12828
J. Phys. Chem. 1994, 98, 12828-12830
Matrix Isolation Studies of the Interactions of Cu Atoms with NO Joseph A. Chiarelli and David W. Ball* Department of Chemistry, Cleveland State University, Cleveland, Ohio 441 15 Received: August 17, 1994; In Final Form: October 7, I994@
Matrix isolation studies of Cu vapors and NO in solid argon at 15 K have identified the 1:l complex Cu(NO). Several isotopically-substituted NO reactants were reacted to better assign the few absorptions of this triatomic complex. Higher concentrations of NO were used to identify Cu(NO),, but for these complexes the vibrational absorptions were relatively weak, unlike similar Cu complexes where CO is the ligand. There was no indication that electron transfer had occurred between Cu and NO. Photolysis and annealing studies showed no changes in the IR spectra, suggesting that the Cu(N0) complex is unreactive with respect to the addition of additional NO ligands.
Introduction
Experimental Section
Matrix isolation studies of metal atom interactions are particularly relevant to catalyst systems, reaction intermediates, and models of ligand systems.’ We have been particularly interested in systems related to corrosion processes, especially where the metal is of commercial importance and the corrodant of interest is a known pollutant gas.2 To that end, we recently reported on the interactions of Cu with NOz3 as well as with H2S and H20.4 (The study of Cu/H20 interaction repeated earlier work5 but corrected some vibrational frequencies and offered a different interpretation based on some density functional theoretical arguments.) We summarize here the interaction of Cu with NO. Notwithstanding NO’S new notoriety as recently discovered neurotransmitter? our interest in NO has a different focus. First, we are interested in the nature of the “bare” complex that would certainly form between a transition metal atom and the stable diatomic radical. Chief among our questions was whether a charge-transfer complex would form, as was detected for the Cu(N02) ~ o m p l e x .This ~ would be very relevant to corrosion and electrical engineers, who see a growing synergistic effect of pollution on the corrosion of electronic parts? Second, since the matrix-isolation technique is well-suited to the identification and study of complexes of varying coordination number,7s8we hope to be able to identify a series of complexes Cu(NO),, where x 2 1. This study would thus allow for an interesting comparison between the nature of Cu/NO complexes and Cu/ CO complexes. Moskovits and Ozin et al. published a very complete study9 of Cu/CO interactions in cryogenic matrices. They identified at least three monocopper complexes and one dicopper complex using a combination of IR and UV-visible spectroscopy. The electronic difference between CO and NO is a single electron. The similarities and differences in their chemistries will make for an interesting juxtaposition. NO as a ligand is well-known, and there are several reports of vibrational frequencies and their relationships to structure. lo NO has been studied previously in inert gas matrices. Published reports include a concentration study to identify NO and (NO), species,” the interaction of NO with Ag clusters,12tetraphenylp~rphinecobalt(II),~~ various metal halides,I4 and alkali and alkaline earth metals.I5 To date, this appears to be the first reported study of transition metal atoms with NO (cf: ref 12).
Details of the apparatus are presented el~ewhere.~ Briefly, a gold-plated copper octagonal block was attached to the cold finger of an APD Displex closed-cycle helium refrigerator. The cold fingerlcopper block assembly was enclosed in a vacuum system whose ultimate precooling pressure was -1 x Torr. Cu was vaporized from a OS-mil-thick Ta tube furnace heated resistively to -1100 “C. NO (Liquid Carbonic, 99.0%), 15N0 (Isotec Inc., 99.9 atom % 15N), 15N180(Isotec Inc., 99.9 atom % 15N,96.2 atom % and Ar (AGA, 99.9995%) were used without further purification and introduced into the vacuum system via stainless steel tubing and needle valves. Cu vapors, NO, and Ar were deposited onto one of eight sides of the octagon, whose temperature ranged from -15 to 17 K during deposition. Depositions lasted for 15-60 min. After deposition, the Fourier transform infrared spectrum was measured using a Nicolet SDXB FTIR spectrometer at 2 cm-’ resolution.
@
Abstract published in Advance ACS Abstracts, November 15, 1994.
0022-3654/94/2098-12828$04.50/0
Results and Discussion When NO was condensed in Ar at the temperature range specified above, not only NO but also (N0)2 was present even at the lowest concentrations. This is consistent with the results of Guillory and Hunter, who found” that, at a level of 0.2% NO in N2, at 4 K “...complete isolation of NO results...”; however, upon “...warming to 15°K...new sharp features appeared ...” which were identified as the (N0)2 species. This suggests that, at the temperatures operative in our experiments (dictated by the mass of the Cu block, inefficiencies of the aluminum heat shield around the block, and the presence of an 1100 “C furnace about 5 cm from the cold finger), there will always be (N0)2 present in our samples. This was confirmed in all our trials; however, the concentration studies performed were consistent with the determined stoichiometry of the assigned products. Nor were identification of products impeded by the presence of (N0)2. When Cu vapors were cocondensed with NO in excess Ar at low concentrations of each reactant (subject to the caveat discussed above), two new peaks were observed. The first appeared at 1610.5 cm-’, and its appearance is illustrated in Figure 1. It is a very strong absorption, occurring in the middle of the pattern of absorptions indicative of matrix-isolated H20 (which is almost always an impurity). Figures 2 and 3 show similar regions for Cu reacting with l5NO and 15N180,respectively. The second absorption of interest is a much weaker absorption occumng at 608.8 cm-’ in Cu/NO matrices, which
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0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 49, 1994 12829
Letters
Cu(N0)
I
h *DO0
A IBSO
le00
1780
ieso
1700
W."."YmaeP
,eo0
lCiCl0
1Sm-l)
-
Figure 1. FlTR spectra of Cu atoms cocondensed with NO in solid argon: bottom spectrum, 1% NO in Ar,top spectrum, same as bottom, but with S0.5% Cu. A = Cu(N0); B = Cu(NO)(NO),. "0
BBO
e40
5eO SO0 -00 W.vllnumbeP Ism-%)
ma0
B10
6BO
Figure 4. Shifting of the Cu-N-0
bend upon isotopic substitution: Top, Cu-N-0 bend; second from top, Cu-lSN-O bend; second from bottom, C U - ' ~ N - ' ~ O bend; bottom, NO in Ar, no Cu.
TABLE 1: Infrared Absorptions (cml) and Approximate Assignments of the Isotopomers of Cu(N0) in Ar at 15 K
$BOO
*SO0
;a00
$700 *.Y.""llb.P
a700
$800
LEO0
LEO0
(em-$)
Figure 2. FTIR spectra of Cu atoms cocondensed with 15N0 in solid argon: bottom spectrum, -1% I5NO in Ar, top spectrum, same as bottom, but with 50.5% Cu.
Cu(N0) 1610.5 608.8
approx assignt N - 0 stretch Cu-N stretch, Cu-N-0
Cu( "NO) 1558.9 602.8
approx assignt lSN-0 stretch Cu-15N stretch, Cu-ISN-O bend?
Cu( l5NI80) 1546.1 592.0
approx assignt lSN-l8O stretch Cu- IsN stretch, Cu- 15N- l8O bend?
TABLE 2: Infrared Absorptions (cm') of Cu(NO)(NO), in Ar at 15 K Cu(NO)(NO), 1586.0 1680.2 1660.8
A
%OS0
1800
1'80
1700 H . " . "m b . P
;-so
Leo0
,580
L800
(Em-*)
Figure 3. FTIR spectra of Cu atoms cocondensed with I5NL8Oin solid argon: bottom spectrum, -1% 1sN180in Ar, top spectrum, same as bottom, but with 50.5% Cu.
is shown in Figure 4. Although appearing in a region where noise is threatening to limit detection, this absorption was consistently observed in the trials. Other spectra in Figure 4 show how this weak absorption shifts demonstrably with isotopic substitution. Table 1 is a complete list of all the vibrational frequencies for the low concentration products. They are assigned to the Cu(N0) species, where it is suggested that the nitrogen atom is bonding to the Cu. This is consistent not only with the bonding arrangement in other nitrosyl complexes1° but also with the presence of the absorption at 608.8 cm-'. If this absorption were assigned to a Cu-0 stretch, it would have shifted less upon 15N substitution, ruling out an 0-bonded
bend?
CU(~~NO)(~~NO),C U ( ~ ~ N ~ ~ O ) ( ~ ~ N ~ ~ O ) , 1558.9 1652.1
1516.9 1608.2 1660.8
complex. (This is also the case were this absorption due to a Cu-0-N bend.) It can be assigned to either a Cu-N-0 bend or a Cu-N stretch. However, such modes are usually coupled,1° and unambiguous description is difficult. If it were assigned to a Cu-N stretch, it might be expected to shift much more upon 15Nsubstitution. Its assignment to a definitive Cu-N-0 bend is difficult because it is somewhat high in frequency for such a mode. Approximating Cu-(NO) as a diatomic, the predicted shifts for a Cu-N stretch agree well with the observed shifts, but we are wary of such an approximation for such relatively large atoms. Concentrations of Cu were kept low to maximize identification of monocopper products; however, the concentration of NO was varied to identify higher NO complexes of Cu. In samples having higher NO concentration, a new set of IR absorptions appeared; these are listed, for all isotopomers of NO, in Table 2. All of the detectable absorptions were located in the N - 0 stretching region of the spectrum (see Figures 1-3). One of these absorptions, appearing at 1586.0 cm-l for l4NI6O, was relatively sharp and strong (though weaker than the primary product absorption at 1610.5 cm-') and appeared even at low concentrations of NO, suggesting that either it was due to a Cu(N0)z complex or an isomeric Cu(0N) complex or it was caused by an alternate matrix site. Given the apparent ubiquitous presence of (NO)z in these samples," we favor the first assignment. All of the other absorptions were relatively weak, even in matrices where the NO concentration was
12830 J. Phys. Chem., Vol. 98, No. 49, 1994
1 . ~ 2 0 %of the total sample. This contrasts with the behavior of higher Cu carbonyl complexes, whose extinction coefficients were showngto be on the order of Cu(C0). What this suggests to us is that the additional NO molecules are not true ligands but instead are perturbing the Cu(N0) complex in the matrix. Annealing and photolysis studies were performed as part of this investigation. Photolysis of CdNO/Ar matrices after deposition with light of various wavelength ranges (Le., 13. > 515 nm, 13. > 400 nm, 280 nm < 13. < 360 nm) showed no change in the infrared spectra. Annealing samples up to -28 K and cooling back to -15 K showed no change in the infrared spectra. There was no increase in the heights of the-peaks assigned to higher NO complexes, nor were any decrease in the intensity of Cu(N0) absorptions observed. These studies show that the Cu(N0) complex is relatively resistant to reaction after its formation in the matrix. The IR spectra indicate that the interaction between Cu atoms and NO is not an electron-transfer-type reaction, unlike the reaction between Cu and N02.3 NO- absorbs15at -1350 cm-'. No new absorptions were detected in this frequency range in any of the cocondensation,photolysis, or annealing studies. The electron affinity16 of NO, 0.026 eV, is low enough to justify this behavior (cfi the electron affinity of NO2 at 2.273 eV, and similar studies3 show the spontaneous formation of Cu+N02-). Although there would be additional energy stabilization due to ion pair formation, NO would have to accept an additional electron into a n*zPantibonding orbital, and this apparently does not occur spontaneously in the presence of a Cu atom. The lone electron of NO does serve to make a strong bond to the Cu atom, creating a system that is relatively inert to the effects of relatively higher temperatures and high-energy photolysis. We conclude from this that the complex Cu(N0) is relatively and unusually stable with respect to accepting additional ligands. Its stability is attributed to the combination of the ground-state *S Cu atom ( [ A r ] 4 ~ ' 3 d ~with ~ ) the radical NO molecule (as'). In the presence of excess NO, only weak features announce
Letters the presence of higher NO complexes, in contrast to the behavior of CO toward Cu under similar conditions. Its unusual interaction toward Cu, as contrasted with those of NO2 and CO, prompt us to continue investigating the interactions of NO with other transition metal atoms. We hope that the applications of these results to direct corrosion processes are obvious. Acknowledgment. This work was supported by Cleveland State University. D.W.B. thanks Dr. R. L. R. Towns for assistance. References and Notes (1) Moskovits, M., Andrews, L., Eds. Physics and Chemistry of Matrix Isolated Species; North-Holland Publishing: Amsterdam, 1989. (2) Chawla, S . K. Ph.D. Thesis, Case Westem Reserve University, 1990. (3) Worden, D.; Ball, D. W. J. Phys. Chem. 1992, 96, 7167. (4) Hoty, J. A.; Worden, D. S.; Ball, D. W. High Temp. Sci., submitted for publication. (5) Kafafi, Z. H.; Hauge, R. H.; Margrave, J. L. In Physics and Chemistry of Matrix-Isolated Species; Moskovits, M., Andrews, L., Eds.; North-Holland Publishing: Amsterdam, 1989; pp 277-302. (6) See, for example: Science 1992, 258, 1862. (7) Poliakoff, M.; Turner, J. J. J. Chem. Soc., Dalton Trans. 1973, 1351. ( 8 ) Poliakoff, M. J. Chem. SOC., Dalton Trans. 1974, 210. (9) Huber, H.; Kiindig, E. P.; Moskovits, M.; Ozin, G. A. J. Am. Chem. SOC. 1975, 97, 2097. (10) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley-Interscience: New York, 1986 and references within. (11) Guillory, W. A.; Hunter, C. E. J. Chem. Phys. 1969, 50, 3516. (12) Frank, F.; Schulze, W.; Tesche, B.; Froben, F. W. Chem. Phys. Lert. 1984, 103, 336. (13) Kozuka, M.; Nakamoto, K. J. Am. Chem. SOC. 1981, 103, 2162. (14) Van Leirsburg, D. A.; DeKock, C. W. J. Phys. Chem. 1974, 78, 134. (15) Tevault, D. E.; Andrews, L. J. Phys. Chem. 1973, 77, 1640. Tevault, D. E.; Andrews, L. J. Phys. Chem. 1973, 77,1646.Tevauk D. E.; Andrews, L. Chem. Phys. Lett. 1977, 48, 103. (16) Lide, D. Ed. CRC Handbook of Chemistry and Physics, 73rd ed.; CRC Press: Boca Raton, FL, 1992, p 10-180.
