Inorganic Chemistry, Vol. 12,No.10, 1973 2269
Tris(N,N-diethyldithiocarbamate)iron(III)
Contribution from the Chemistry Department, State University of New York at Buffalo, Buffalo, New York 14214
Correlation between Structure- and Temperature-DependentMagnetic Behavior of Iron Dithiocarbamate Complexes. Crystal Structure of Tris(N,N-diethyldithiocarbamato)iron(III) at 297 and 79°K J. G. LEIPOLDT and P. COPPENS*
ReceivedAprill6, 1973 The crystal structure of tris(N,N-diethyldithiocarbamato)iron(III), Fe(S,CNEt,), , has been determined from three-dimensional counter data at 297 and at 79°K to correlate the temperature dependence of its magnetic moment (4.3 and 2.2 BM at the two temperatures, respectively) with structural changes. The space group of the crystals changes on cooling from P2,/c to C2/n. Lattice constants are a = 14.29 (l),b = 10.37 ( l ) , c = 17.87 (1) A, p = 116.6 (1)" at 297°K and a = 13.96 ( l ) , b = 10.24 ( l ) , c = 17.71 (1) A, p = 116.5 (1)" at 79'K. 2 = 4 at both temperatures. The data have been refined by least-squares to R(F) = 5.4% (297'K) and 6.8%(79°K). At room temperature the iron atoms are located on a pseudo-twofold axis, which becomes a true twofold axis in the low-temperature structure. The change in space group is accompanied by a contraction of the Fe-S distances of 0.05 A and a small but significant decrease in distortion from octahedral symmetry of the FeS, group. When the average Fe-S distances in all known iron dithiocarbamato complexes are plotted against their magnetic moments, a smooth curve is obtained indicating strong correlation between geometry and magnetic behavior. Differences of 0.018-0.028 A between the averages over the two sets of Fe-S distances related by the pseudothreefold axis observed in earlier studies of low-spin complexes are not present in the structure at 79°K. Thus, these differences are probably not due to Jahn-Teller and spin-orbit interactions in the low-spin state as proposed earlier. Slight differences in the geometry of the ligand at the two temperatures are compatible with a mechanism by which the effect of the substituent on the crystal field is transmitted through the conjugated system of the ligand. Analysis of the temperature parameters at room temperature indicates that the present diffraction data cannot distinguish between a mixed-spin state and the existence of a mixture of two different spin states as an explanation for the variability of the magnetic moment.
Introduction Earlier studies have shown that the tris(N,N-dialkyldithiocarbamato)iron(III) complexes with general formulas Fe(S2CNR2)$ have a distorted octahedral geometry with a temperature-dependent magnetic moment.' The temperature dependence has been explained through the existence of two nearly equienergetic ground states of symmetry 6A and 'T, but Mossbauer3 and proton nmr studies failed to show evidence for the simultaneous population of two electronic levels. Though it is possible to explain the spectroscopic results by a rapid crossover between the levels (>IO7 sec-'), there is an alternative explanation based on crystal field calculations by Harris?~' according to which the ground state may be a mixed-spin state, the variable magnetic moment being due to a change in character of the ground state with temperature. The calculations by Harris served to explain the difference in magnetic moments of the ferric ion in hemoglobin derivatives and in different heme proteins. Thus, the iron dialkyldithiocarbamates may be magnetic analogs of heme iron proteins and an elucidation of their magnetic behavior is of considerable importance. Whether the compound is high spin or low spin at a given temperature depends on the substituent R2 although R is on the periphery of the molecule. It has been proposed that R affects the crystal field due to the sulfur atoms through the conjugated system of the ligands.' If the complexes change from the high-spin state to the low-spin state, one would expect a contraction of the FeS6 core. Evidence for this contraction is obtained (i) from pressure-dependent studies' and (ii) from structural data available for different complexes.6-8 9'
(1) A. H. Ewald, R. L. Martin, I. G. Ross, and A. H. White, Proc. Roy. Soc., Ser. A , 2 8 0 , 235 (1964). (2) A. H. Ewald, R. L. Martin, E. Sinn, and A. H. White, Inorg. Chem., 8 , 1837 (1969). (3) P. B. Merrithew and P. G. Rasmussen, Znorg. Chem., 11, 325 (1972). (4) G. Harris, Theor. Chim. Acta, 5 , 379 (1966). (5) G. Harris, Theor. Chim. Acta, 1 0 , 119 (1968). (6) B. F. Hoskins and B. P. Kelly, Chem. Commun., 1517 (1968).
(i) It was calculated from pressure-dependent studies (in solution) that the Fe-S distance decreases by about 0.07 on going from the high-spin to the low-spin state if the entire volume change is due to the change in the radius of the FeS6 molecular core. This assumption is supported by the absence of a volume change with increasing pressure for the isomorphous cobalt dithiocarbamates. (ii) Crystallographic evidence for this contraction can be obtained from the available structural data of tris(N,N-di-nbutyldithiocarbamato)iron(III) (A),6 tris(o-ethyl xanthate) iron(II1) (B),7 tris( 1-pyrrolidinedithiocarbamato)iron(III) (C),' and tris(N-methyl-N-phenyldithiocarbamato)iron(III) (D).' The mean Fe-S distance is 2.41 for compounds A and C which are predominantly in the high-spin state, with magnetic moments respectively 5.3 and 5.9 BM, while the mean Fe-S distance is 2.3 15 A . for compounds B and D which are predominantly in the low-spin state, with magnetic moments 2.7 and 2.9 BM, respectively. Thus, there appears to be a contraction of about 0.08 A in the Fe-S distances on going from the high-spin to the low-spin state. The structural data for compounds A-D also show that there is a significant change in the geometry of the FeS6 polyhedron. The structural information as summarized above is from room-temperature (film) data on four different compounds. Because the geometry of the FeS6 polyhedron could be affected by the variation of R, we decided to collect diffractometer data for one compound at both room temperature and 79°K. Tris(N,N-diethyldithiocarbamato)iron(III) was selected; it is predominantly in the high-spin state at room temperature with a magnetic moment of 4.3 BM and predominantly in the low-spin state at 79°K with a magnetic moment of 2.2 B M . ~ By studying one compound at different temperatures the geometric change accompanying the variation in magnetic moment can be measured?and information can be obtained
a
a
(7) B. F. Hoskins and B. P. Kelly, Chem. Commun., 45 (1970). (8) P. C. Healy and A. H. White, J. Chem. Soc., Dalton Trans., 1163 (1972).
2270 Inorganic Chemistry, Vol. 12, No. 10, I973 that is not accessible through studies confined to room temperatures. Experimental Section
J. G. Leipoldt and P. Coppens Table I. Crystal Data for Fe(S,CNEt,), at 297 and 79°K a, A
297°K 14.29 (1) 10.37 (1) 17.87 (1) 116.6 (1) 4 1.404 P2,k
79°K 13.96 (1) 10.24 (1) 17.71 (1) 116.5 (1) 4 1.467 C2/n
b, A The compound was prepared by adding 100 ml of a concentrated C, a solution of the sodium salt of the Ligand (0.03 mol) to 100 ml of a D>deg concentrated aqueous solution of ferric chloride (0.01 mol). The 2 precipitate was filtered and extracted with chloroform. Small black d,, g cm-3 crystals precipitated on slow addition of ethanol to the separated Space group chloroform layer. They can be recrystallized from benzene. The crystals are not air sensitive and can be stored for prolonged periods. The same crystal (with dimensions 0.17 X 0.17 X 0.26 mm3, elongated parallel to c) was used for both the high- and low-temperature data collection. The three-dimensional diffraction data were collected on an automated Picker diffractometer. The c axis was approximately parallel to the $I axis of the diffractometer. For low-temperature data collection, cooling was achieved with the selfD regulating X-ray cryostat CT 38.' The crystal is mounted on a copper block which is continuously cooled by cryogen supplied through a flexible transfer line. Calibration of the cryostat with a sample of TbVO, showed that at the phase-transition temperature of 32°K the sample is not more than 2" above the temperature of the copper block. In the present experiment the copper block was kept at 78"K, so the sample temperature will be described as 79 c 1°K. To prevent thermal shock damage to the crystal special care was taken to keep the temperature constant during refill of the intermediate supply dewar. The systematic absences as obtained from room-temperature Weissenberg film data are hOI, 1 = 2n + 1, and OkO, k = 2n - 1. The cell dimensions at room temperature and at 79°K are given in Table I. 2 The reflections of the type h t k = 2n + 1 were very weak, indicating that the lattice is pseudo-C-centered. Precession photographs -~ Figure 1. Schematic drawing of the FeS, polyhedron, at about -140" showed that all reflections of the type h ik = 2n + 1 type h t k = 2n were used in the next two cycles. The R value for disappeared to give the space group C2/c. It follows that the molecule these data was reduced to 2% with little difference between the two has pseudo-twofold symmetry at room temperature and a true twogroups of reflections, indicating satisfactory convergence. fold symmetry axis at low temperature. This phase change is reversThe structure was then further refined with all data using anisoible as shown by subsequent precession photographs of the same tropic thermal parameters for all the atoms. The final R value was crystal at room temperature. 5.4%( R w = [ Z w ( F 0 - klFc~)2/Z~Fo2]"2 = 6.1%). In the room-temperature data collection (Mo Ka: radiation) the To use the same atom parameters for the refinement of the lowintensities were measured by the moving-crystal, moving-countcr temperature data, *i2, ' i 2 ,0 was added to the equivalent positions of technique using a 2(1.5 t 0.692 tan 8) scan in 28 at a rate of l"/min, the room-temperature space group P2,/c. This means that the lowwith 20ma, = 45". The background intensities were estimated by temperature space group was C2/n, rather than the conventionally the counting for 10 sec at the beginning and end of each scan. A set listed space group C2/c. The final R value for the low-temperature of symmetry-equivalent reflections up to 28 = 20' was also collected. structure was 6.8% ( R , = 8.3%). Three standard reflections were measured after every 25 reflections In both refinements the weights w ( F ) were obtained with the to monitor long-range changes in the beam intensity or the crystal expressions 0 2 ( ( F 2 = ) a2(counting) + (0.03F2)2and a ( F ) = a(F2)/2F. reflectivity. No such changes were observed during data collection. The estimate of the proportionality constant (0.03) is based on the The low-temperature data were obtained in the same way except comparison of the intensities of symmetry-related reflections. Rethat the step-scan method was used with a step size of 0.06". The flections with F
