Nitric Oxide Reduction of the Copper(II) Complex Cu(dmp)22+ (dmp = 2,9-Dimethyl-1,10-phenanthroline)

Full text of DOI:10.1021/ic9511175

Inorg. Chem. 1996, 35, 2411-2412
2411
2+
Nitric Oxide Reduction of the Copper(II) Complex Cu(dmp)2 (dmp )
2,9-Dimethyl-1,10-phenanthroline)
Dat Tran and Peter C. Ford*
Department of Chemistry, University of California, Santa Barbara, California 93106
ReceiVed August 24, 1995
Nitric oxide (NO) has important roles in both environmental1
and biological chemistry,2 and the recent discoveries of a variety
of bioregulatory and immune response roles for NO in the latter
area have made it the subject of intensive experimental research.
The bioregulatory functions of NO are generally attributed to
interactions between NO and Fe(II/III) centers of certain heme
proteins, but reactions with other metal centers are of concern.3
NO has been demonstrated to be a reductant in reactions with
Co(II) and Fe(II) halides as well as Fe(III) porphyrins and
ferrihemoproteins.4 Given the importance of Cu(I/II) couples
in biological redox systems,5 the Cu-NO interaction may also
be of interest; indeed the NO reactions of different types of
copper proteins have been qualitatively described.6 In this
context, the redox reactions between NO and certain Cu(II)
complexes are being examined in this laboratory. Described
here is a preliminary report of the nitric oxide reduction of Cu-
(dmp)22+ (I, dmp ) 2,9-dimethyl-1,10-phenanthroline) in solu-
tion, which we believe to be the first quantitative demonstration
of NO reduction of a cupric complex. Of further interest,
reaction with I may have potential as a chemical NO sensor.
of the cuprous complex was clearly observed by the character-
istic metal to ligand charge transfer band centered at 454 nm in
the optical absorption spectrum. The reaction was determined
to be quantitative from the spectral changes as calculated from
the known extinction coefficients.9 II was recovered as its
triflate (OTf ) CF3SO3-) salt from the reaction of I plus NO
1
in methanol, and its identity was confirmed by H NMR.10
Dichloromethane solutions of this salt displayed the character-
istic emission spectrum of II (λmax ∼ 680 nm).11 Notably, when
the reaction was carried out in CH2Cl2 solution with low
concentrations of MeOH (<1%) under PNO ) 1 atm, the reaction
solutions themselves were observed to be emissive, with the
emission intensity increasing with time as the reduction
proceeded.
Analysis of the methanol reaction solution by GC-MS
demonstrated the presence of methyl nitrite (CH3ONO; m/z 44,
46, 60, 61). Furthermore, GC analysis of the products from
the reaction in mixed CH2Cl2/CH3OH solution quantitatively
showed that the mole ratio of CH3ONO/Cu(dmp)2+ formed was
unity. When the reaction was instead carried out in an
unbuffered aqueous solution, the solution pH was observed to
decrease from pH 6 to pH 3 in a manner consistent with acid
formation according to eq 1. Nitrite, NO2-, was also detected
2+
The reduction potential for Cu(dmp)2 (0.58 V vs NHE in
water)7 is substantially more positive than that for most other
cupric complexes. This apparently reflects steric effects of the
2,9-dimethyl substituents of the dmp ligand, which force I out
Cu(dmp)22+ + ROH + NO f
Cu(dmp)2+ + RONO + H+ (1)
of a planar geometry, making it a stronger oxidant than the 1,10-
2+
phenanthroline analog Cu(phen)2
(0.08 V).7 A similar
coordination geometry argument has been invoked to rationalize
the positive redox potentials of “blue” copper proteins.8
In deaerated solutions of neat H2O, neat CH3OH, or methanol/
dichloromethane mixtures, Cu(dmp)22+ was very rapidly reduced
to Cu(dmp)2+ (II) upon addition of excess NO. The formation
as a product of the reaction in aqueous solutions by the Griess
analytical method.12 Thus the overall reaction is as shown in
(1).
No reaction was observed in neat CH2Cl2 or neat CH3CN
solution, indicating the importance of the hydroxylic solvent in
providing a reagent that can react with the coordinated NO (Vide
infra). Similar trends were observed qualitatively by Olson and
Wayland4b for the reductive nitrosylation of the ferric tetraphen-
ylporphyrin complex Fe(TPP)Cl.
Preliminary rate measurements carried out spectrophotometri-
cally in mixed CH2Cl2/MeOH solutions indicate the reaction
kinetics to be dependent on the concentration of methanol. At
low methanol concentrations, there is a linear relationship
(1) Schwartz, S. E.; White, W. H. in Trace Atmospheric Constituents:
Properties, Transformation and Fates; Schwartz, S. E., Ed.; Wiley:
New York, 1983; pp 1-117.
(2) (a) Butler, A. R.; Williams, D. L. H. Chem. Soc. ReV. 1993, 233-
241. (b) Moncada, S.; Palmer, R. M. J.; Higgs, E. A. Pharm. ReV.
1991, 43, 109-141. (c) Palmer, R. J. J.; Ferrige, A. G.; Moncada, S.
Nature 1987, 327, 524-526. (d) Barinaga, M. Science 1991, 254,
1296-1297. (e) Ignarro, L. J. J. NIH Res. 1992, 4, 59-62. (f) Wink,
D. A.; Ford, P. C. Methods: A Companion to Methods in Enzymology;
Academic: London, 1995; Vol. 7, pp 14-20.
(3) (a) Hoshino, M.; Ozawa, K.; Seki, H.; Ford, P. C. J. Am. Chem. Soc.
1993, 115, 9568 and references therein. (b) Traylor, T. G.; Duprat,
A. F.; Sharma, V. S. J. Am. Chem. Soc. 1993, 115, 810-811. (c) Tsai,
A.-L. FEBS Lett. 1994, 341, 141-145. (d) Waldman, S. A.; Murad,
F. Pharmacol. ReV. 1987, 39, 163-196. (e) Yu, A. E.; Hu, S.; Spiro,
T. G.; Burstyn, J. N. J. Am. Chem. Soc. 1994, 116, 4117-4118.
(4) (a) Gwost, D.; Caulton, K. G. Inorg. Chem. 1973, 12, 2095-2099.
(b) Wayland, B. B.; Olson, L. W. J. J. Chem. Soc., Chem. Commun.
1973, 897-898. (c) Kon, H.; Kataoka, N. Biochemistry 1969, 8, 4741.
(d) Dickinson, L. C.; Chien, J. C. W. J. Am. Chem. Soc. 1971, 93,
5036-5040. (e) Chien, J. C. W. J. Am. Chem. Soc. 1969, 91, 2166-
2168.
(5) Bioinorganic Chemistry of Copper; Karlin, K. D., Tyekla´r, Z., Eds.;
Chapman & Hall, Inc.; New York, 1993.
(6) Gorren, A. C. F.; de Boer, E.; Wever, R. Biochim. Biophys. Acta 1987,
916, 38-47.
(7) (a) James, B. R.; Williams, R. J. P. J. Chem. Soc. 1961, 2007-2019.
(b) Hawkins, C. J.; Perrin, D. D. J. Chem. Soc. 1963, 2996-3002. (c)
Lei, Y.; Anson, F. C.; Inorg. Chem. 1994, 33, 5003-5009.
(8) Vallee, B. L.; Williams, R. J. P. Proc. Natl. Acad. Sci. U.S.A. 1968,
59, 498.
(9) (a) The extinction coefficients for I at λmax ) 760 nm in H2O and
CH3OH are ꢀ760 ) 100 M-1 cm-1 and ꢀ760 ) 180 M-1 cm-1
,
respectively. The corresponding values for II at λmax ) 454 nm are
ꢀ454 ) 6160 M-1 cm-1 and ꢀ454 ) 7530 M-1 cm-1 in the same
solvents.9b (b) Sundararajan, S.; Wehry, E. L. J. Phys. Chem. 1972,
76, 1528-1536.
1
+
(10) (a) The H NMR spectrum of the Cu(dmp)2 product isolated from
2+
the reaction of Cu(dmp)2 with NO in CD3OH gave a spectrum
identical to that for an independently prepared Cu(dmp)2PF6 sample:
δ 8.65, 8.61 (2H, d); δ 8.13 (2H, s); δ 7.90, 7.85 (2H, d); δ 2.45 (6H,
s). The free ligand dmp gave δ 8.31, 8.27 (2H, d); δ 7.82 (2H, s); δ
7.64, 7.59 (2H, d); and δ 2.87 (6H, s). These values are consistent
with literature values.11b (b) Pallenberg, A. J.; Koenig, K. S.; Barnhart,
D. M. Inorg. Chem. 1995, 34, 2833-28440.
+
(11) (a) The emission of Cu(dmp)2 is quenched in donor solvents such
as CH3OH and CH3CN.10b (b) Kirchhoff, J. R.; Gamache, R. E., Jr.;
Blaskie, M. W.; Del Paggio, A. A.; Lengel, R. K.; McMillin, D. R.
Inorg. Chem. 1983, 22, 2380-2384.
(12) Archer, S. FASEB J. 1993, 7, 349-360.
0020-1669/96/1335-2411$12.00/0 © 1996 American Chemical Society

2412 Inorganic Chemistry, Vol. 35, No. 9, 1996
Communications
of a NO complex with I in the nonreactive solvent CH2Cl2 were
not successful, probably because the equilibrium constant K is
small at room temperature.
Step ii has some precedent in reactions of Ru(II)-coordinated
NO with hydroxide to give the corresponding nitro complexes
and of Ir(III) coordinated with alcohols to give isolable alkyl
nitrite complexes.15 The dependence of the overall rate on
[MeOH] when the reaction was carried out in a mixed
dichloromethane/methanol solution is consistent with step ii
being rate-limiting. Since only a small change in the rate
constant (kobs) was observed when the reaction was carried out
with added trifluoroacetic acid (CF3CO2H),16 methanol rather
than methoxide ion must be involved in this key step.
Figure 1. Dependence of kobs for formation of Cu(dmp)2+ on methanol
concentration in dichloromethane at 25 °C (PNO ) 1 atm). Inset:
representative kinetic trace and its first-order fit.
2+
For comparison purposes, the reaction of Cu(phen)2 with
NO was briefly examined. This proved to be very sluggish
even in neat methanol (kobs < 10-6 s-1), and only partial
reduction was observed for a solution of Cu(phen)22+ under NO
atmosphere for several days. The behavior of Cu(phen)22+ can
be rationalized by its reduction potential, which is 0.5 V less
positive than that of Cu(dmp)22+. The potential difference is
reflected in the ability of the methyl substituents in dmp to
destabilize the Cu(II) state through steric interactions, while little
repulsive steric interaction is expected for the tetrahedral Cu(I)
state. Yandell et al. invoked the same argument to rationalize
the substantially larger rate constant observed for the oxidation
Scheme 1. Proposed Mechanism for the Reduction of
2+
Cu(dmp)2 by NO in Hydroxylic Solvents
of ferrocytochrome c by Cu(dmp)22+ (ca. 5 orders of magnitude)
2+ 17
than that observed for the oxidation by Cu(phen)2
.
The
between the observed first-order rate constants (kobs) for the
formation of Cu(dmp)2+ and [MeOH] (Figure 1). However, at
concentrations above ∼3%, the kobs values increased in a
nonlinear fashion, suggesting that the activity of the MeOH is
affected by changes in the bulk solvent as methanol becomes a
significant fraction.
The mechanism of this reaction is unlikely to follow an outer-
sphere redox pathway since the calculated potential for simple
electron transfer reaction between Cu(dmp)22+ and NO is quite
unfavorable (∆E ) -0.63 V in aqueous solution). Thus, the
sequence of reactions shown in Scheme 1 is proposed as a likely
mechanism. This involves three key steps: (i) the reversible
formation of an inner-sphere adduct between NO and Cu(II);
oxidizing power of Cu(dmp)22+ was also reflected in its ability
to be reduced by nitrite ion, NO2-, in methanol, while no
2+
reaction was observed for the analog Cu(phen)2
.
2+
The affinity of Cu(dmp)2 for NO suggested possibilities
of applying it as a NO sensor in a solid state matrix. In this
context, [Cu(dmp)2](OTf)2 was incorporated into a SiO2 sol-
gel matrix and the resulting solid material was exposed to NO.18
The result was a rapid color change from yellow to green. This
system could be “recycled” repeatedly (between NO and Ar or
air) without any apparent decomposition. Unlike the reaction
in solution, there were no permanent redox changes in the solid
state reaction. The Cu oxidation state of the green product
appears to be +2, as evident by the persistence of the ligand
field absorption band (∼730 nm) in the visible region. Our
preliminary EPR studies to probe the nature of the Cu(II)-NO
interaction in the solid state have not been conclusive. Detailed
studies of this solid state system and of the solution phase
(ii) reaction of this adduct with ROH to give an N-coordinated
+
nitrite derivative; (iii) dissociation to the products, Cu(dmp)2
,
RONO, and H+.
In the solid state Cu(II) is frequently found to be five-
coordinate;13 indeed, there are two structures of salts of I which
indicate the presence of a fifth ligand, nitrate in the case of the
salt [Cu(dmp)2(NO3)]NO313b and H2O in the case of [Cu(dmp)2-
(H2O)](OTf)2.13c Therefore, an equilibrium between NO and
solvent complexes as depicted in step i would be reasonable.
In this context, Sykes and co-workers14 suggested interconver-
sion of a 5-coordinate Cu(II) to its 4-coordinate analog prior to
2+
reductions of Cu(dmp)2 by NO and nitrite are in progress.
Acknowledgment. This research was supported by the U.S.
National Science Foundation (Grant No. CHE-94000919 to
P.C.F.). D.T. thanks Dr. Ivan Lorkovic for helpful discussions.
IC9511175
(15) (a) Rhodes, M. R.; Barley, M. H.; Meyer, T. J. Inorg. Chem. 1991,
30, 629-635 and references therein. (b) Reed, C.; Roper, W. J. Chem.
Soc., Dalton Trans. 1972, 1243.
4-
electron transfer as being rate limiting in the Fe(CN)6
2-
reduction of CuIIL2 in aqueous solution (where L ) 2,9-
(16) Reaction carried out in 2% methanol/dichloromethane (v/v) in the
presence of 2.6 mM CF3CO2H yielded a kobs of 6.7 × 10-3 s-1
compared to 8.4 × 10-3 s-1 without CF3CO2H.
dimethyl-4,7-bis((sulfonyloxy)phenyl)-1,10-phenanthroline). None-
theless, our attempts (by IR spectroscopy) to observe formation
(17) Augustin, M. A.; Yandell, J. K. Inorg. Chem. 1979, 18, 577-583.
(18) (a) Sol-gel matrices were prepared with tetraethyl orthosilicate (Si-
(OEt)4, TEOS) from Aldrich and [Cu(dmp)2](CF3SO3)2] ∼1 mM in
CH3OH/H2O. Samples were allowed to solidify at room temperature,
dried at 90 °C overnight, and purged with Ar rigorously before NO
(1 atm) was introduced. (b) van Wu¨llen, L.; Tran, D.; Ford, P. C.
Studies in progress.
(13) (a) Hathaway, B. J. Struct. Bonding (Berlin) 1973, 14, 49. (b) van
Meerssche, M.; Germain, G.; Declercq, J. P.; Willputte-Steinert, L.
Cryst. Struct. Commun. 1981, 10, 47. (c) White, A. H. Unpublished
results.
(14) Al-Shatti, N.; Lappin, A. G.; Sykes, A. G. Inorg. Chem. 1981, 20,
1466-1469.