF4)2 (V), and
[Cu(1-BB)(DMSO-O)(NO2)](ClO4) (VI)
I
II
III
IV
V
VI
formula
fw
system
space group
a (Å)
C21H27Cl2CuN5O9 C20H23ClCuN8O4
C22H26ClCuN7O6
583.49
triclinic
C22H31Cl2CuN5O10S2 C22H31B2CuF8N5O2S2 C20H28ClCuN6O7S
627.92
538.45
724.09
698.80
triclinic
P1h (No. 2)
9.304(5)
10.428(4)
16.834(8)
96.85(3)
97.25(3)
108.21(2)
2
592.54
triclinic
P1h (No. 2)
7.533(2)
8.936(1)
19.168(2)
97.66(1)‘
98.62(1)
101.06(1)
2
triclinic
P1h (No. 2)
8.493(3)
10.846(7)
14.484(5)
93.71(4)
103.13(3)
100.61(4)
2
monoclinic
P21/n (No. 14)
12.024(3)
12.588(5)
15.408(2)
90
101.90(2)
90
4
triclinic
P1h (No. 2)
9.400(1)
10.494(2)
16.760(2)
96.67(1)
97.10(1)
108.45(1)
2
P1h (No. 2)
7.402(2)
12.500(1)
14.660(2)
68.14(1)
88.02(2)
78.61(1)
2
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
Z
V (Å3)
1270(1)
1.643
2282(1)
1.567
1233.0(4)
1.572
1534.8(5)
1.567
1517(1)
1.530
1234.4(7)
1.602
D
calc (Mg m-3
)
cryst size (mm)
crystal descripn
no. of refl meas
no. of unique refl
no. of obs refl
criterion for obs
Rint
0.25 × 0.15 × 0.05 0.10 × 0.10 × 0.25 0.11 × 0.06 × 0.03 0.55 × 0.45 × 0.36
0.30 × 0.20 × 0.20
0.20 × 0.20 × 0.10
plate
needle
4777
plate
prism
7215
prism
prism
4924
4264
2612
I > 2σ(I)
0.038
70
4813
4427
2088
I > 2σ(I)
0.033
70
7224
5068
3388
I > 2σ(I)
0.041
70
-11,11
-12,12
-3,20
0.075
0.076
5513
4272
3426
I > 2σ(I)
0.041
70
-9,9
-10,10
-3,23
0.081
0.111
1.790
325
4126
2620
I > 2σ(I)
0.039
70
5482
4438
I > 2σ(I)
0.029
70
θ
max (deg)
min,hmax
min,kmax
h
0,10
-1,14
0,9
-11,11
-12,12
-3,20
0.058
0.060
1.358
388
k
-13,13
-17,17
0.073
0.053
1.144
0,15
-15,15
-17,17
0.070
0.073
1.462
319
l
min,lmax
-18,18
0.075
0.074
1.725
311
Ra
b
Rw
GOFc
2.134
397
no. of refined param 352
2
a R ) ∑(|Fo| - |Fc|)/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑|Fo| ]1/2
.
c GOF ) [∑(|Fo| - |Fc|)2/(No - Nv)]1/2
.
exist for the near-UV band,37 as discussed for the nitrite
complexes of other metal ions.
same amount of NO was detected in each case, indicating that
protonation and dehydration of bound nitrite, according to the
first step of reaction 1, must be faster than its reduction. As
expected, neither the copper(II) complexes nor the preformed
copper(II)-nitrite adducts are active in the nitrite reductase
reaction, confirming the absence of nonspecific reactions in the
Nitrite Reductase Reactivity. In the attempt to mimic the
nitrite reduction process occurring at the type 2 center of the
enzyme, some preliminary experiments were carried out by
reacting the copper(I) complexes with NaNO2 anaerobically.
When diluted solutions of the reagents in methanol were used,
a slow reaction occurred to produce green copper(II) species.
In dry acetonitrile the reaction was very slow unless small
amounts of acid were added. Under these conditions, a rapid
evolution of NO took place. In order to prevent the occurrence
of complex reactions between nitrogen oxide species and the
Cu(I)/Cu(II) couple, we found it useful to apply a weak stream
of inert gas to remove the NO produced in the reaction from
contact with the solution. On passing the gas stream through a
cool and anaerobic Fe(EDTA) solution, it was possible to detect
and quantitate NO as the strong [Fe(EDTA)(NO)] complex.39
Under the gas flow conditions employed, the recovery of NO
is obviously lower than the theoretical amount; therefore, the
yields of NO were determined relative to standard nitrite-
ascorbate solutions. Some comparative data on the reactivity
of the various copper(I) complexes in the nitrite reduction are
reported in Table 3. Relatively short reaction times were
considered because the prolonged flow of inert gas through the
Fe(EDTA) solution led to a loss of [Fe(EDTA)(NO)] adduct.
Thus, the order of reactivity found
-
copper(I) systems and strong binding of NO2 to the Cu(II)
centers.
The Cu(II) species produced in the nitrite reductase reactions
described above are the [Cu(ligand)]2+ complexes. This was
indicated by the UV-vis spectral properties of the final reaction
solutions and confirmed by isolating the product of the reaction
between [Cu(1-BB)]PF6, NaNO2, and HBF4. The formulation
of this product corresponded to [Cu(1-BB)](BF4)(PF6), which
upon crystallization from a DMSO solution, deposited crystals
of [Cu(1-BB)(DMSO-O)2](BF4)2, whose structure was discussed
in a previous section. We have tried to observe some reaction
intermediates by following the reactions between either the Cu(I)
-
species and NO2 + 2H+ or between Cu(I) species and NO+
at lower temperature (-20 to -40 °C) through EPR, after
freezing the solutions in liquid nitrogen. In some instances, in
the initial phase of the reaction it was possible to observe a
weak and structured EPR signal (∼12 G), superimposed on more
prominent Cu(II) signals, due to some intermediate species that
we are currently trying to characterize. Several attempts were
also made to isolate nitrite adducts of the copper(I) complexes,
but we have been unable to obtain them in sufficiently pure
form, even working at low temperature. The necessity to use
nonprotic but polar solvents like acetonitrile to dissolve the Cu(I)
complexes is a problem because the affinity of nitrite for the
Cu(I) species is low and using an excess of nitrite salt inevitably
leads to partial oxidation of the copper(I) species.
[Cu(2-BB)]+ > [Cu(1-BB)]+ > [Cu(TB)]+ > [Cu(AcBB)]+
indicates relative rates rather than overall chemical conversions.
Blank experiments showed that the amount of NO produced in
the absence of the Cu(I) complexes was almost negligible, while
some nitrite reductase activity was exhibited by [Cu(MeCN)4]-
PF6. Interestingly, by reacting the copper(I) complexes with
NO+BF4- under the same conditions as with NO2- + 2H+, the
Conclusion. We have reported a series of complexes of
potential interest in the context of copper nitrite reductase
biomimetic chemistry. The copper(II) complexes form O-
bonded nitrite adducts. This seems a common characteristic
(39) Fanning, J. C. Coord. Chem. ReV. 1991, 110, 235.

1108 Inorganic Chemistry, Vol. 35, No. 5, 1996
Casella et al.
Table 6. Selected Positional Parameters and Their Estimated
Table 5. Selected Positional Parameters and Their Estimated
Standard Deviations for [Cu(1-BB)(DMSO-O)2](ClO4)2
Standard Deviations for [Cu(1-BB)(DMSO-O)2](BF4)2
atom
x
y
z
Beqa (Å2)
atom
x
y
z
Beqa (Å2)
Cu
S1
S2
S2′
O1
O2
N1
N2
N3
N4
N5
C1
C2
C3
C4
C5
C6
-0.01479(8)
-0.2114(1)
-0.1192(3)
-0.2240(4)
-0.2098(4)
-0.0950(5)
0.2059(4)
0.1386(5)
-0.0012(4)
0.2244(5)
0.0415(4)
0.2681(5)
0.1371(5)
0.2717(7)
-0.0088(5)
-0.0964(5)
-0.2530(6)
-0.3170(6)
-0.2291(6)
-0.0736(6)
0.2981(5)
0.73649(7)
0.5491(1)
0.9631(3)
0.9045(3)
0.6060(3)
0.8409(4)
0.8565(4)
1.0932(4)
0.8874(4)
0.5485(4)
0.6083(4)
0.9507(5)
0.9795(5)
1.2165(5)
1.0745(5)
0.9450(4)
0.8944(5)
0.9779(6)
1.1062(5)
1.1570(5)
0.7724(5)
0.6425(5)
0.5515(6)
0.4471(5)
0.4840(5)
0.4053(5)
0.2886(6)
0.2542(5)
0.3308(6)
0.3942(6)
0.4822(7)
0.914(1)
0.74846(4)
0.84512(8)
0.6141(2)
0.6602(2)
0.7647(2)
0.6497(3)
0.7474(3)
0.9023(3)
0.8340(2)
0.6261(3)
0.6747(2)
0.8250(4)
0.8542(3)
0.9319(4)
0.9135(3)
0.8707(3)
0.8682(3)
0.9108(4)
0.9531(4)
0.9561(3)
0.7232(4)
0.6750(3)
0.6150(4)
0.5917(3)
0.6220(3)
0.5982(3)
0.5406(3)
0.5097(4)
0.5343(4)
0.8234(4)
0.8488(4)
0.594(1)
3.69(1)
4.19(3)
4.70(6)
5.11(8)
4.23(8)
7.6(1)
4.08(9)
3.96(9)
3.66(9)
4.6(1)
3.88(9)
4.5(1)
3.8(1)
5.6(2)
3.8(1)
3.6(1)
4.3(1)
5.1(1)
5.0(1)
4.7(1)
4.6(1)
4.0(1)
6.4(2)
4.4(1)
4.0(1)
4.5(1)
5.3(1)
5.7(2)
5.6(1)
5.6(2)
6.1(2)
17.6(5)
8.7(2)
Cu
S1
S2
S2′
O1
O2
N1
N2
N3
N4
N5
C1
C2
C3
C4
C5
C6
0.9854(1)
0.7918(2)
0.7706(4)
0.8827(6)
0.7910(5)
0.8970(2)
1.2053(6)
1.1421(7)
1.0026(6)
1.2253(7)
1.0419(6)
1.2693(9)
1.1412(8)
1.277(1)
0.9965(8)
0.9071(8)
0.7514(8)
0.6888(9)
0.780(1)
0.9334(9)
1.2997(9)
1.1890(8)
1.374(1)
1.0894(9)
0.9711(9)
0.8164(9)
0.786(1)
0.902(1)
1.053(1)
0.834(1)
0.597(1)
0.907(1)
0.685(1)
0.2366(1)
0.0494(2)
0.4019(4)
0.4620(5)
0.1058(5)
0.3331(5)
0.3540(6)
0.5954(5)
0.3895(5)
0.0453(6)
0.1068(5)
0.4513(7)
0.4825(7)
0.7199(8)
0.5782(7)
0.4471(6)
0.3990(7)
0.4820(8)
0.6117(8)
0.6608(7)
0.2711(8)
0.1385(7)
0.0441(9)
-0.0549(7)
-0.0177(7)
-0.0986(7)
-0.2145(8)
-0.2495(8)
-0.1729(8)
-0.1034(8)
-0.0208(9)
0.5910(9)
0.409(1)
0.24828(7)
0.3463(1)
0.1579(2)
0.1149(3)
0.2664(3)
0.1495(3)
0.2453(4)
0.4012(4)
0.3334(3)
0.1258(4)
0.1752(3)
0.3225(5)
0.3534(4)
0.4317(5)
0.4152(4)
0.3713(4)
0.3710(5)
0.4154(5)
0.4578(5)
0.4581(5)
0.2223(5)
0.1745(4)
0.1130(6)
0.0915(4)
0.1219(4)
0.0977(4)
0.0394(5)
0.0075(5)
0.0325(5)
0.3239(6)
0.3506(6)
0.1891(6)
0.077(1)
4.11(3)
4.77(5)
6.2(1)
6.4(1)
4.5(1)
7.5(2)
4.3(2)
4.4(2)
3.9(1)
5.3(2)
4.2(1)
5.2(2)
4.2(2)
6.6(3)
4.1(2)
3.8(2)
4.8(2)
5.8(2)
6.0(2)
4.9(2)
5.3(2)
4.5(2)
7.4(3)
5.0(2)
4.6(2)
5.1(2)
6.1(2)
6.8(3)
6.4(2)
6.6(3)
7.3(3)
9.9(3)
18.0(6)
C7
C8
C9
C7
C8
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
C21
C22
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
C21
C22
0.1890(5)
0.3762(7)
0.0885(6)
)0.0266(6)
-0.1789(6)
-0.2097(7)
-0.0917(8)
0.0577(7)
-0.1742(7)
-0.4084(6)
-0.315(1)
-0.083(1)
1.0936(7)
0.6894(4)
a Beq ) (1/3)∑i∑jBijai*aj*ai‚aj.
a Beq ) (1/3)∑i∑jBijai*aj*ai‚aj.
of copper(II) complexes with nitrogen donor ligands15,17c and
is actually the mode in which the substrate binds to copper(II)
in nitrite reductase.7a Whether the bidentate chelate or mono-
dentate coordination type is adopted seems largely dictated by
steric factors, the chelation mode being preferred in the absence
of steric restrictions. Besides the IR technique, also the near-
reactivity of the enzyme.20,40 From the data in Table 3 it is
evident that the increase in the number of donor groups in the
ligand enhances the activity of the Cu(I) complex in the reaction,
-
in spite of the lower accessibility of the Cu(I) center to NO2
or NO+. It is also clear that the increase of donor strength of
the nitrogen donors (1-BB or 2-BB Vs TB) facilitates electron
transfer from Cu(I) to the bound NO+ residue. In all these
regards, the tris(imidazole) donor set for the type 2 Cu site of
the enzyme represents an optimal arrangement. The higher
reactivity of [Cu(2-BB)]+ with respect to [Cu(1-BB)]+ must be
attributed to the higher flexibility of the ligand, a requisite that
is certainly important to accommodate the strucutral changes
involved in the complex upon the Cu(I) f Cu(II) redox change.
-
UV charge transfer band associated with the Cu(II)-NO2
chromophore may be sensitive to the coordination type of the
anion. The mode of nitrite binding to the copper(I) complexes
with the same ligands may be different, though, and this problem
remains unsolved at the moment. Tolman and co-workers found
-
that on treatment of an excess of NO2 with the copper(I)
complex of a triazacyclononane ligand in methanol, a dimeric
adduct containing a µ2-(η1-N:η1-O) nitrite was formed,16a and
Experimental Section
-
this originated a mononuclear N-bonded Cu(I)-NO2 adduct
All reagents from commercial sources were of the highest purity
available and were used as received. Acetonitrile for spectroscopic
and reactivity measurements was successively distilled from potassium
permanganate, sodium carbonate, and calcium hydride. The ligands
bis[2-(1-methylbenzimidazol-2-yl)ethyl]amine (2-BB), N-acetyl-2-BB
(AcBB), and tris[2-(1-methylbenzimidazol-2-yl)ethyl]nitromethane (TB)
were prepared as described previously.21,22 [Cu(MeCN)4]PF6 was
prepared by a literature method,41 while the copper(II) complex [Cu-
(TB)][ClO4]2 was also reported recently by us.38 The synthesis and
manipulation of the copper(I) complexes were performed with Schlenk
techniques. Elemental analyses were obtained at the microanalytical
laboratory of the Chemistry Department in Milan. Infrared spectra were
recorded on a Jasco FT-IR-5000 instrument. NMR spectra were
recorded on Bruker WP-80 and AC-200 spectrometers, operating at
by treatment with PPh3.16b The present copper(I) complexes
are apparently much more reactive toward nitrite, and isolation
of their adduct appears difficult. On the other hand, the data
we currently have on the Cu(I)-NO2- adduct and the copper(II)
species formed in the initial phase of the Cu(I) + NO2-/2H+
or Cu(I) + NO+ reaction indicate they may not contain any
N-bonded NxOy residue.
The anaerobic reactions between the Cu(I) complexes reported
-
here and NO2 or NO+ represent the first systematic study of
the nitrite reductase reaction in model systems. While the
activation of nitrite bound to Cu(I) on protonation was reported
by Halfen and Tolman,16b we show here that protonation and
dehydration of bound nitrite occur faster than its reduction.
Although the existence of an intermediate Cu(I)-NO+ species
in the enzymatic reaction has never been directly proved, there
is mechanistic evidence for its formation from the in Vitro
(40) (a) Ye, R. W.; Toro-Suarez, J.; Tiedje, J. M.; Averill, B. A. J. Biol.
Chem. 1991, 266, 12848. (b) Jackson, M. A.; Tiedje, J. M.; Averill,
B. A. FEBS Lett. 1991, 291, 41.
(41) Kubas, G. J. Inorg. Synth. 1979, 19, 90.

Model Complexes of Copper Nitrite Reductase
Inorganic Chemistry, Vol. 35, No. 5, 1996 1109
Table 7. Selected Positional Parameters and Their Estimated
Standard Deviations for [Cu(1-BB)(DMSO-O)(NO2)](ClO4)
Table 8. Selected Positional Parameters and Their Estimated
Standard Deviations for [Cu(2-BB)(MeOH)(ClO4)(ClO4)
atom
x
y
z
Beqa (Å2)
atom
x
y
z
Beqa (Å2)
Cu
S2
O1
O2
O3
N1
N2
N3
N4
N5
N6
C1
C2
C3
C4
C5
0.1619(2)
-0.3244(3)
0.1222(8)
-0.1463(7)
0.4103(9)
0.173(1)
0.2435(9)
0.1974(8)
0.2706(9)
0.1988(8)
0.270(1)
0.247(1)
0.227(1)
0.270(1)
0.228(1)
0.198(1)
0.171(1)
0.181(1)
0.218(1)
0.239(1)
0.258(1)
0.239(1)
0.319(1)
0.242(1)
0.201(1)
0.163(1)
0.166(1)
0.212(1)
0.250(1)
-0.338(1)
-0.291(1)
0.2481(1)
0.1366(2)
0.0381(6)
0.2361(6)
0.0971(9)
0.4643(7)
0.2864(6)
0.1998(6)
0.6003(7)
0.3675(6)
0.0026(8)
0.4808(8)
0.3215(8)
0.3917(9)
0.1277(8)
0.0739(8)
-0.0815(8)
-0.1795(9)
-0.1217(9)
0.0305(9)
0.5890(8)
0.5172(8)
0.7664(9)
0.4899(9)
0.3452(8)
0.2137(9)
0.229(1)
0.30372(5)
0.24154(9)
0.3267(3)
0.2839(3)
0.3572(3)
0.2775(3)
0.1033(3)
0.2041(2)
0.4712(3)
0.4014(3)
0.3503(3)
0.2125(4)
0.1723(3)
0.0517(4)
0.0895(3)
0.1526(3)
0.1556(4)
0.0940(4)
0.0319(4)
0.0275(3)
0.3380(3)
0.4038(3)
0.4924(4)
0.5154(3)
0.4725(3)
0.5005(3)
0.5735(4)
0.6164(4)
0.5899(4)
0.2643(5)
0.1533(4)
5.15(2)
5.99(4)
7.1(2)
6.8(1)
9.7(2)
6.5(2)
5.4(1)
4.6(1)
5.8(1)
5.3(1)
8.4(2)
5.5(2)
4.9(2)
7.1(2)
5.2(2)
4.9(2)
5.9(2)
7.2(2)
6.8(2)
6.2(2)
5.7(2)
5.2(2)
7.5(2)
5.7(2)
5.4(2)
6.2(2)
7.5(2)
7.5(2)
7.2(2)
7.6(2)
7.8(2)
Cu
Cl1
O1
O2
O3
O4
O9
N1
N2
N3
N4
N5
C1
C2
C3
C4
C5
0.6619(1)
0.8156(3)
0.8402(7)
0.6442(8)
0.882(1)
0.896(1)
0.7341(8)
0.5674(7)
0.3818(8)
0.7881(8)
0.8625(9)
0.454(1)
0.410(1)
0.220(1)
0.530(1)
0.649(1)
0.572(1)
0.738(1)
0.859(1)
0.816(1)
0.736(1)
0.854(1)
1.004(1)
0.957(1)
1.168(1)
0.280(1)
1.233(1)
1.072(1)
0.316(1)
0.277(1)
0.465(2)
0.425(2)
0.560(1)
0.719(2)
0.2410(1)
0.1520(2)
0.2525(6)
0.1122(9)
0.0525(7)
0.1961(9)
0.3588(6)
0.3709(6)
0.4815(7)
0.1254(6)
-0.0532(6)
0.1062(8)
0.3725(8)
0.518(1)
0.5552(8)
0.4872(8)
0.6748(9)
0.7228(9)
0.6550(9)
0.5370(8)
0.0024(8)
-0.1870(8)
0.0384(8)
0.1520(8)
0.0323(9)
0.144(1)
0.24292(9)
0.0650(2)
0.1413(5)
0.0260(6)
0.1044(6)
-0.0038(5)
0.3729(4)
0.1758(5)
0.1209(5)
0.3097(5)
0.3477(5)
0.1981(8)
0.1538(6)
0.0927(8)
0.1203(6)
0.1569(6)
0.0938(7)
0.1032(7)
0.1409(7)
0.1670(6)
0.3106(6)
0.3613(8)
0.3730(6)
0.3495(6)
0.4155(7)
0.4317(7)
0.4068(7)
0.3666(6)
0.1323(9)
0.1599(7)
0.200(1)
4.24(3)
4.94(5)
5.9(2)
9.2(3)
9.4(3)
9.3(2)
6.0(2)
4.2(2)
5.0(2)
4.2(2)
4.9(2)
9.4(3)
4.7(2)
7.0(3)
4.5(2)
4.2(2)
5.9(3)
6.4(3)
5.8(3)
4.9(2)
4.4(2)
6.4(3)
4.5(2)
4.4(2)
5.7(2)
6.2(3)
5.8(3)
4.8(2)
7.2(3)
6.2(3)
6.0(5)
8.0(6)
6.2(3)
8.2(3)
C6
C7
C8
C9
C6
C7
C8
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
C21
C22
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19a
C19b
C20
C21
0.2561(9)
0.2631(8)
0.139(1)
0.263(1)
-0.020(2)
0.004(2)
0.373(1)
0.504(1)
-0.050(1)
0.090(1)
0.243(2)
0.2766(8)
0.3951(8)
a Beq ) (1/3)∑i∑jBijai*aj*ai‚aj.
-0.0692(9)
0.485(1)
80 and 200 MHz, respectively. Optical spectra were obtained on a
HP 8452A diode array spectrophotometer. EPR spectra were measured
in frozen solutions using a Varian E-109 spectrometer operating at
X-band frequencies.
a Beq ) (1/3)∑i∑jBijai*aj*ai‚aj.
Table 9. Selected Positional Parameters and Their Estimated
Standard Deviations for [Cu(2-BB)(N3)](ClO4)
Bis[(1-methylbenzimidazol-2-yl)methyl]amine Dihydrate (1-BB‚-
2H2O). A mixture of iminodiacetic acid (22.5 mmol), N-methyl-o-
phenylenediamine (45 mmol), and 6 M HCl (80 mL) was refluxed for
3 d. After cooling, concentration to a small volume of the solution
under vacuum and cooling in ice yielded a precipitate of the
hydrochloride salt of the product. This was collected by filtration. Upon
addition of acetone to the filtrate, further material precipitated. The
unified mixture of crude hydrochloride salts was dissolved in the
minimum amount of warm water, and the solution was treated with
excess dilute ammonia under stirring. The free base thus precipitated
was filtered off, washed with small amounts of cold water, and dried
under vacuum (yield 60%). Anal. Calcd for C18H23N5O2: C, 63.32;
H, 6.79; N, 20.51. Found: C, 63.13; H, 6.99; N, 20.49. IR (Nujol,
cm-1): 3385 br, 3054 w, 1622 m, 1526 m, 1483 s, 1400 m, 1315 s,
1284 s, 1238 m, 1149 w, 1118 m, 1009 m, 930 w, 917 w, 878 w.
NMR (CDCl3): δ 2,08 (s, NH + H2O), 3.77 (s, 6H, N - CH3), 4.17
(s, 4H, CH2), 7.2-7.4 and 7.7-7.8 (m, 8H, benzimidazole C-H).
Copper(I) Complexes. The synthesis of the copper(I) complexes
was performed under argon as follows. To a solution of the ligand (1
mmol) in 40 mL of degassed ethanol (1-BB), dichloromethane-ethanol
(2-BB), or dichloromethane (AcBB and TB) was added solid [Cu-
(MeCN)4]PF6 (1 mmol). The pale yellow solution was stirred for about
1 h at room temperature, concentrated under vacuum, and cooled in
ice. The precipitate thus formed was collected by filtration and dried
under vacuum.
atom
x
y
z
Beqa (Å2)
Cu
N1
N2
N3
N4
N5
N6
N7
N8
C1
C2
C3
C4
C5
C6
C7
0.08555(9)
0.1042(5)
0.1715(5)
0.1106(5)
0.1783(5)
0.2097(5)
-0.0790(5)
-0.1369(5)
-0.2005(6)
0.1781(6)
0.2415(8)
0.0878(6)
0.0446(6)
0.0451(7)
-0.0412(8)
-0.0824(8)
-0.0392(7)
0.1665(6)
0.2405(7)
0.1265(6)
0.0851(6)
0.1137(7)
0.0594(7)
0.0181(7)
0.0318(7)
0.2164(7)
0.2591(7)
0.1831(8)
0.2178(7)
0.9681(1)
0.8847(5)
0.8308(6)
0.9874(5)
1.0500(5)
1.0716(5)
0.9527(7)
0.9596(6)
0.9640(8)
0.9054(7)
0.8232(9)
0.7593(7)
0.7953(6)
0.6749(7)
0.6180(8)
0.6495(7)
0.7362(7)
1.0664(6)
1.1182(8)
0.9537(6)
0.9124(6)
0.8976(7)
0.8029(8)
0.7597(8)
0.8171(7)
1.0937(7)
0.9962(7)
1.1700(7)
1.1588(7)
0.83920(7)
0.9488(4)
1.0861(4)
0.7185(4)
0.6066(4)
0.8869(4)
0.8110(4)
0.7418(4)
0.6732(5)
1.0244(5)
1.1769(6)
1.0512(5)
0.9637(5)
1.0874(5)
1.0339(6)
0.9446(6)
0.9094(6)
0.6914(5)
0.5558(6)
0.5788(5)
0.6491(5)
0.4979(5)
0.4935(6)
0.5653(6)
0.6449(5)
0.9832(6)
1.0388(5)
0.8330(6)
0.7436(6)
4.72(2)
4.5(1)
5.1(2)
4.8(1)
4.9(1)
4.9(1)
6.5(2)
5.5(2)
7.9(2)
4.9(2)
7.2(3)
4.8(2)
4.7(2)
5.9(2)
6.8(2)
6.3(2)
5.6(2)
4.7(2)
6.2(2)
4.7(2)
4.5(2)
5.5(2)
6.2(2)
6.0(2)
5.2(2)
6.2(2)
5.8(2)
5.8(2)
5.7(2)
C8
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
[Cu(1-BB)]PF6. Anal. Calcd for C18H19N5CuPF6: C, 42.07; H,
3.73; N, 13.63. Found: C, 42.15; H, 4.00; N, 13.45. IR (Nujol, cm-1):
3355 m, 1618 m, 1499 s, 1487 s, 1400 w, 1330 w, 1296 w, 1263 w,
1247 w, 1228 w, 1087 m, 1009 w, 988 w, 942 w, 841 s, 775 w, 746
s.
[Cu(2-BB)]PF6. Anal. Calcd for C20H23N5CuPF6: C, 44.33; H,
4.28; N, 12.92. Found: C, 44.05; H, 4.22; N, 12.62. IR (KBr, cm-1):
3310 m, 3062 w, 1615 w, 1545 w, 1485 s, 1412 m, 1344 w, 1328 m,
a Beq ) (1/3)∑i∑jBijai*aj*ai‚aj.
1296 m, 1236 m, 1178 m, 1151 w, 1125 m, 1108 m, 1058 w, 1021 m,
1007 w, 975 w, 936 s, 835 vs, 748 s.

1110 Inorganic Chemistry, Vol. 35, No. 5, 1996
Casella et al.
Table 10. Selected Positional Parameters and Their Estimated
Standard Deviations for [Cu(2-BB)(NO2)](ClO4)(MeCN)
Table 11. Selected Bond Distances (Å) with Estimated Standard
Deviations for [Cu(1-BB)(DMSO-O)2](ClO4)2 (IV),
[Cu(1-BB)(DMSO-O)2](BF4)2](BF4)2 (V), and
atom
x
y
z
Beqa (Å2)
[Cu(1-BB)(DMSO-O)(NO2)](ClO4) (VI)
Cu
O5
O6
N1
N2
N3
N4
N5
N6
C1
C2
C3
C4
C5
C6
C7
0.9123(2)
0.620(1)
0.7048(9)
0.8434(9)
0.794(1)
1.0525(9)
1.2077(9)
1.026(1)
0.578(1)
0.849(1)
0.776(2)
0.753(1)
0.784(1)
0.690(1)
0.661(1)
0.694(1)
0.758(1)
1.145(1)
1.318(1)
1.152(1)
1.055(1)
1.175(1)
1.103(1)
1.008(1)
0.986(1)
0.898(1)
0.908(1)
1.046(1)
1.191(1)
0.6507(1)
0.7286(7)
0.7677(5)
0.5225(5)
0.3458(6)
0.7725(5)
0.8816(6)
0.5423(5)
0.7851(8)
0.4134(7)
0.2235(8)
0.4142(7)
0.5269(7)
0.3895(8)
0.4794(9)
0.5929(8)
0.6162(7)
0.7812(7)
0.9175(8)
0.9467(1)
0.8775(7)
1.0553(8)
1.0932(8)
1.0245(8)
0.9144(8)
0.4590(7)
0.3659(7)
0.6108(8)
0.6878(8)
0.74420(9)
0.6484(5)
0.7661(4)
0.8573(5)
0.9518(5)
0.6724(5)
0.5543(5)
0.6755(5)
0.7015(7)
0.8607(6)
0.9810(8)
1.0062(5)
0.9466(6)
1.1026(6)
1.1357(6)
1.0781(6)
0.9828(6)
0.5922(6)
0.4669(7)
0.6133(6)
0.6890(6)
0.6080(7)
0.6789(8)
0.7574(8)
0.7627(6)
0.6828(7)
0.7854(7)
0.5660(6)
0.5515(6)
3.29(3)
7.5(2)
4.4(2)
3.2(2)
3.8(2)
3.4(2)
3.7(2)
3.5(2)
6.3(3)
3.6(2)
6.0(3)
3.7(2)
3.3(2)
4.4(2)
4.8(3)
4.4(2)
3.7(2)
3.2(2)
4.8(3)
3.5(2)
3.4(2)
4.7(3)
5.4(3)
5.2(3)
4.3(2)
4.7(2)
4.4(2)
4.2(2)
4.5(2)
IV
V
VI
Cu-O1
Cu-O2
Cu-O3
Cu-N1
Cu-N3
Cu-N5
N1-C1
N1-C10
N2-C2
N2-C3
N2-C4
N3-C2
N3-C5
N4-C11
N4-C12
N4-C13
N5-C11
N5-C14
C1-C2
1.982(3)
2.268(5)
1.980(4)
2.242(6)
1.962(6)
2.275(6)
2.671(8)
2.049(6)
1.970(5)
1.978(5)
1.45(1)
1.468(8)
1.348(8)
1.46(1)
1.388(9)
1.316(9)
1.396(8)
1.363(8)
1.44(1)
1.39(1)
1.306(9)
1.403(8)
1.494(9)
1.393(9)
1.40(1)
1.37(1)
1.40(1)
1.40(1)
1.35(1)
1.50(1)
1.39(1)
1.41(1)
1.35(1)
1.38(1)
1.38(1)
1.34(1)
2.062(4)
1.969(4)
1.961(4)
1.469(6)
1.477(8)
1.355(6)
1.469(6)
1.376(7)
1.326(5)
1.391(7)
1.357(7)
1.453(8)
1.383(5)
1.317(6)
1.398(5)
1.483(8)
1.400(6)
1.388(8)
1.392(7)
1.391(9)
1.388(7)
1.381(8)
1.489(6)
1.391(8)
1.390(7)
1.387(7)
1.394(7)
1.41(1)
2.040(5)
1.967(5)
1.958(6)
1.478(9)
1.47(1)
1.343(9)
1.477(8)
1.36(1)
1.321(8)
1.39(1)
1.35(1)
1.43(1)
1.368(8)
1.305(9)
1.401(8)
1.47(1)
1.414(8)
1.37(1)
1.38(1)
1.38(1)
1.40(1)
1.35(1)
1.501(9)
1.41(1)
1.41(1)
1.40(1)
1.39(1)
1.38(1)
1.36(1)
1.530(5)
1.767(9)
1.744(9)
1.570(8)
1.948(9)
1.51(2)
1.563(9)
1.66(1)
1.75(1)
C8
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
C4-C5
C5-C9
C5-C6
C6-C7
C7-C8
C8-C9
C10-C11
C13-C14
C13-C18
C14-C15
C15-C16
C16-C17
C17-C18
S1-O1
a Beq ) (1/3)∑i∑jBijai*aj*ai‚aj.
[Cu(AcBB)]PF6‚H2O. Anal. Calcd for C22H27N5O2CuPF6: C,
43.89; H, 4.52; N, 11.63. Found: C, 44.02; H, 4.43; N, 11.43. IR
(KBr, cm-1): 3450 w, 3060 w, 2954 w, 1642 s, 1487s, 1415 m, 1338
w, 1245 s, 1158 w, 1011 m, 841 vs, 748 s.
[Cu(TB)]PF6‚H2O. Anal. Calcd for C31H35N7CuO3PF6: C, 48.85;
H, 4.63; N, 12.86. Found: C, 48.83; H, 4.26; N, 12.65. IR (KBr,
cm-1): 3648 w, 3064 w, 1618 m, 1543 s, 1489 s, 1412 m, 1336 m,
1282 m, 1241 m, 1156 w, 1127 w, 1102 w, 1062 w, 1009 m, 839 vs,
746 s.
Copper(II) Complexes. The synthesis of these complexes was
carried out according to the following general procedure. To a solution
of the ligand in dichloromethane (2-BB, AcBB) or dichloromethane-
ethanol (1-BB) was added the stoichiometric amount of copper(II)
perchlorate hexahydrate dissolved in methanol. The product was
precipitated by addition of diethyl ether, eventually after concentration
of the solution under vacuum, and precipitation was completed by
cooling in a refrigerator. The solid was filtered off, washed with small
amounts of diethyl ether, and dried under vacuum.
[Cu(1-BB)][ClO4]2. Anal. Calcd for C18H19N5CuCl2O8: C, 38.07;
H, 3.37; N, 12.33. Found: C, 37.98; H, 3.92; N, 12.06. IR (Nujol,
cm-1): 3422 m, 3232 m, 1615 m, 1541 m, 1508 m, 1460 s, 1425 w,
1321 m, 1294 m, 1251 w, 1207 w, 1091 s, 1013 m, 946 m, 895 w, 770
s, 754 s, 721 w, 702 w, 625 s.
[Cu(2-BB)][ClO4]2. Anal. Calcd for C20H23N5CuCl2O8: C, 40.31;
H, 3.89; N, 11.75. Found: C, 40.78; H, 4.02; N, 11.78. IR (KBr,
cm-1): 3440 m, 3060 w, 2930 w, 1618 m, 1495 s, 1462 s, 1412 m,
1359 w, 1332 m, 1294 w, 1261 w, 1238 w, 1148 s, 1118 s, 1081 s,
1009 w, 973 w, 940 w, 862 w, 768 s, 627 s.
[Cu(AcBB)][ClO4]2‚2MeOH. Anal. Calcd for C24H33N5CuCl2-
O11: C, 41.06; H, 4.74; N, 9.98. Found: C, 41.15; H, 4.00; N, 10.04.
IR (KBr, cm-1): 3425 m, 3060 w, 2946 w, 1630 vs, 1485 m, 1435 s,
1340 w, 1296 w, 1245 w, 1216 w, 1189 w, 1154 w, 1010 br, 932 w,
880 w, 748 s, 638 s, 625 m.
1.361(8)
1.536(4)
1.774(7)
1.772(6)
1.543(6)
1.73(1)
1.668(7)
1.576(7)
1.35(2)
S1-C19
S1-C20
S2-O2
S2-C21
S2-C22
S2′-O2
S2′-C21
S2′-C22
O1-N6
O3-N6
1.508(5)
1.76(1)
1.754(8)
1.970(7)
1.25(1)
1.20(1)
The product was then filtered off, washed with small amounts of cold
water and diethyl ether, and dried under vacuum.
[Cu(1-BB)(NO2)]ClO4. Anal. Calcd for C18H19N6CuClO6: C,
42.03; H, 3.72; N, 16.34. Found: C, 41.16; H, 3.21; N, 16.09. IR
(KBr, cm-1): 3616 m, 3532 m, 3208 m, 3068 w, 2960 m, 1620 m,
1597 m, 1526 s, 1499 s, 1485 s, 1458 s, 1425 s, 1390 m, 1321 m,
1294 m, 1272 w, 1249 w, 1098 s, 1011 m, 995 m, 942 m, 845 w, 775
m, 741 s, 700 w, 625 s.
[Cu(1-BB)(N3)]ClO4. Anal. Calcd for C18H19N8CuClO4: C, 42.36;
H, 3.75; N, 21.95. Found: C, 41.99; H, 3.43; N, 21.42. IR (Nujol,
cm-1): 3628 w, 3410 w, 3096 w, 2110 s, 2060 w, 1615 w, 1506 m,
1458 s, 1350 w, 1319 w, 1294 w, 1251 w, 1094 s, 1011 m, 951 w, 938
w, 746 s, 623 s.
[Cu(2-BB)(NO2)]ClO4‚MeOH. Anal. Calcd for C21H27N6CuCl-
O7: C, 43.91; H, 4.74; N, 14.63. Found C, 43.69; H, 4.20; N, 14.31.
IR (KBr, cm-1): 3424 m, 3256 m, 2932 w, 2882 w, 1618 w, 1499s,
1462 s, 1417 m, 1334 m, 1307 w, 1294 w, 1257 w, 1238 w, 1183 m,
1100 s, 1015 m, 975 m, 934 w, 913 w, 861 w, 832 w, 768 s, 750 s,
625 s.
The azide and nitrite adducts of the copper(II) complexes were
obtained by adding a methanol solution of the sodium salt of the anion
to an equimolar solution of the copper(II) complex in acetonitrile. When
no spontaneous precipitation occurred even after cooling in a refrigera-
tor, a small amount of diethyl ether was added to the cold solution,
and precipitation was completed upon further cooling in the refrigerator.
[Cu(2-BB)(N3)]ClO4‚2MeOH. Anal. Calcd for C22H31N8CuCl-
O6: C, 43.85; H, 5.19; N, 18.60. Found: C, 43.30; H, 4.88; N, 18.07.
IR (KBr, cm-1): 3370 w, 3224 m, 3060 w, 2922 w, 2054 s, 1613 w,
1495 m, 1483 w, 1458 m, 1415 m, 1332 w, 1294 w, 1259 w, 1241 w,
1191 w, 1098 s, 1011 w, 986 w, 930 w, 909 w, 864 w, 828 w, 768 s,
758 m, 623 m.

Model Complexes of Copper Nitrite Reductase
Inorganic Chemistry, Vol. 35, No. 5, 1996 1111
Table 12. Selected Bond Angles (deg) with Estimated Standard
Deviations for [Cu(1-BB)(DMSO-O)2](ClO4)2 (IV),
[Cu(1-BB)(DMSO-O)2](BF4)2 (V), and
Table 13. Selected Bond Distances (Å) with Estimated Standard
Deviations for [Cu(2-BB)(MeOH)(ClO4)](ClO4) (I),
[Cu(2-BB)(N3)](ClO4) (II), and [Cu(2-BB)(NO2)](ClO4)(MeCN)
(III)
[Cu(1-BB)(DMSO-O)(NO2)](ClO4) (VI)
IV
V
VI
I
II
III
O1-Cu-O2
O1-Cu-O3
O1-Cu-N1
O1-Cu-N3
O1-Cu-N5
O2-Cu-N1
O2-Cu-N3
O2-Cu-N5
O2-Cu-O3
N1-Cu-N3
N1-Cu-N5
N1-Cu-O3
N3-Cu-N5
N3-Cu-O3
N5-Cu-O3
Cu-O1-S1
Cu-O1-N6
Cu-O2-S2
Cu-O2-S2′
Cu-O3-N6
Cu-N1-C1
Cu-N1-C10
Cu-N3-C2
Cu-N3-C5
Cu-N5-C11
Cu-N5-C14
C1-N1-C10
C2-N2-C3
C2-N2-C4
C3-N2-C4
C2-N3-C5
C11-N4-C12
C11-N4-C13
C12-N4-C13
C11-N5-C14
N1-C1-C2
N2-C2-N3
N2-C2-C1
N3-C2-C1
N2-4-C5
N2-C4-C9
C5-C4-C9
N3-C5-C4
N3-C5-C6
C4-C5-C6
C5-C6-C7
C6-C7-C8
C7-C8-C9
C4-C9-C8
N1-C10-C11
N4-C11-N5
N4-C11-C10
N5-C11-C10
N4-C13-C14
N4-C13-C18
C14-C13-C18
N5-C14-C13
N5-C14-C15
C13-C14-C15
C14-C15-C16
C15-C16-C17
C16-C17-C18
C13-C18-C17
O1-S1-C19
O1-S1-C20
C19-S1-C20
O2-S2-C21
O2-S2-C22
C21-S2-C22
O2-S2′-C21
O2-S2′-C22
C21-S2′-C22
O1-N6-O3
101.6(2)
100.8(2)
90.9(2)
51.3(2)
173.9(3)
98.8(2)
99.3(2)
83.0(2)
99.5(2)
93.3(2)
141.3(2)
82.0(2)
81.3(2)
134.8(2)
157.6(2)
94.8(2)
86.5(2)
Cu-O1
2.333(7)
2.105(6)
1.954(7)
1.958(7)
2.021(8)
Cu-O9
169.5(2)
98.0(2)
96.9(1)
88.8(2)
91.5(2)
95.7(2)
168.9(2)
98.3(2)
97.0(2)
90.3(2)
92.9(2)
95.2(2)
Cu-N1
1.962(6)
1.959(6)
2.003(6)
1.946(6)
1.965(6)
1.964(7)
2.017(7)
Cu-N3
Cu-N5
Cu-N6
Cu-O5
2.439(8)
2.008(6)
Cu-O6
Cl1-O1
Cl1-O2
Cl1-O3
Cl1-O4
O9-C21
N6-N7
N7-N8
O5-N6
O6-N6
N1-C1
1.453(7)
1.409(7)
1.403(9)
1.392(9)
1.42(1)
81.9(2)
81.5(2)
82.0(2)
80.8(2)
161.8(2)
117.3(2)
161.0(2)
117.9(3)
1.150(8)
1.172(9)
1.23(1)
1.29(1)
1.34(1)
1.38(1)
1.38(1)
1.46(1)
1.36(1)
1.32(1)
1.42(1)
1.34(1)
1.47(1)
1.40(1)
1.51(1)
1.53(1)
112.1(5)
141.3(3)
154.9(2)
120.3(3)
122.7(4)
151.1(3)
1.30(1)
1.40(1)
1.35(1)
1.48(1)
1.36(1)
1.33(1)
1.39(1)
1.35(1)
1.47(1)
1.37(1)
1.45(1)
1.40(2)
1.33(2)
1.51(1)
1.39(1)
1.38(1)
1.39(1)
1.39(2)
1.40(2)
1.37(1)
1.51(1)
1.40(1)
1.40(1)
1.37(1)
1.36(1)
1.39(2)
1.38(1)
1.49(2)
1.41(2)
1.53(2)
1.336(9)
1.38(1)
1.35(1)
1.48(1)
1.37(1)
1.32(1)
1.41(1)
1.36(1)
1.47(1)
1.39(1)
1.50(1)
1.49(1)
N1-C4
78.1(5)
112.5(5)
113.1(4)
113.8(4)
140.8(4)
113.8(4)
140.6(5)
114.2(6)
127.9(6)
106.4(6)
125.7(5)
105.4(5)
128.0(6)
104.8(5)
127.2(6)
105.6(5)
107.0(6)
113.3(6)
125.6(6)
121.0(6)
106.2(6)
130.8(6)
123.0(7)
108.6(6)
130.9(6)
120.5(6)
116.9(7)
121.4(7)
122.1(7)
116.0(7)
105.2(5)
114.1(6)
123.7(6)
122.1(6)
107.5(6)
131.6(7)
120.9(7)
107.9(6)
130.7(6)
121.3(6)
117.3(7)
121.4(8)
122.2(7)
116.9(7)
N2-C1
109.9(3)
111.0(3)
113.3(4)
139.5(3)
113.7(3)
139.6(4)
115.4(4)
125.9(5)
107.2(3)
126.7(4)
105.7(4)
126.5(4)
106.7(4)
126.8(5)
106.6(4)
106.1(4)
112.3(5)
127.7(4)
120.0(4)
106.1(4)
132.2(4)
121.7(4)
108.6(4)
130.2(4)
121.1(5)
116.8(4)
121.7(5)
121.9(6)
116.9(4)
106.3(4)
112.1(4)
126.3(5)
121.6(5)
107.0(4)
131.0(6)
122.0(5)
107.7(4)
130.4(5)
121.9(4)
116.0(6)
121.2(5)
122.5(5)
116.4(6)
104.9(3)
102.7(3)
99.3(3)
109.4(5)
112.2(4)
114.3(5)
138.9(4)
114.7(4)
139.0(5)
114.8(6)
125.8(7)
108.7(5)
125.4(6)
105.5(5)
128.6(6)
106.2(6)
125.1(7)
106.2(6)
107.6(6)
112.2(7)
129.3(6)
118.5(6)
104.8(6)
132.9(5)
122.2(7)
108.7(6)
131.5(5)
119.7(7)
117.8(6)
121.2(7)
121.7(8)
117.4(6)
105.7(6)
113.6(5)
125.9(7)
120.5(7)
107.4(6)
132.5(8)
120.0(7)
106.7(6)
131.5(8)
121.8(6)
115.9(8)
122.5(7)
121.8(7)
117.9(9)
104.4(4)
103.3(4)
99.3(4)
N2-C2
N2-C3
N3-C9
N3-C12
N4-C9
N4-C10
N4-C11
N5-C17
N5-C19a
C5-C19b
C1-C18
C3-C4
1.49(1)
1.42(1)
1.35(1)
1.39(1)
1.38(1)
1.42(1)
1.37(1)
1.48(1)
1.38(1)
1.41(1)
1.35(1)
1.35(1)
1.41(1)
1.40(1)
1.52(1)
1.53(1)
1.45(1)
1.41(1)
1.41(1)
1.38(1)
1.36(2)
1.42(1)
1.40(1)
1.48(1)
1.40(1)
1.38(1)
1.37(1)
1.35(2)
1.42(1)
1.39(2)
1.52(1)
1.54(2)
C3-C5
C4-C8
C5-C6
C6-C7
C7-C8
C9-C20
C11-C12
C11-C13
C12-C16
C13-C14
C14-C15
C15-C16
C17-C18
C19a-C20
C19b-C20
[Cu(AcBB)(NO2)]ClO4‚2MeOH. Anal. Calcd for C24H33N6Cu-
ClO9: C, 44.45; H, 5.13; N, 12.96. Found: C, 44.29; H, 4.70; N,
12.82. IR (KBr, cm-1): 3422 m, 2958 w, 2816 w, 1630 s, 1485 s,
1425 s, 1377 m, 1360 w, 1296 w, 1245 w, 1216 w, 1154 m, 1081 w,
1011 s, 934 w, 841 w, 748 s 679 m.
[Cu(AcBB)(N3)]ClO4. Anal. Calcd for C22H25N8CuClO5: C, 45.52;
H, 4.34; N, 19.30. Found: C, 45.22; H, 4.01; N, 19.15. IR (Nujol
mull, cm-1): 3080 sh, 2066 s, 1659 s, 1618 m, 1597 w, 1495 s, 1410
m, 1350 m, 1338 w, 1294 m, 1278 m, 1247 m, 1170 w, 1154 w, 1114
vs, 1062 s, 1025 w, 1002 w, 980 w, 932 w, 841 w, 754 s, 621 m.
[Cu(TB)(NO2)]ClO4‚MeOH. Anal. Calcd for C32H37N8CuClO9:
C, 49.49; H, 4.80; N, 14.43. Found: C, 49.00; H, 4.80; N, 14.18. IR
(KBr, cm-1): 3520 w, 3064 w, 2974 w, 1618 w, 1545 s, 1491 s, 1458
s, 1415 w, 1383 w, 1292 w, 1245 w, 1096 s, 1011 w, 932 w, 882 w,
845 w, 748 s, 623 s.
[Cu(TB)(N3)]ClO4‚MeOH. Anal. Calcd for C32H37N10CuClO7: C,
49.74; H, 4.83; N, 18.13. Found: C, 49.40; H, 4.56; N, 18.00. IR
(KBr, cm-1): 3452 w, 3064 w, 2952 w, 2078 s, 2060 sh, 1618 w,
1543 s, 1493 s, 1458 s, 1412 w, 1338 w, 1290 w, 1241 w, 1098 s,
1011 w, 932 w, 882 w, 748 s, 623 s.
101.6(5)
109.4(3)
99.6(6)
119.8(7)
94.7(4)
97.8(5)
106.7(4)
107.4(4)
97.3(5)
113.1(7)
100.3(6)
111.2(5)
101.4(6)
102.9(7)
Small samples of the nitrite adducts were prepared similarly using
101.0(5)
Na15NO2.
118.5(8)

1112 Inorganic Chemistry, Vol. 35, No. 5, 1996
Casella et al.
Table 14. Selected Bond Angles (deg) with Estimated Standard Deviations for [Cu(2-BB)(MeOH)(ClO4)](ClO4) (I), [Cu(2-BB)(N3)](ClO4)
(II), and [Cu(2-BB)(NO2)](ClO4)(MeCN) (III)
I
II
III
I
II
III
O1-Cu-O9
O1-Cu-N1
O1-Cu-N3
O1-Cu-N5
O9-Cu-N1
O9-Cu-N3
O9-Cu-N5
N1-Cu-N3
N1-Cu-N5
N3-Cu-N5
N1-Cu-N6
N3-Cu-N6
N5-Cu-N6
O5-Cu-O6
O5-Cu-N1
O5-Cu-N3
O5-Cu-N5
O6-Cu-N1
O6-Cu-N3
O6-Cu-N5
Cu-O1-Cl1
Cu-O9-C21
Cu-N1-C1
Cu-N1-C4
Cu-N3-C9
Cu-N3-C12
Cu-N5-C17
Cu-N5-C19a
Cu-N5-C19b
Cu-N6-N7
Cu-O5-N6
Cu-O5-N6
O1-Cl1-O2
O1-Cl1-O3
O1-Cl1-O4
O2-Cl1-O3
O2-Cl1-O4
O3-Cl1-O4
N6-N7-N8
O5-N6-O6
C1-N1-C4
119.1(2)
87.2(3)
86.7(3)
114.5(4)
91.5(3)
86.6(3)
126.3(4)
171.6(3)
94.5(3)
93.2(3)
C1-N2-C2
126.6(7)
108.0(7)
125.4(8)
106.8(7)
127.4(7)
107.9(7)
124.6(8)
119(1)
127.3(7)
109.0(6)
123.7(7)
108.1(6)
126.1(6)
107.5(6)
126.3(7)
111.1(6)
124.9(8)
108.5(7)
126.6(8)
105.7(7)
126.5(8)
108.1(7)
125.4(8)
107.0(7)
C1-N2-C3
C2-N2-C3
C9-N3-C12
C9-N4-C10
C9-N4-C11
C10-N4-C11
C17-N5-C19a
C17-N5-C19b
N1-C1-N2
150.5(3)
95.0(2)
91.1(3)
94.0(3)
98.5(3)
142.5(3)
156.8(3)
93.4(3)
93.7(3)
118.(1)
111.7(7)
123.8(8)
124.4(8)
106.0(7)
132.0(9)
121.9(8)
107.8(7)
131.5(8)
120.8(8)
116.8(9)
121.4(9)
121.2(9)
117.9(9)
111.3(7)
125.9(8)
122.8(8)
106.3(7)
131.4(8)
122.4(7)
107.7(7)
131.5(8)
120.8(8)
115.7(9)
121.8(9)
122.8(8)
116.6(9)
114.6(3)
112.3(9)
125.(1)
110.7(7)
125.2(7)
124.1(6)
104.9(7)
131.6(7)
123.4(7)
108.6(6)
132.5(7)
118.8(7)
117.6(7)
120.1(9)
121.4(8)
118.5(7)
110.6(6)
127.4(7)
122.0(7)
107.3(7)
132.0(8)
120.7(7)
106.5(7)
130.9(8)
122.6(7)
116.7(8)
122.9(8)
119.3(8)
117.8(8)
110.5(7)
113.6(6)
111.1(7)
109.3(8)
128.0(7)
122.7(8)
106.6(7)
131.6(8)
121.8(9)
107.7(8)
132.0(7)
120.3(8)
116.8(8)
122.6(9)
120(1)
118.4(8)
112.8(8)
124.8(7)
122.2(8)
105.6(8)
132.8(8)
121.5(9)
107.8(8)
131.4(8)
120.8(9)
117.9(9)
122.(1)
N1-C1-C18
N2-C1-C18
N2-C3-C4
55.5(3)
100.4(3)
100.6(3)
97.2(3)
91.0(3)
92.8(3)
152.6(3)
N2-C3-C5
C4-C3-C5
N1-C4-C3
N1-C4-C8
C3-C4-C8
C3-C5-C6
C5-C6-C7
120.1(3)
131.2(6)
124.2(6)
128.5(5)
126.6(5)
125.6(5)
117.3(7)
119.2(9)
122(1)
C6-C7-C8
C4-C8-C7
125.7(6)
127.5(4)
125.7(5)
125.8(5)
111.7(5)
106.2(4)
124.8(6)
127.3(6)
127.2(7)
126.6(6)
108.2(5)
111.3(5)
N3-C9-N4
N3-C9-C20
N4-C9-C20
N4-C11-C12
N4-C11-C13
C12-C11-C13
N3-C12-C11
N3-C12-C16
C11-C12-C16
C11-C13-C14
C13-C14-C15
C14-C15-C16
C12-C16-C15
N5-C17-C18
C1-C18-C17
N5-C19a-C20
N5-C19b-C20
C9-C20-C19a
C9-C20-C19b
124.6(4)
105.1(6)
85.9(6)
107.8(5)
108.0(4)
109.7(4)
110.4(5)
111.2(5)
109.7(6)
120.(1)
117.7(8)
111.2(8)
113.6(7)
110.5(7)
176.3(9)
106.8(6)
121.(2)
114.(1)
119(1)
113.5(8)
107.9(6)
113.8(7)
115.1(8)
106.5(7)
Caution! Although we did not have problems working with small
amounts of the perchlorate complexes reported in this study, extreme
care should be exercised in handling materials containing this shock-
sensitive anion.
Structure Determinations. Crystals of [Cu(1-BB)(DMSO-O)2]-
(ClO4)2 and [Cu(1-BB)(NO2)(DMSO-O)](ClO4) were obtained by
dissolution of the complex in DMSO followed by addition of an equal
volume of diethyl ether and a half-volume of THF. Crystals of [Cu-
(2-BB)(MeOH)(ClO4)](ClO4) were obtained upon slow evaporation of
a methanolic solution of the complex. Crystals of [Cu(2-BB)(N3)]-
(ClO4) and [Cu(2-BB)(NO2)](ClO4) were grown by diffusion of diethyl
ether in a MeCN solution of the complex. Crystals of [Cu(1-BB)-
(DMSO-O)2](BF4)2 from the reaction of [Cu(1-BB)]PF6 with nitrite were
obtained as follows. The Cu(I) complex (13.9 mg, 0.027 mmol)
dissolved in MeCN (2 mL) was reacted with a 0.03 M methanol solution
of NaNO2 (0.027 mmol) and a 7.45 M diethyl ether solution of HBF4
(0.054 mmol) under a stream of argon. The reaction was continued
for about 1 h, after which the product [Cu(1-BB)](BF4)(PF6) was
precipitated by addition of diisopropyl ether {IR (cm-1): 1080 vs, br,
ν(BF4); 839 s, ν(PF6). UV-vis and EPR spectra were identical with
those of [Cu(1-BB)]2+}. Crystallization from a mixture of DMSO-
diethyl ether-THF as described above deposited crystals of [Cu(1-
BB)(DMSO-O)2](BF4)2.
Nitrite Reductase Activity. The reactions between [Cu(1-BB)]+,
[Cu(2-BB)]+, [Cu(AcBB)]+, [Cu(TB)]+, and nitrite were performed at
room temperature as follows. To a degassed solution of Cu(I) complex
(2.7 × 10-5 mol) in dry MeCN (1 mL) were added a few microliters
of degassed solutions of NaNO2 (2.7 × 10-5 mol) in methanol and
HBF4‚Et2O (5.4 × 10-5 mol) in diethyl ether. A weak argon stream
bubbled through the solution delivered the gas evolved from the reaction
into a degassed 1 × 10-2 M Fe(EDTA) solution in 1 M aqueous citrate
buffer, pH 5.0, which was kept cooled in an ice bath. After interruption
of the argon flow, the amount of NO evolved in the reaction was
estimated from the intensity of the characteristic absorption of the [Fe-
(EDTA)(NO)] complex at 432 nm (ꢀ ) 590 M-1 cm-1). The reactions
between the Cu(I) complexes and (NO+)(BF4-) were performed
similarly. The extinction coefficient used for the [Fe(EDTA)(NO)]
complex refers to the operation conditions of the experiments and was
determined by bubbling NO gas through the degassed 0.01 M
Fe(EDTA) solution for 5 min and reading its absorbance at 432 nm.
Blank determinations of NO recovery from reduction of nitrite by
ascorbate under the gas flow conditions used in the reactions were
carried out on methanol-acetonitrile (1:1, v/v) solutions containing
NaNO2 (0.01 M) and yielded an average of 83% NO (as [Fe(EDTA)-
(NO)] complex) after 5 min. The yields of NO reported in Table 3
are relative yields calculated on the basis of the effective recovery of
NO in the blank experiments. The experiments were run in duplicate
with high reproducibility (differences were less than 5%).
Data were collected on an Enraf-Nonius CAD-4 diffractometer with
graphite-monochromated Cu KR radiation (λ ) 1.541 84 Å) at 293(2)
K in the 2-70° θ range. Calculations were performed on a MicroVax
3100 computer with SDP and MolEN software.42,43 Crystal data and
pertinent experimental details are given in Table 4. Atomic scattering
(42) SDP Structure Determination Package; B. A. Frenz and Associates
and Enraf-Nonius: College Station, TX, and Delft, The Netherlands,
1990.
(43) MolEN: An InteractiVe Structure Solution Procedure; Enraf-Nonius:
Delft, The Netherlands, 1990.

Model Complexes of Copper Nitrite Reductase
Inorganic Chemistry, Vol. 35, No. 5, 1996 1113
factors, with anomalous dispersion corrections, were taken from ref
44. All the structures were solved by direct methods (MULTAN80).45
Besides the exceptions listed below, in all the structures the non-
hydrogen atoms were anisotropically refined by full-matrix least squares,
while the hydrogen atoms were not refined. The positions of the
nonacidic hydrogen atoms were calculated, with the exception of those
of [Cu(2-BB)(N3)](ClO4), where they were experimentally found.
In [Cu(2-BB)(MeOH)(ClO4)](ClO4), one of the carbon atoms (C19)
was found statistically disordered between two slightly different
positions (multiplicity of each position ) 0.5). The hydrogen atoms
bonded to N5, C19, and C20 were disregarded. In [Cu(2-BB)(N3)]-
(ClO4), the hydrogen atom of the amine N-H group was isotropically
refined. In [Cu(2-BB)(NO2)](ClO4), a disordered MeCN solvation
molecule is present, which was refined isotropically; in the final ∆F
map positive and negative residuals were found near this molecule.
Permutation of the labels of the C and N terminal atoms of this MeCN
molecule produced inconsistent results. Also the perchlorate anion
seems quite disordered, because of the high thermal parameters of its
oxygen atoms. In both [Cu(1-BB)(DMSO-O)2](ClO4)2 and [Cu(1-BB)-
(DMSO-O)2](BF4)2, the dimethyl sulfoxide ligand in the apical position
is disordered, with the sulfur atom statistically distributed on opposite
sides of the plane defined by the atoms O2, C21, and C22; moreover,
in both structures the counterion farthest from the complex (the
perchlorate of Cl2 and the tetrafluoroborate of B2) seems quite
disordered; in the final ∆F maps postive and negative residuals are
found near these molecules. Selected positional parameters are given
in Tables 5-10. Selected bond distances and angles are reported in
Tables 11-14. Further details are presented in the Supporting
Information.
Acknowledgment. This work was supported by the Euro-
pean Community, allowing regular exchange of preliminary
results with several European colleagues, under Contract ER-
BCHRXCT920014, and by the Italian MURST.
Supporting Information Available: A view of the [Cu(1-BB)-
(DMSO-O)2]+ cation in the [Cu(1-BB)(DMSO-O)2](BF4)2 crystal and
tables of complete atomic coordinates, anisotropic thermal parameters,
bond lengths and angles, and the details of the crystallographic analyses
of the copper(II) structures (44 pages). Ordering information is given
on any current masthead page.
(44) International Tables for X-ray Crystallography; Kynoch: Birmingham,
U.K., 1974; Vol. IV.
(45) Main, P.; Hull, S. E.; Lessinger, L.; Germain, G.; Declerq, J. P.;
Woolfson, M. M. MULTAN80: A System of Computer Programs for
the Automatic Solution of Crystal Structures from X-ray Diffration
Data; Universities of York and Louvain: York, England, and Louvain,
Belgium, 1980.
IC950392O