J. P h y ~ Chem. . 1982, 86, 387-370
R=
co-0 x 100% co-0 + Co-t (2)
where C = total cobalt concentration. For catalysts calcined a t 400 OC, evaluation of Figure 5 indicates that, within experimental uncertainty (dR/dC)4wc = k
creases as the metal loating was increased up to a limiting value. For example, N = 4.2 was observed for a 2% Co/A1203catalyst calcined a t 400 “C, increasing to = 5.2 for a 16% catalyst calcined at the same temperature. The limit corresponds to the average coordination of Co ions in Co304( N = 5.3). In addition, at constant metal loading a lower average coordination number was observed for a catalyst calcined at 600 OC vs. one calcined at 400 “C.
m
and the slope of the reduction curve would be dR/dC = slope
367
(3)
where k is a constant at the calcination temperature; Le., R increases linearly as a function of Co concentration. In contrast to the above, (dR/dC)6000C = 0 up to 1-270 Co for catalysb calcined at 600 OC; Le., initially all Co ions occupy tetrahedral lattice sites. Above this concentration, R varies linearly with concentration or (dR/dC),., = k’for C > 1% (4) The magnitudes of k and k’ are related to the filling of octahedral sites vs. tetrahedral sites since these quantities depend on eq 1. From Figure 5 it is apparent that k > k’. This result indicates that at the lower calcination temperature (400 “C), filling of the octahedral sites is more favorable than at 600 “C. In addition, as the metal loading is increased for a given calcination temperature the formation of Co-o becomes more probable. This is particularly evident for catalysts calcined at 600 OC since at concentrations below 1% Co, only Co-t is formed or (dR/dC) = 0. This behavior is quite consistent with the sequential impregnation experiments of Chung and Mas~0th.~ Preliminary results obtained from EXAFS experiments at the Stanford Synchrotron Radiation Laboratory are in agreement with the results presented above.24 It was observed that for catalysb calcined at a fixed temperature, the average coordination number (2y) of the Co ions in(24)R. B. Gregor, F. W. Lytle, R. L. Chin, and D. M. Hercules, J. Phys. Chem.,86,1232 (1981).
Conclusions It is evident from the above study that many factors affect the structural and chemical properties of C0/A1203 catalysts. Of primary importance is the interaction between the metal ions and support which renders a portion of the metal ions chemically inactive. The existence of this interaction appears to be a general characteristic of most y-A1203-supportedsystems. It was shown that the interaction may lead to tetrahedrally and/or octahedrally coordinated c o ions. At low metal concentrations, the cobalt is predominantly in tetrahedral lattice sites of the support (Co-t). Formation of this species is favored by high calcination temperatures; the percentage of cobalt ions which exist as Co-t is greatest at low metal loadings. At higher cobalt concentrations an octahedrally coordinated interaction species (Ceo) is produced. Eventually a “bulk-like” Co304phase segregates on the surface of the catalyst. The extent of the interaction has a pronounced effect on catalyst reducibility. It has been established that the ability of the catalysts to undergo reduction is hindered by strong metal-support interaction. This is attributed to formation of the chemically inert Co-t species. Its influence on catalyst reducibility is especially prevalent for catalysts of low metal loadings and high calcination temperatures. Acknowledgment. The authors would like to thank Dr. M. Wu for valuable discussions and suggestions. The spectroscopic work was supported by the National Science Foundation under Grant No. CHE78-00876 and the reduction studies by the US.Department of Energy under Grant No. DE-AC02-79ER10485. A000.
Paramagnetic Impurities in Cr(CO)6 Crystals.’ 2. The EPR Spectrum of Mn(C0)4NOT.
J. R. Morton, and K. F. Preston’
Divlslon of Cbmlstty, Natlonal Research Councll of Canada, Ottawa, Canada K1A OR9 (Received: June 10, 1981; I n Flnal Form: September 30, 1981)
An anisotropic EPR spectrum detected in y-irradiated single crystals of Cr(CO)6doped with MXI(CO)~NOis attributed to the radical anion of the latter. The unpaired electron in this molecule occupies an antibonding u* orbital composed primarily of Mn(Bd,z) and N(2p,) atomic orbitals. There is some evidence suggesting that the Mn-N-0 moiety in the free radical is bent.
Introduction In recent studies of the products of irradiation of certain transition-metal carbonyl nitrosyls we detected the EPR spectra of a number of electron-excess, or “19-electronn, species: Fe(N0)2(CO)2-,Fe(NO),CO, Mn(NO),CO-, and (1)NRCC No. 19813. (2) NRCC Reeearch Associate 1980. O022-3654/82/2086-0387~01.2510
C O ( N O ) ~ ( C O ) ~The , ~ *composition ~ of the semioccupied molecular orbital (SOMO) in these paramagnetic nitrosyls is somewhat unusual in that contributions from the transition-metal atomic orbitals are very small. The unpaired (3)J. R. Morton, K. F. Preston, and S. J. Strach, J. Phys.Chem.,84, 2478 (1980). (4)C. Couture, J. R.Morton, K. F. Preeton, and 5.J. Strach, J. Magn. Reson. 41,88 (1980).
0 1982 American Chemical Society
388
The Journal of Physical Chemistry, Vol. 86, No. 3, 7982
,
Lionel et ai. 100 GAUSS
.I
Flgure 2. Firstderivatlve EPR spectrum of Mn(CO),NO- impurlty In a single crystal of Cr(CO),, at 90 K for H, parallel to the a axis and a microwave frequency of 9350 MHz.
Figure 1. b axis projection of the Cr(CO),, crystal structure. Atom nwnberhrg is identical wlth that in ref 9, from which the crystabgaphic data were obtained. Cr and atoms numbered 1 and 2 lie In the ac plane; atoms numbered 3 and 4 lie above the plane and are related by reflection In ac to equivalent atoms 3a and 4a below the plane.
electron occupies a “a”orbital, analogous to the aFSOMO of N;, which consists principally of equal 2p atomc orbital components on two geometrically equivalent nitrogen nuclei. This tends to,suggest that the presence of two NO ligands confers stability on the electron-excess molecule through a delocalization of the unpaired electron. Our success in employing single crystals of Cr(CO)6 as a host for paramagnetic metal carbonyls5 encouraged us to attempt the generation of electron-excess nitrosyls in that matrix. Although we have been frustrated in our efforts to observe the above-mentioned dinitrosyl radicals in Cr(CO)&we have detected a new manganese nitrosyl in y-irradiated Cr(CO)6containing MXI(CO)~NOimpurity. We believe the new radical to be the electron-excess center MII(CO)~NO-in which the unpaired electron is localized in a Mn-N Q* orbital.
discrete Cr(CO)6 molecules (4 per unit cell) whose StrUCtures are very nearly regular octahedral. A projection in the ac “mirror” plane (Figure 1)shows the salient features of the crystal structure. An axis, O(l)-C(l)-Cd(2)-0(2), of each Cr(CO)6 molecule lies in the ac plane approximately f34” from c, with the remaining four carbonyl ligands (3 and 3a, 4 and 4a) disposed symmetrically above and below that plane. Two types of substitutional impurity centers have been detected in Cr(CO)6by EPR5J0J1“mirror-plane” species and “skew” species. In the former, the principal directions of the g2tensor are in ac parallel to Cr-C(l or 2), the b axis, and in ac perpendicular to Cr-C(l or 2). For species of this type, the only nonzero off-diagonal elements of g2 in the crystal axis system are the ac elements, and two sites are observed for an arbitrary alignment of the crystal with respect to Ho.The “skew” species have one principal direction parallel to Cr-C(3,3a, 4, or 4a), all off-diagonal elements of the g2tensor are nonzero, and four sites are observed for an arbitrary direction of Ho.
Results For certain orientations of the crystal with respect to particularly simple EPR the static magnetic field (Ho), spectra (Figure 2) were observed. These consisted of a sextet of triplets having an effective g value not far from free spin. Evidently, the paramagnetic center in irradiated Cr(C0)6:Mn(C0)4N0possessed an unpaired electron which interacted with one ssMn(l= 5/2) nucleus and one Experimental Section 14N(I = 1)nucleus. Such simple spectra were observed MII(CO)~NOwas prepared by the nitrosylation of Mnthroughout a plane of the crystal and also for the direction (CO)5H with N-methyl-N-nitroso-p-toluenesulfonamide perpendicular to that plane. This plane and the direction (Aldrich) in diethyl ether.6 Mn(C0)5Hwas obtained via perpendicular to it were identified as the crystallographic M I I ( C O ) ~ Nby ~ the reduction of Mn2(CO)lo (Strem ab plane and the c axis, respectively, of the orthorhombic Chemicals, Inc.) with sodium amalgam in tetrahydrofuran? ~ certain other directions each of the Cr(CO)6~ r y s t a l .For Small amounts (-0.05%) of Mn(C0)4N0were doped lines of a coalesced spectrum split into four, corresponding into Cr(CO)6 by cosubliming a mixture of the two comto four magnetically distinguishable sites for the impurity pounds at 50 “C onto a cold (10 “C) surface. Suitably sized center in the crystal. As indicated above, four-site impurity single crystals were irradiated for several hours at 77 K centers in Cr(CO)6usually exhibit site splitting in all three in a 9000-Ci 6oCogamma cell, and were then mounted crystallographic planes. It was clear at this point, thereunder liquid N2 on a two-circle goniometer? EPR spectra fore, that we were dealing with a manganese nitroxyl for were recorded and measured at 90 K with a Varian E-12 which the ab elements of the tensors of g2,ahIn2, and aN2, spectrometer and accessories. were accidentally zero. Since the off-diagonal elements for all three tensors were Crystallography zero in the ab plane, it was necessary to examine that plane Chromium hexawbonyl aystallizes in the orthorhombic first in order to establish goniometer readings for the axes ~ lattice consists of system with space group P n r n ~ .The a and b. These were identified as the turning points in plots of g2and a2against angle in the ab plane. Spectra (5)M. P. Boyer, Y. Le Page, J. R. Morton, K. F. Preston, and M. J. Vuolle, Can. J. Spectrosc., in press. (6) P. M. Treichel, E. Pitcher, R. King, and F. Stone, J. Am. Chem. SOC.,83,2593 (1961). (7)K.J. Reimer and A. Shaver, Znorg. Synth., 19, 162 (1979). (8)G.Roberta and W. Derbyshire, J. Sci. Instrum., 38,511 (1961).
(9)A.Whitaker and J. W. Jeffrey, Acta. Crystallogr., 23,977(1967). (10)T. Lionel, J. R. Morton, and K. F. Preston, J. Chem. Phys., in press. (11)T. Lionel, J. R. Morton, and K. F. Preston, Chem. Phys. Lett., 81,17 (1981).
The Journal of Physical Chemlstty, Vol. 86, No.
EPR Spectrum of Mn(CO),NO-
3, 1982 309
TABLE I: g2 and Hyperfine Interaction-Squared" Tensors for Mn(CO),NO- in Cr(CO), principal values" and direction cosines in abc
tensors6 a
g2
aMn2
aN2
" Units are gauss'.
1 I
5817 L37
-?*O 11.1
b
C
2
X
Y
0 4.0667 0.0103
0.02923 0.0103 4.0289
0.5475 0.1439 - 0.8243
- 0.5547 0.8000 -0.2288
0.6266 0.5825 0.5178
0 7327 793
;xi7 4254
0.5629 0.1361 -0.8152
-0.6401 0.6957 -0.3528
0.5229 0.7053 0.4787
0 30.7 - 5.0
-11.1 -5.0 47.2
1 1
55.3
33.0
0.5304 0.1692 -0.8307
-0.6894 0.6564 -0.3064
27.5 0.4934 0.7352 0.4648
Sign choice for the off-diagonal corresponds to one of four distinguishable sites.
TABLE 11: Angle between Cr-C Directions in Pure Cr(CO), Crystal and Principal Directions of the Impurity Center direction cosines bond cr-C( 1 ) cr-C( 3 ) cr-C(4)
a 0.557 -0.590 0.585
b 0.000 0.704 0.712
C
-0.830 -0.395 0.389
were then recorded at small angular intervals (usually loo) throughout the remaining two crystallographic planes. As expected, the spectra in those planes revealed the presence of two distinguishable sites. Best-fit tensor elements for g2,afi2, and aN2were obtained by least-squares fitting sinusoidal curves (Figure 3) to magnetic parameters (g2, a2) derived from second-order analyses of individual spectra. The tensors in the crystal axis system are assembled in Table I along with their principal values and directions.
Discussion The principal values and directions of the three tensors given in Table I provide us with detailed information concerning the orientation of the paramagnetic nitrosyl in the host lattice, its geometry, and its electronic structure. We note first that the tensors are orthorhombic (no two principal values equal) and their principal directions are parallel within experimental error (one standard deviation elo).The departure from axiality is small but well outside experimental error for both g2and aMn2.This is confirmed by the observation (Table 11) of a strong correlation between the three principal axes of each tensor and Cr-C bond directions in the crystal lattice. Had the tensors been strictly axial (two principal values equal), we would not have expected the directions of the perpendicular components ( x , y ) to correspond to a particular crystal direction. This was the case, for example, for the *skew" species Fe(C0)5+and Mn(CO)&l- in Cr(CO)&loJ1 The most important fact which emerges from Table I is that the directions ( 2 ) of g(min), aMn(min),and aN(max) are parallel. From this fact alone, one can infer that the unpaired electron residues in an orbital composed of Mn(34z) and N(2pJ atomic orbitals. Bearing this in mind, the only physically reasonable principal values of the &Mn and 14Nhyperfine tensors are (in G ) (-83.4, -88.7, +50.8) and (-5.7, -5.2, +7.4), respectively. Using the appropriate 0 ) r(r3)for the one-electron parameters (8 ~ r / 3 ) # ~ ( and valence atomic orbitals,12 we estimated unpaired spin (12)J. R.Morton and K. F. Preston,J. Magn. Reson., 30,577 (1978).
tensor principal axis 2 X
Y
4.06
angle, deg g 8.4 11.2 10.7
aMn
8.0 5.0 6.1
aN 10.0 8.2 6.8
4 fl
0
a
15
30 45
60
7 5 90 c_
Figure 3. Plot of g2 vs. angle in the ac crystal plane. Experimental
data are represented by squares:the curves are the best ieast-squares fit for the two dlstlngulshable sites.
populations of 72% for Mn(3d2a) and 22% for N(2p2), together with very small negative values of -2% for Mn(4s) and -0.2% for N(2s). The latter contributions are of a size and sign consistent with core polarization of inner atomic s orbitals. The description of the SOMO for the manganese nitrosyl provided by the data of Table I is then clear: the unpaired electron occupies an antibonding Mn-N u* molecular orbital. The SOMO thus resembles that of the radical Mn(CO)&-, in which the unpaired electron is essentially located in a Mn-C1 u* orbital."J3 A comparison of magnetic parameters for the two species is given in Table 111. The close resemblance of the spectroscopic data for the two radicals leads us to identify the manganese nitrosyl as the electron-excess species Mn(C0)4NO-. (13)0.P.Anderson, S. A. Fieldhouse, C. E. Forbes, and M. C. R. Symons, J. Chem. SOC.,Dalton Trans., 1329 (1976).
370
The Journal of Physical Chemistry, Voi. 86, No. 3, 1982
TABLE 111: Comparison of g and Hyperfinea Tensors for the Radicals Mn(CO),CI- and Mn(CO),NOMn(CO),CI2.0067 2.0076 1.9997 I X
-51.7
aMn{ Y
-51.1 42.8
2
{:
a c l o r aN Y a Units
(r )21.6
(+ )20.0 46.2
Mn(CO),NO2.0159 2.0189 2.0019 -83.4 - 88.7 50.8
- 5.1 - 5.2 7.4
are gauss.
The principal difference between the two manganese radicals is the lower symmetry of the nitrosyl: its g and afi tensors are orthorhombic (no two principal values equal) whereas those of the chloride are axial (two principal values equal). This difference might have been predicted from the symmetries of the neutral manganese impurity molecules, had these been preserved upon electron capture: Mn(CO)&l has C , symmetry, M~I(CO)~NO has C% symmetry only. However, we believe that the nitrosyl tensors are orthorhombic because the Mn-Na moiety in the free radical is bent. Evidence strongly supporting this contention comes from a consideration of the orientation of the manganese nitrosyl in the Cr(CO)6 lattice. The data of Table I1 show that the z axes of the tensors lie 8-10' from the Cr-C(l) direction of the undamaged host lattice. Hitherto, all spectroscopic tensors which we have examined5J0J1had a principal value which lay, within either along one of the four experimental error (fl'), equivalent "skew" Cr-C(3,3a, 4, or 4a) directions or along one of the "mirror-plane'! (ac) Cr-C(l, 2) directions." Paramagnetic impurities in Cr(CO)6have thus fallen into one of two classes: four-site (skew) species, or two-site (mirror-plane) species, whose tensors contain six or two nonzero off-diagonal elements, respectively. The manganese carbonyl nitrosyl reported here is a four-site species, for which z is oriented almost along the mirror-plane directions Cr-C(l, 2). In view of the established behavioFJO (14) Although Cr-C(l) and Cr-C(2) are crystallographically distinguishable, we have never observed spectra which enable us to make the distinction.
Lionel et ai.
of radicals such as VO(CO)52+and Fe(C0)5+in CI-(CO)~, we have every reason to believe that for Mn(C0)4NOhaving a linear Mn-N-0 group z would lie exactly along Cr-C( 1, 2). We conclude, therefore, that upon electron capture the N atom lifts out of the ac plane and that the Mn-N-0 group in the nitrosyl anion is bent. Without W-hyperfine interaction data we are, of course, unable to determine the positions of the carbonyl ligands with respect to the central manganese atom. In the original nitrosyl, the carbonyls are equivalent in pairs in a trigonal bipyramidal structure.15 The same arrangement could be present in the anion radical, in which case a Mn-C (axial) bond would lie within a few degrees of Cr-C(3,3a, 4,4a). It is more likely, however, that in the Cr(CO)Goctahedral host the manganese nitrosyl is approximately square-pyramidal. Enemark and Feltham16have discussed the structures of metal nitrosyls in terms of molecular orbital theory. Metal-nitrosyl interactions are considered to dominate the overall ligand field, and an orbital scheme was produced in which the effective number of d electrons is determined by considering nitrosyl ligands bound as NO+. Thus, MII(CO)~NO-would be reformulated as Mn2-(CO),(NO+), Le., {MnNOI9or an effective de species. Unfortunately, Enemark and Feltham's schemes for five-coordinate mononitrosyls predict a SOMO having a(N0) rather than a(N0) character for a de species. We feel that their ordering of the a*(NO)d,,,d,,, and d,z,a(NO) orbital should be reversed, leading to a scheme similar to that of Gray, Bernal, and Billig" in which the ninth d-electron occupies a a*(d,z,a(NO)) orbital. Such a scheme, in which the a*(NO) levels lie immediately above the highest d orbital (d,z), offers a simple explanation of the a-type structures found for electron-excess dinitrosyls such as CO(NO)~(CO)~ species whose This radical may be classed as a (CO(NO)~I" metal d shell is filled and whose last electron enters a a*(NO) orbital. The presence of trans nitrosyls in such radicals permits ?r delocalization of the unpaired electron and results in considerable stabilization. (15) B. A. Frenz, J. H. Enemark, and J. A. Ibers, Inorg. Chern., 8,1288 (1969). (16) J. H. Enemark and R. D. Feltham, Coord. Chern. Rev., 13, 339 (1974). (17) H. B. Gray, I. Bernal, and E. Billig, J.Am. Chern. SOC.,84,3404 (1962).
