J. Am. Chem. SOC.1981,103, 1932-1941
Structural Characterization of the [ Fe4(S2C2(CF3)2)4(p3-S),12- Dianion. Stereochemical Relationship between the Cubanelike Fe4S4Cores of This Balch Dithiolene Dianion and the [ Fe4(S ~ - C ~ H ~ ) ~ ( ~ ~ - S ) ~ ] * ' Dication and Resulting Electronic Implications Timothy H. Lemmen, Joseph A. Kocal, Frederick Yip-Kwai Lo, Michael W. Chen, and Lawrence F. Dahl* Contributionfrom the Department of Chemistry, University of Wisconsin-Madison, Madison. Wisconsin 53706. Received June 2, 1980
Abstract An X-ray crystallographicanalysis of the tetrakis(cis-l,2-bis(tritluoromethyl)ethylene 1,2-dithiolato)tetrairon tetrasulfide dianion as the tetraphenylarsoniumsalt has provided (1) unequivocal proof that its central cubanelike Fe4S4core experimentally conforms to a regular tetragonal D r 4 2 m geometry whose Fe-Fe and Fe-S dimensions closely resemble those of the Fe4S4 core in the [Fe4($-C,H,)4(p3-S)4]2+dication and (2) the correct coordination (previously assumed to be a distorted trigonal-bipyramidal arrangement) of five sulfur atoms about each chemically equivalent iron atom. The Fe4S4framework of the dianion possesses a uniformly compressed iron tetrahedron (along the S4-4 axis) with four shorter lengths of mean 2.855 A and two longer lengths of mean 3.442 A; the four vertical Fe-S bonds (approximatelyparallel to the S4axis) of mean 2.153 A are 0.09 A shorter than the other eight equivalent Fe-S bonds of mean 2.247 A. The disposition of the four S2C2(CF3)2 ligands about the Fe4S4core gives rise to a nearly regular square-pyramidalsulfur environment about each iron atom. Inclusion of the dithiolene ligands results in a small but significant sterically induced angular distortion which lowers the symmetry for the entire dianion to an approximate S4configuration. A geometrical comparison of the four equivalent (Fe-Fe)-bonded Fe2S2 fragments in the dianion is made with the planar Fez& fragment possessed by the [Fe2(SzCz(CF3)2)4]monoanion and other [M2(S2X)4]ndimers (M = Fe, Co), and the resulting bonding implications are discussed. The geometrically similar Fe4S4cores in the [Fe4(S2C2(CF3)2)4(p3-S)4]2dianion and [Fe4(q5-C5H5)4(p3-S)4]2+ dication provide evidence for equivalent Fe-Fe interactions and thereby furnish a basis for the application of a qualitative metal cluster model which not only correlates the electronic configuration of the dianion with its geometry but also enables a prediction of probable geometrical changes series. [AsPh4 2+[Fe4(S2C2expected for the other two isolated members ( n = 0, -1) of the [Fe4(S2Cz(CF,)2)4(p3-S)41n (CF )2)4(p3-S)4]2-crystallizes in a monoclinic unit cell of €9/ n symmetry with lattice constants a = 13.832 (3) ,b = 37.279 (7) c = 14.973 (3) A, @ = 101.27 (4)O, and V = 7572.0 pcpld = 1.78 g/cm3 for 2 = 4. Least-squares refinement (with anisotropic thermal coefficients for the As, Fe, and S atoms, isotropic temperature factors for the C and F atoms, and a rigid-group constraint for each of the eight phenyl rings) gave R,(F) = 12.6% and Rz(F) = 11.7% for 3899 independent diffractometry collected data with I > 2 4 Q .
A,
b,
A3;
The work presented herein is an outgrowth of our structural quently concluded3that under assumed D u symmetry the resulting investigations of the [Fe4(~5-C5H5)4(p3-S)4]n clusters (n = 0,' 1: mean distances and mean bond angles corresponded in a gross 23) for the purpose of establishing a correlation between geometry fashion to those of the [Fe(q5-C5H5)4(p3-S)4]2+ dication. We, and electronic configuration in this cubanelike Fe4S4series. A therefore, proposed3 that the idealized Fe4S4architecture in the crystallographic analysis of the dication revealed an unanticipated iron dithiolene anion is structurally and electronically similar to tetragonal D u geometry for its Fe4s4core with four relatively short that in the iron cyclopentadienyl dication. In order to provide an operational test of this hypothesis, it was necessary to perform and two long F e F e distances. Of particular interest is the overall an X-ray diffraction study on the tetraphenylarsonium salt of the resemblance of this architecture with that of the bis(trifluor0methyl)-l,2-dithiolene dianion of the [Fe4L4(p3-S)4]nseries (L [Fe4(S2C2(CF3)z)4(p3-S)4]2dianion which was prepared and isolated via the Balch p r o c e d ~ r e . ~The results given here are = S2C2(CF3)2, n = 0, -1, -2) whose members were synthesized, relevant not only in connection with the Mossbauer data reported isolated, and physicochemically characterized by B a l ~ h . An ~ ~ )Good ~ ) ~ -and ChandraSa and later by Reiff and co-workerssbfor X-ray diffraction examination of [ N B u ~ ] ~ + [ F ~ ~ ( S ~ C ~ ( C Fby (p3-S)4]2-was carried out by Bernal and Davis? who were unable the [Fe4(S,C2(CF3),),(p3-S),1" tetramers ( n = 0, -1, -2 as the to refine the crystal structure due to a crystallographic disorder [AsPh4]+salts) but also with respect to the remarkably different problem which involved the nonresolution of the two independent structural-bonding influences found when other terminal ligands tetra-n-butylammonium cations. Although the large observed such as SR,6 C1', NO: and (CO)? are coordinated with each iron variations in the Fe-Fe, Fe-S, and S-23 distances of the Fe4S4 series atom of an Fe4S4core. The fact that the [Fe4(SR)4~3-S)4]n framework (e.g., the six reported Fe-Fe distances are 2.58, 2.75, ( n = -1, -2, -3) has been shown from extensive studies of their 2.78, 2.80, 3.19, and 3.26 A with esd's of 0.02 A) prevented an structural and physicochemical properties by Holm, Ibers, and assignment5 of any idealized geometry to this cluster, we subseco-workers1° to be the first well-defined synthetic analogues of (1) (a) Wei, C. H.; Wilkes, G. R.; Treichel, P. M.; Dahl, L. F. Inorg. Chem. 1966,5,900-905. (b) Schunn, R. A.; Fritchie, C. J., Jr.; Prewitt, C. T. Ibid. 1966, 5, 892-899. ( 2 ) Trinh-Toan; Fehlhammer, W. P.; Dahl, L. F. J. Am. Chem. Soc. 1977, 99,402-407. (3) Trinh-Toan; Teo, B. K.; Ferguson, J. A.; Meyer, T. J.; Dahl, L. F. J . Am. Chem. SOC.1977, 99,408-416. (4) Balch, A. L. J. Am. Chem. SOC.1969, 91,6962-6967. (5) (a) Bernal, I.; Davis, B. R.; Good, M. L.; Chandra, S. J . Coord.Chem. 1972, 2, 61-65. (b) Frankel, R. B.; Reiff, W. M.; Bernal, I.; Good, M. L. Inorg. Chem. 1974, 13,493-494.
0002-7863/8 1/1503-1932$01.25/0
(6) (a) Laskowski, E. J.; Reynolds, J. G.; Frankel, R. G.; Foner, S.; Papaefthymious, G.C.; Holm, R. H. J . Am. Chem. SOC.1979,201,6562-6570 and references cited therein. (b) Berg, J. M.; Hodgson, K. 0.;Holm, R. H. Ibid. 1979, 101, 4586-4593 and references cited therein. (7) Bobrik, M. A.; Hodgson, K. 0.;Holm, R. H. Inorg.Chem. 1977,16, 1851-1858. (8) (a) Gall, R. S.; Chu, C. T.-W.; Dahl, L. F. J. Am. Chem. SOC.1974, 96,4019-4023. (b) Chu, C. T.-W.; Lo, F. Y.-K.; Dahl, L. F., submitted for
publication. (9) Nelson, L. L.; Lo, F. Y.-K.; Rae, A. D.; Dahl, L. F., unpublished research.
0 198 1 American Chemical Society
J. Am. Chem. SOC.,Vol. 103, No. 8, 1981 1933
lFe4(s2C2(CF,134(~~-s)~12-
A
the Fe4S4cluster units in bacterial ferredoxins and in the highpotential iron proteins has stimulated considerable interest concerning structural differences in these Fe4S4complexes.
Experimental Section Crystal Data. [ASP~~]~+[F~~(S~C~(CF~]~)~(~~-S)~]~; (fw = 2023.0) was synthesized and black crystals were obtained as descnbed previously! A suitable parallelepiped-shaped crystal of dimensions 0.40 X 0.28 X 0.25 mm was mounted and sealed in a glass capillary under an argon atmosphere. Intensity data were obtained with Mo K a radiation on a Syntex Pi diffractometer via the w scan method. The detailed procedures of crystal alignment, data collection, and treatment of the diffraction data are given elsewhere.” An analytical absorption correction12was applied to the intensity data in that the calculated transmission coefficients (based upon a calculated linear absorption coefficient of 21.1 cm-I for Mo Ka radiation) ranged from 0.52 to 0.68. Lattice constants measured at ca. 22 OC for the chosen monoclinic unit cell are a = 13.832 (3) A, b = 37.279 ( 7 ) A, c = 14.973 (3) A, and @ = 101.27 (4)’; the unit cell volume of 7572.0 A’ gives rise to a calculated density of 1.775 g/cm3 for z = 4. Structural Determination and Refmement. Systematic absences of {hod for h I odd and {OkO)for k odd uniquely define the probable space group to be P2,ln. The crystallographically independent unit consists of two tetraphenylarsonium cations and one dianion; this corresponds to 2 arsenic, 4 iron, 12 sulfur, 24 fluorine, and 64 carbon atoms each oc- y, cupying the fourfold set of general positions i ( x , y, r; ‘ I 2+ x ,
+
I12 + 2).
A three-dimensional Patterson mapI3 provided initial coordinates for the two arsenic and four iron atoms. Successive Fourier and difference Fourier syntheses” eventually yielded coordinates for all 106 nonhydrogen atoms. Least-squares refinement”” was performed with anisotropic thermal parameters utilized for the arsenic, iron, and sulfur atoms and isotropic temperature factors for the carbon and fluorine atoms. Each of the eight phenyl rings was constrained to its well-known geometry and refined as a rigid group. The final refinement converged at R,(F) = 12.6% and R 2 ( n = 1 1.7%15 for 3899 independent reflections with I > 2 4 ; the “goodness-of-fit’’ value15 was 1.70, while all shift-over-error ratios were less than 1.2. A final difference Fourier map revealed no anomalous features. The relatively high discrepancy factors are attributed mainly to the composite effects of residual overlap of adjacent diffraction maxima (due to the long b axial length) and presumed anisotropic thermal behavior of the dithiolene carbon and fluorine atoms as evidenced by their large isotropic temperature factors. Our arbitrary use of an isotropic thermal model for all carbon and fluorine atoms was based upon program-size limitations (at the time of the refinement) coupled with a reluctance to lower the data-to-parameter ratio15 of 9.311. Our satisfaction with the refined crystal structure stems from our primary interest in the structural features of the Fe4S4core, for which the corresponding distances and bond angles are equivalent within experimental error under assumed tetragonal Du symmetry. The positional and thermal parameters from the output of the final full-matrix least-squares cycle are given in Table I. Interatomic distances and bond anglesI8 are presented in Table 11, while selected (IO) Ibers, J. A.; Holm, R. H. Science (Washington, D.C.)1980, 209, 223-235 and references cited therein. (1 1) Byers, L. R.; Dahl, L. F. Inorg. Chem. 1980, 19, 680-692 and references cited therein. (12) Blount, J. F. “DEAR, A Local FORTRAN Absorption-Correction Program”, 1965, based on the method given by: Busing, W. R.; Levy, H. A. Acta Crystallogr. 1957, 10, 180-182. (13) Calabrese, J. C. “MAP, A FORTRAN Fourier Summation and Molecular Assemblage Program”; University of Wisconsin-Madison: Madison, WI, 1972. (14) “ORFLSR, A Local Rigid-Body Least-Squares Program” (adapted from the Busing-Martin-Levy ORFLS); Report ORNL-TM-305; Oak Ridge National Laboratory, Oak Ridge, TN, 1963. (15) The unweighted and weighted discrepancy factors used are R ( = [zllFOl - l F c 1 1 / ~ 1 ~ o lxI 100 and R2(F) = [zwillFoI - I ~ c 1 l 2 / z w r l ~ 0I’I x 100. All least-squaresrefinements were based on the minimization of zwillFd - lfcl12 with individual weights of w, = l/02(Fo)assigned on the basis of the esd s of the observed structure factors. The standard deviation in an observation of unit weight (“goodness-of-fit”) is defined by [zwi(lFol- IFJ)*/(m - n)]1/2,where the number of reflections (m)was 3899 and the number of parameters varied in the refinement (n) was 419, from which m / n = 9.3/1. (16) Atomic scattering factors for neutral atoms were Anomalous dispersion were applied .. to the scattering factors of the As, Fe, and s atoms. (17) (a) Cromer, D. T.; Mann, J. B. Acta Crystallogr.,Sect. A 1968, A24, 321-324. (b) Stewart, R. F.; Davidson. E. R.: Simuson. W. T. J. Chem. Phvs. 1965, 42, 3175-3187. (c) Reference 12, p 149. .
11
Fe -Fe [4] 2.730i [2] 3.224
s(4,)
S * * - S 121 2.855i (41 3.442
Fe-S
141 2.153i [SI 2.247 W
Figure 1. [Fe4(S2C2(CF3)2)4(p3-S)4]2dianion of idealized tetragonal S4-4 configuration. Its Fe4S4core experimentally conforms to D r 42m symmetry. least-squares planes and interplanar angled9 are given in Table 111. Observed and calculated structure factors are listed as supplementary material. All molecular configurations were computer generated and computer drawn.20
Results and Discussion General Description of the Crystal Structure. The solid-state 2)4(p3-S)4]2- is expectedly structure of [AsPh4]2+ [Fe4(SZC2(CF3) composed of discrete monocations and dianions. There is no evidence of unusual interionic interactions in that the shortest interionic F-C(pheny1) contacts are ca. 3.1 .& (possibly involving weak F-H-C bonding). Figure 1 displays the configuration of the [Fe4(S2C2(CF3)z)4(p3-S)4]2-dianion which possesses within experimental error a cubanelike Fe4S4core of tetragonal D2d symmetry that is reduced to S4symmetry by inclusion of the dithiolene ligands (vide infra). Although the overall architecture is similar in a gross fashion to that of the [Fe4(S2),(p3-S),]” fragment illustrated in a figure by Bernal and DavisSafor the partially solved (uncooperative) crystal structure of the [NBu4]+salt, the large apparent uncertainties in the positional parameters of the iron and sulfur atoms in this latter structure preclude any physically meaningful comparison between the two structures. The two independent tetraphenylarsonium cations each possess normal As-C distances with identical means of 1.89 .& and expected variations of the six tetrahedral C-As-C bond angles within 107.5-1 11.Oo and 106.4-1 13.4’ ranges. The [Fe4(S2C2(CF3)2)4(p3-S)4]2Dianion. (a) Previously Determined Physicochemical Properties. The air-stable [ Fe4(S2C2(CF3)z)4(p3-S)4]2dianion was first reported in 1969 by Balch4 who prepared the tetraphenylarsonium and tetrabutylammonium salts of the dianion by reduction of the neutral parent: this reduction was achieved by dissolution of the neutral parent in dimethyl sulfoxide as a basic solvent. In turn, the neutral parent was synthesized4 by the reaction of Fe2(C0)6(S2C2(CF3)2) and sulfur in refluxing xylene. Polarographic measurements of this dianion in dichloromethane solution revealed two anodic and two cathodic waves at nearly the same voltages as those obtained from a polarographic examination of the neutral parent which exhibited four successive cathodic waves. This electrochemical indication for the existence of a corresponding tetrameric monoanion led to its isolation by Balch4 from the reaction of the neutral parent with [ASP~~]~+[F~~(S~C~(CF~)~)~(~~-S)~]~-. Because of their presumed (18) Busing, W. R.; Martin, K. 0.;Levy, H. A. “ORFFE, A FORTRAN CrystallographicFunction and Error Program”; Report ORNL-TM-306; Oak Ridge National Laboratory: Oak Ridge, TN, 1964. (19) “PLANES”. A revised version of “PLANE 1” written by: Smith, D. L. Ph.D. Thesis, University of Wisconsin-Madison, 1962; Appendix IV. (20) Johnson, C. K. “ORTEP-11, A FORTRAN Thermal-Ellipsoid Plot Porgram for Crystal Structure Illustrations”;Report ORNL-5138; Oak Ridge National Laboratory: Oak Ridge, TN, 1976.
1934 J. Am. Chem. SOC.,Vol. 103, No. 8, 1981
Lemmen et al.
Table I. Atomic Parameters for [ AsPh, ] 2+[ Fe,(S,C,(CF,),), atom
104~ 6457 (3) 5091 (3) 7036 (3) 5263 (3) 4830 (6) 6409 (6) 6906 (6) 5698 (6) 6099 (7) 7714 (7) 5228 (6) 3501 (6) 8619 (7) 7354 (6) 3931 (7) 5295 (7) 7045 (25) 7761 (23) 6980 (48) 8646 (41) 4038 (24) 3316 (23) 4031 (33) 2180 (29) 9173 (26) 8630 (22) 10320 (46) 9013 (36) 3678 (20) 4268 (25) 2730 (52) 4182 (44) 6116 (23) 7274 (23) 7544 (26) 85 13 (20) 9021 (20) 9411 (20) 3776 (17) 3302 (16) 4808 (18) 2060 (18) 1847 (18) 1624 (16) 10781 (19) 10494 (22) 10673 (22) 8293 (18) 9677 (19) 9448 (19) 2774 (20) 2038 (21) 2388 (20)
24(3) 29(3) 17(3) 27 (3) 29 (6) 40(6) 38 (6) 31 (6) 56 (7) group C,H5(1) C,jH5(2) C,H,(3) C6H,(4)
104y 4166 (1) 3810 (1) 3660 (1) 3588 (1) 4112 (3) 4177 (3) 3624 (3) 3310 (2) 4396 (3) 45 16 (3) 3777 (3) 3779 (3) 3748 (3) 3350 (3) 3261 (3) 3547 (3) 4698 (9) 4735 (9) 4873 (18) 4977 (16) 3706 (9) 3701 (8) 3633 (12) 3659 (12) 3553 (9) 3379 (9) 3608 (19) 3186 (14) 3177 (8) 3326 (9) 2898 (18) 3284 (17) 485 1 (8) 5201 (9) 4718 (9) 5305 (8) 4973 (8) 4891 (7) 3287 (7) 3833 (6) 3708 (6) 3415 (7) 3975 (7) 3613 (7) 3353 (7) 3884 (9) 3689 (8) 3078 (6) 3372 (7) 2868 (8) 2753 (8) 3182 (8) 2845 (8)
A. Positional and Isotropic Temperature Factors 104z B, Az atom 104x
109
7146 (3) b 3294 (20) W22) 5376 (3) b 4748 (23) F(23) b 6042 (3) 4304 (21) ~(24) 7143 (3) b 10142 (3) As(1) 6607 (6) 7234 (3) b As@) 10012 C( 1-1) b 5631 (6) 9224 C(1-2) 75 10 (6) b C(1-3) 9 142 595 1 (6) b 9848 C( 1-4) 8382 (6) b C( 1-5) 10636 7307 (7) b 10718 C(1-6) 3955 (6) b 9292 C(2-1) 4928 (6) b C(2-2) 9615 6491 (7) b C(2-3) 895 1 4900 (6) b 7964 C(2-4) 6925 (7) b C(2-5) 7641 8583 (6) b 8305 C(2-6) 8816 (24) 3.0 (8) 11454 C(3-1) 8355 (21) 2.4 (7) 11720 C(3-2) 9742 (42) 9.6 (17) 12657 C(3-3) 8695 (39) 8.1 (15) 13328 C(3-4) 3346 (23) 2.9 (8) 13062 (33-5) 3788 (21) 2.3 (7) 12125 C( 3-6) 2373 (29) 5.0 (10) 9883 3292 (27) 4.1 (9) C(4-1) 9492 5702 (23) 3.0 (8) (34-2) 9 344 4962 (21) 2.1 (7) C(4-3) 9587 5844 (44) 10.0 (18) (34-4) 9978 4234 (34) 6.5 (12) C(4-5) 10126 C(4-6) 7945 (20) 1.6 (7) C(5-1) 7975 8721 (24) 3.0 (8) C(5-2) 8511 8053 (42) 13.3 (18) C(5-3) 9020 9668 (41) 8.8 (16) C(5-4) 8995 9882 (20) 10.0 (9) C(5-5) 8459 9767 (21) 11.2 (10) C(5-6) 10391 (23) 7950 12.4 (11) 7361 C(6-1) 8385 (18) 8.6 (8) C(6-2) 6572 9494 (20) 9.0 (8) C( 6-3) 8250 (18) 6729 8.9 (8) C(6-4) 7677 2111 (16) 7.2 (7) C(6-5) 1801 (15) 8466 5.7 (6) (36-6) 2052 (16) 8309 7.0 (6) C(7-1) 5892 2679 (17) 7.9 (7) C(7-2) 2871 (17) 555 1 7.7 (7) 4565 C(7-3) 3888 (15) 5.7 (6) C(7-4) 3920 5564 (17) 7.7 (7) 426 1 C(7-5) 5344 (21) 10.8 (9) 5247 C(7-6) 6668 (21) 10.2 (9) 7762 C(8-1) 3528 (16) 6.9 (7) 7319 C(8-2) 3870 (17) 7.6 (7) 7754 C(8-3) 4574 (17) 8.8 (8) C(8-4) 8630 8647 (20) 12.8 (8) C(8-5) 907 3 8066 (19) 13.3 (9) 8638 (38-6) 7241 (19) 12.0 (8) B. Anisotropic Thermal Parameters (X 104)a1b
5 (0) 5 (0) 6(1) 5 (1) 6 (1) 6 (1) 4 (1) 3 (1) 7 (1)
32 (3) 19(3) 32(3) 27 (3) 37 (6) 31 (6) 21 (5) 40(6) 23 (5)
O(1) -3(1) -2(2) -2 (2) 2(2) 2 (2) 0 (2)
104xb 104yb
104Zb
C. p,deg
3753 2562 2635 2510
-142 1655 90 -1985
70 1372 -2391 265
(p3-S)4]l-
-2(1) 1(1)
0 (2)
-1 (1)
3350 (7) 3444 (8) 2939 (9) 7127 (1) 4857 (1) 6618 6444 6073 5876 6050 642 1 7316 7392 75 15 7560 7484 7361 7254 7613 7724 7476 7117 7006 7326 7670 7834 7654 7310 7 146 4949 4679 475 1 5093 5 363 5291 5272 5443 575 1 5888 5717 5409 4774 4830 477 1 465 4 4598 4657 4438 4105 3800 3828 4161 4466
38 (7) 6 (1) 23(6) 7 (1) -1 (1) 30(6) 9 (1) 0(1) 34(7) 8 (1) -4(2) 26 (6) 7 (1) -3 (2) 41 (7) l O ( 1 ) 1(2) 40(7) 9(1) -2(2) 29 (3) 5 (0) -4 (2) 47 (3) 8 (0) Rigid-Body Parameters for the Eight Phenyl Ringf ~ , d e g p , deg group 104Xb 104yb 3(2) 2(3) -4(2) 8 (5) -7 (5) -4(4) 15 (5) lO(5)
87.1 136.2 -86.8 149.0 16.3 -168.0 17.7 148.5
-3(1)
176.7 -110.8 -5.3 99.5
C6H,(5) C6H,(6) C,H,(7) C6H,(8)
8485 7519 4906 8196
5021 5580 4714 4133
1042 9865 (17) 10296 (21) 9987 (19) 10104 (3) 3029 (3) 10103 9555 9594 10181 10729 10690 9103 8301 7543 7587 8389 9147 9995 10122 10038 9825 9698 9782 11189 11159 11955 12781 12811 12015 4207 4722 5599 5961 5446 4569 2318 1765 1291 1370 1923 2397 3030 3833 3859 3082 2279 2254 2599 2633 2350 2033 1999 2282
B, A 2
8.1 (7) 10.8 (10) 9.8 (9) b b
2.5 (7) 2.9 (8) 3.9 (8) 4.3 (9) 4.8 (10) 4.7 (9) 3.5 (8) 4.6 (10) 6.8 (12) 4.9 (10) 6.9 (11) 3.8 (9) 2.7 (7) 2.8 (7) 5.4 (10) 6.4 (12) 5.9 (11) 5.1 (10) 3.0 (8) 3.6 (8) 4.7 (9) 4.3 (10) 3.9 (9) 3.2 (8) 3.8 (8) 2.9 (7) 3.1 (8) 4.2 (9) 6.4 (12) 4.6 (9) 3.4 (8) 3.4 (9) 5.0 (10) 4.6 (10) 8.5 (14) 5.7 (11) 4.7 (9) 6.9 (12) 6.3 (11) 6.0 (11) 4.8 (10) 5.3 (11) 5.8 (12) 8.3 (14) 11.5 (18) 8.9 (17) 7.2 (13) 5.8 (11)
47 (7) 38(6) 27 (6) 53 (7) 40 (6) 30(6) 29(5) 31 (2) 47 (3) 104Zb
p,deg
U , deg
p,deg
5084 17.7 1844 -93.3 3056 -176.4 2316 -55.0
174.6 -157.3 105.6 167.6
-111.5 32.9 185.8 162.2
J. Am. Chem. SOC.,Val. 103, No. 8, 1981 1935 Anisotropic thermal parameters of the form a In this and the following tables, estimated standard deviations are given in parentheses exp[-(hzpll t k2p,, + 12& t 2hkp1, + Ship,, + 2kZp,,)] were used for the Fe, S, and As atoms. Each of the eight rings in the two independent tetraphenylarsonium cations was refined as a rigid group with hydrogens excluded. The six carbon atoms, C(n-m)(where m = 1-6), in a given phenyl ring n (where n = 1-8) were assigned individual isotropic temperature factors which were varied during the refinement. Each ring was assumed to have D s h symmetry with C C bond lengths of 1.39 A. The internal orthogonal axial system x‘, y‘, z ’ , for each phenyl ring n was defined as follows: the origin xb,yb, Zb was placed at the center of the carbon framework;x‘ was chosen along C(n-l)-C(n-4), y’ along the perpendicular bisectors of C(n-2)-C(n-3) and C(n-S)-C(n-6),and z‘ along x ‘ x y ’ . The three angles 0 , 6,and p refer to the orientation of the internal axial system with respect to an external orthogonal system by rotations about y ’ , x’, and z ‘ , respectively. The orthogonal axes are defined in our program relative to the crystal axes as: a, = a, bo = c, X a, co = a X b. oxygen sensitivity, no effort was made by Balch4 to isolate the n = -3 and n = -4 members of this electron-transfer series. The formulation by Balch4 of the dianion as a tetramer in preference to an alternate dimeric formulation, [ Fe2(S2C2(CF3),),SZ]-, was based upon (1) a conductivity study of the [NBu4]+ salt indicating that it behaves as a 2:l electrolyte in acetonitrile solution and (2) magnetic susceptibility measurements which established the [NBu4]+ salt to be diamagnetic (instead of paramagnetic which would be required for a dimeric formulation) in acetone solution as well as in the solid state. Balch4 also showed this tetrameric formulation for the dianion to be consistent with the corresponding tetrabutylammonium salt of the [Fe4(S2Cz(CF3)2)4S4]monoanion being paramagnetic. Because of their much lower solubilities, similar but less extensive studies were made by Balch4 on the corresponding phenyl-substituted [Fe4(S2C2Ph2)4S4]n series (n = 0, -1). Evidence for its tetrameric nature was provided by isolating [AsPh4]+[Fe4(SzCzPh2)4S4]-, shown from magnetic susceptibility and ESR measurements to be paramagnetic. In addition, polarographic measurements of both the neutral parent and monoanion indicated the monoanion to be the first reduced member of this electrontransfer series. It is especially noteworthy that the phenyl-substituted neutral parent of this series was originally reported in 1966 by Schrauzer and co-workers,zl who prepared this sulfur-rich iron dithiolene complex from (1) the thermal degradation of Fe(SzC2Ph2)2,(2) the reaction of metallic iron or iron carbonyls with sulfur and diphenylacetylene, and (3) the reaction of Fez(C0)6(SzC2Ph2)with sulfur. Their incorrect formulation of this neutral compound as a dimer, Fe2(SzCzPh2)2S2,was apparently made on a chemically intuitive basis. Balch4 noted the considerable chemical stability of the ironsulfur framework in [Fe4(SzCz(CF3)2)4S4]n (n = 0, -2) and in Fe4(SZCzPhz)4S4, as evidenced by these tetramers being unlike the bis(dithio1ene) iron dimers in that they are not cleaved by triphenylarsine and triphenylstibine, even though these bases do reduce the neutral CF,-substituted parent (but not the Ph-substituted analogue) to the monoanion. On the other hand, triphenylphosphine was found4 to rupture the iron-sulfur bonds (by formation of triphenylphosphine sulfide) with the production of Fe(S2C2R2)2PPh3from the neutral complexes and of [Fez(S2C2(CF3)2)4]z-from the dianion. The presence of “acid labile” dianion was also demonsulfur in the [ Fe4(S2Cz(CF3)2)4S4]2strated4 by its reactions with acid and mercuric ion. On the basis of the above data, Balch4suggested two reasonable atomic arrangements for the members of these two series-one involving a cubanelike Fe4S4framework and the other involving bridging by dithiolene ligands as well as by the “free” sulfur atoms. The former model was ascertained from the previously mentioned crystallographic study reported in 1972 by Bema1 and DavisSaon the [Fe4(SzCz(CF3)2)4(p3-S)4]zdianion as the [NBu4]+ salt. Although the overall configuration of the [Fe4(S2)4(p3-S)4]zfragment was established, its detailed structural features were not resolved because of a presumed crystal disorder of the counterions. At the same time, Good and ChandraSapresented the results of a temperature-dependent Mossbauer investigation of the [Fer (S2C2(CF3)z)4(p3-S)4]n series (n = 0, -1, -2 as the [AsPh4]+salts); this zero-(applied field) spectral study was shortly followed by magnetic-field Mossbauer measurements of these compounds by Reiff and c o - w o r k e r ~ .Room-temperature ~~ Mossbauer spectra (21) Schrauzer, G. N.; Mayweg, V. P.; Finck, H. W.; Henrich, W. J. Am. Chem. SOC.1966,88, 4604-4609.
performed by Ktinding on members of both series were reported earlier by Balch! These Miissbauer data are of special importance with respect to the nature of bonding of these particular Fe4S4 clusters (vide infra). (b) The FeS4 Fragment. This structural determination ascertained that the Fe4S4core experimentally conforms to a tetragonal D ~ 4 2 geometry. m Table I1 shows that the six Fe-Fe distances in the iron tetrahedron separate under this symmetry into four shorter equivalent lengths of range 2.713 (7)-2.738 (7) A and mean 2.730 A and two longer equivalent lengths of range 3.222 (7)-3.225 (7) A and mean 3.224 A. Likewise, the six S-S distances in the nonbonding sulfur tetrahedron divide into two shorter equivalent lengths of range 2.844 (12)-2.866 (13) A and mean 2.855 A and four longer equivalent lengths of range 3.437 (13)-3.446 (13) A and mean 3.442 A. These variations correspond to the iron tetrahedron being uniformly compressed via Fe-Fe bonding (vide infra) along the S4-4 axis from a nonbonding cubic Td geometry into a tetragonal DZdgeometry. This distortion concomitantly gives rise to an expansion of the four equivalent S--S edges (bisecting the Fe-Fe bonds) relative to the other two Sa-S edges perpendicular to the S4 axis. The four equivalent Fe-S bonds, which are approximately parallel to the S4axis, of range 2.150 (1 1)-2.155 (10) A and mean 2.153 A are markedly longer by 0.09 A than the other eight equivalent Fe-S bonds of range 2.235 (10)-2.257 (10) A and mean 2.247 A. This bond-length difference may be ascribed to the particular electronic nature of the Fe4S4core in the dianion (vide infra). These observed bond-length variations expectedly produce correspondingly large separations of the 12 Fe-S-Fe and 12 S-Fe-S bond angles into two sets each. The four equivalent (Fe-Fe)-bonded Fe2S2 fragments possess eight acute Fe-S-Fe bond angles of 76.7O (average) and eight obtuse S-Fe-S bond angles of 102.9’ (average), while the two equivalent (Fe-Fe)nonbonded F%S2 fragments havefour obtuse Fe-S-Fe bond angles of 91.7’ (average) and four acute S-Fe-S bond angles of 78.9’ (average). In contradistinction to each of the four (Fe-Fe)-bonded Fe2S2fragments being essentially planar with a torsional angle t9 of 171.4O (average) (Le., the mean of 171.4, 171.4, 171.6, and 171.0’) along the Fe-Fe line between the two planes each formed by one bridging S atom and the two Fe atoms, the two (FeFe)-nonbonded Fe2S2fragments are each markedly nonplanar as indicated by corresponding torsional angles of fl = 131.3 and 131.8’. This geometry of the Fe4S4 core in the [Fe4(SZCz(CF3)J4(p3-S),12- dianion resembles that of the Fe4S4core in the [Fer (~s-C,Hs)4(p3-S),]z+dication (vide infra) in sharp contrast to the different geometries of the Fe4S4cores in other structurally known [Fe4L4(p3-S)4]ntetramers (viz., L = C5H5,n = 0,’ 12;L = SR, n = -2, -3;6 L = C1, n = -2;’ L = NO, n = 0, -1;8 L = (CO),, n = 09).
(c) The Dithiolene Ligands and Their Arrangement about the Fe4S4Core. Figures 1 and 2 show the disposition of the four S2Cz(CF3)zligands about the central Fe4S4 framework. This particular ligand arrangement results in each of the four chemically equivalent iron atoms adopting a localized squarepyramidal sulfur coordination. Figure 3 depicts the localized environment about one of the iron atoms, while Table IV presents the appropriate bond angles and S . 4 contacts in its coordination sphere. These distances and bond angles are averaged under assumed S4-a symmetry for the entire dianion. The fact that each iron atom is displaced by 0.60 A (average) from its mean basal plane
1936 J . Am. Chem. SOC.,Vol. 103, No. 8, 1981
Lemmen et al.
Table 11. Interatomic Distances (A) and Bond Angles (Deg) for [ AsPh,] ,+[Fe,(S,C,(CF,),),(p,-S),] *A. Intraanion Distances and Bond Angles within the Fe4(p3-S), Core (Averaged under Assumed Dd-42m Symmetry) Fe( 1)-Fe(3) 2.731 (7) Fe( l b S ( 1) 2.153 (11) 3.444 (13) Fe(lbS(3) 2.246 (10) S(1). * G(3) Fe( 1)-Fe(4) 2.713 (7) Fe( 1)-S(2) 2.155 (10) 2.257 (10) S(1). * *S(4) 3.437 (13) ~ e m - s ( 4 ) Fe(2)-Fe( 3) 2.738 (7) Fe(2)-S( 1) 2.153 (11) 2.249 (10) S(2). * 4(3) 3.446 (13) Fe( 3)-S(2) Fe( 2)-Fe(4) 2.737 (7) Fe(2)-S(2) 3.439 (13) Fe(4)-S( 1) 2.150 (11) 2.251 (10) S(2). * 4(4) average 2.730 Fe(3)-S(3) 2.245 (10) average 3.442 average 2.153 2.246 (10) ~ ( 1 )...s(2) Fe(1). .Fe(2) 3.225 (7) Fe( 3)-S(4) 2.866 (13) 2.235 (10) ~ ( 3 ) .. .s(4) 2.844 (12) Fe(3). . sFe(4) 3.222 (7) Fe(4)-S(3) Fe(4)-S(4) 2.246 (10) average 2.855 average 3.224 average 2.247 Fe( l)-S(2)-Fe(3) 76.5 (4) 103.0 (4) 79.1 (4) S(l)-Fe(l)-S(2) S(lFFe(l)-S(3) Fe( 1)-S( 1)-Fe(2) 91.7 (4) Fe( l)-S(3)-Fe(3) 79.1 (4) 76.7 (3) 103.5 (4) S( l)-Fe(4)-S(3) S(l)-Fe(2)-S(2) 91.3 (4) Fe( 1)-S(2)-Fe(2) Fe(2)-S(2)-Fe(3) 78.6 (4) 76.8 (4) S(3)-Fe(3)-S(4) S(2)-Fe( 1)-S(3) 102.8 (4) Fe( 3)-S( 3)-Fe(4) 92.0 (3) 78.8 (4) 76.9 (3) Fe(2)-S(4)-Fe( 3) 103.2 (4) S(2)-Fe(3)-S( 3) S(3)-Fe(4)-%4) Fe(3)-S(4)-Fe(4) 91.6 (4) average 78.9 76.9 (4) Fe(2)-S( 1)-Fe(4) 102.6 (4) S(2)-Fe(2)-S(4) 91.7 average Fe( 2)-S(4)-Fe(4) 76.9 (3) 102.8 (4) S(2bFe(3)-S(4) Fe( 1)-S( 1)-Fe(4) 76.2 (4) S(lj-Fe(2j-S(4) 102.6 (4j Fe( l)-S(3)-Fe(4) 76.4 (3) S(l)-Fe(4)-S(4) 102.8 (4) aver age 76.7 average 102.9 B. Intraanion Distances within Each Fe(S,C,(CF,),) Fragment Fe( 1)-S( 11) 1.26 (6) 2.182 (10) C( 11)-C( 13) 1.55 (6) S(ll)-C(ll) 1.75 (4) C( 13)-F( 1) 1.29 (6) Fe( 1)-S( 12) 2.150 (10) C( 12)-C(14) 1.53 (6) S(12)-C(12) 1.76 (3) C( 13FF(2) C(13)-F(3) 2.176 (11) 1.26 (6) Fe(2)-S(21) S(2 1)-C(21) 1.74 (3) C(21)-C(23) 1.48 (5) 1.31 (5) Fe(2)-S(22) C( 14)-F(4) 2.174 (10) C(22)-C(24) 1.61 (4) S(22)-C(22) 1.70 (3) C( 14)-F(5) 2.186 (10) 1.21 (5) Fe(3)-S(31) C( 3 1)-C( 33) 1.57 (6) S( 31 ) C (31) 1.69 (3) 1.39 (5) C( 14)-F(6) 2.180 (10) C( 32)-C(34) 1.49 (5) S( 32)-C( 32) 1.75 (3) Fe(3)-S( 32) 1.37 (4) C(23)-F(7) 2.181 (11) Fe(4)-S(41) C(4 1)-C(43) 1.70 (7) S( 4 1)-C(4 1) 1.66 (3) 2.155 (10) C(23)-F(8) 1.40 (4) Fe(4)-S( 42) 1.45 (6) S(42)-C(42) 1.69 (3) C(42)-C(44) 1.29 (4) average 1.55 average 1.72 2.173 C(2 3)-F(9) average 1.28 (4) C(24)-F( 10) S(ll)*.*S(12) 3.031 (14) C(ll)-C(12) 1.32 (4) C(24)-F( 11) 1.37 (4) S(21). * aS(22) 3.031 (13) C(21)-C(22) 1.30 (4) C(24)-F( 12) 1.30 (4) 3.052 (13) S(31). * eS(32) C(31)-C(W 1.37 (4) C( 3 3)-F( 13) 1.26 (6) 3.005 (14) C(4 1)-C(42) 1.40 (4) S(41). . .S(42) C(33)-F( 14) 1.32 (6) average average 1.35 3.030 C(33)-F(15) 1.27 (6) C(34)-F( 16) 1.36 ( 5 ) C(34)-F( 17) 1.35 ( 5 ) 1.38 (5) C(34)-F( 18) 1.03 (6) C(4 3)-F( 19) 1.43 (6) C(43)-F(20) 1.23 (6) C(43)-F(21) C(44)-F(22) 1.34 (5) C(44)-F(23) 1.25 (5) 1.37 (5) C(44)-F(2 4) C. Intraanion Bond Angles within Each Fe(S,C,(CF,),) Fragment S( 11)-Fe( 1)-S( 12) 88.8 (4) 111 (6) S(ll)-C(ll)-C(l2) 117 (3) C( ll)-C( 13)-F( 1) 110 (5) F( 1)-C(13)-F(2) S(12)-C(12)-C( 11) 121 (3) C( 1l)-C( 13)-F(2) 111 (5) S(21)-Fe(2)-S(22) 88.3 (4) 108 (6) F( 1)-C(13)-F(3) S(3 l)-Fe(3)-S(32) 88.7 (4) 106 (5) S(2l)-C(21)-C(22) 111 (5) 119 (3) C( ll)-C( 13)-F(3) F(2)-C(13)-F(3) S(41)-Fe(4)-S(42) 87.8 (4) 111 (5) S(22)-C(22)4(2 1) 113 (4) 122 (3) F(4)-C( 14)-F(5) C( 12)-C(14)-F(4) average 121 (3) S(3 1)-C(31)-C(32) 88.4 119 (5) C( 12)-C( 15)-F(5) 96 (4) F(4)-C(14)-F(6) 105 (5) F(5)-C( 14)-F( 6) 110 (4) 117 (3) C ( 1 2 W 16)-F(6) S(32)-C(32)-C(31) Fe( 1)-S( 11)-C( 11) 106.9 (13) 114 (4) 120 (4) C(2 1)-C(23)-F(7) S(41)-C(4 1)-C(42) 102 (3) F(7)-C(23)-F(8) Fe( 1)-S( 12)-C( 12) 105.6 (12) 106 (4) 112 (3) 118 (3) C(2 l)-C( 23)-F(8) S(42)-C(42)-C(41) F(7)-C(23-F(9) Fe(2)-S(21)-C(2 1) 105.5 (12) 102 (3) F(8)-C(2 3)-F(9) 119 (4) 119 C(2 1)-C(23)-F(9) Fe(2)-S(22)-C(22) 105.3 (11) average 107 (3) C(22)-C( 24)-F( 10) 112 (3) F( 10)4(24)-F(11) Fe(3)-S(31)-C(31) 106.5 (13) C(ll)-C(12)-C(14) 121 (4) 113 (4) F( 10)-C(24)-F( 12) C(22)-C(24)-F(ll) 110 (3) Fe(3)-S( 32)-C(32) 106.3 (12) 127 (4) C( 12)-C(1l)-C( 13) 104 (3) F( 1l)-C(24)-F( 12) 111 (3) C(22)-C(24)-F( 12) Fe(4)-S( 41)-C(4 1) 107.0 (11) C(2 1)-C(22)-C(24) 123 (3) 104 (5) F( 13)-C(33)-F( 14) 115 (5) C(3 l)-C( 33)-F( 13) Fe(4)-S(42)-C(42) 107.7 (13) 130 (4) C(22)-C(21)
