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An extremely large 57Fe Mössbauer quadrupole splitting for a distorted tetrahedral high-spin iron(II) complex
7 June 1996
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 255 (1996) 134-136
An extremely large 57Fe MSssbauer quadrupole splitting for a distorted tetrahedral high-spin iron(II) complex David J. Evans Nitrogen Fixation Laboratory, John lnnes Centre, Norwich Research Park, Colney, Norwich, NR4 7UH, UK
Received 26 February 1996; in final form 29 March 1996
Abstract The zero-field, solid state, 57Fe M~3ssbauer spectrum of the trinuclear high-spin iron(II) complex [Fe3(SC6H2~Pr3 2,4,6)4{N(SiMe3)2} 2] has been recorded at 77 K. 'Unusually' large quadrupole splitting values (> 3.5 mm s -~) are observed for both the distorted trigonal planar (3.60 mm s-J) and distorted tetrahedral iron atoms (4.55 m m s ]) of the complex. The quadrupole splitting of the distorted tetrahedral iron atom is extremely large and equals that previously reported as the largest. 'Unusually' large quadrupole splittings for high-spin iron(If) are not that unusual and, in fact, are found for several coordination numbers and geometries. 1. Introduction High-spin iron(II) complexes with 'unusually' large 57Fe MSssbauer quadrupole splittings (AEQ > 3.5 mm s -1) have been reported and include the five-coordinate high-spin ferrous heme-like moiety found in N i t r o s o m o n a s europeae [1 ], five-coordinate high-spin ferrous porphyrins with an anionic oxygen or nitrogen donor or an halide ion as an axial ligand of iron [2-7], five-coordinate bis(dithiocarbamate) iron(II) complexes [8], six-coordinate polymeric iron(II) thiosemicarbazide [9], and iron(II) complexes with polythioether ligands [10,11]. In most of the five-coordinate complexes the geometry about the iron is square pyramidal, however, it has been noted that a large quadrupole splitting cannot be used to differentiate between high-spin square pyramidal and square planar coordination [11]. The largest AEQ reported [12] to date (4.54 mm s - t at 77 K) is for tetrakis(1,8-naphthyridine)iron(II) perchlorate, which has been shown by X-ray crystallography [13] to be eight-coordinate.
2. Experimental i
[Fe3(SC6H2Pr3-2,4,6)4{N(SIMe3)~}2] was prepared by the published procedure [14]. M~Sssbauer spectra were recorded on an ES-Technology MS105 spectrometer with a 925 MBq 57Co source in a rhodium matrix. Spectra were referenced against a 25 p~m natural iron foil at 298 K.
3. Results and discussion The trinuclear high-spin iron(II) complex i [Fe3(SC6H 2 Pr3-2,4,6)4{N(SiMe3)2} 2] has recently been prepared and its structure determined by X-ray crystallography (Fig. 1) [14]. The central iron atom has distorted tetrahedral geometry, coordinated by four thiolate ligands. The two terminal iron atoms, coordinated by two bridging thiolate and one amide ligand, have distorted trigonal planar geometry. There is no crystallographic evidence for any other interactions between, for example, the substituted aromatic
0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved PII S0009-2614(96)00366- 1
D.J. Evans/Chemical Physics Letters 255 (1996) 134-136
ring of bridging thiolate and the central iron atom which would increase the coordination number of, and change the geometry about, this central iron atom. The solid state 57Fe M6ssbauer spectrum of the trinuclear complex recorded at 77 K, in zero field, is shown in Fig. 2. It is clear that the spectrum is, as expected, not a simple quadrupole split doublet but arises from the overlap of two quadrupole split doublets, one of which has a very large line-width (Iv'l/2 = 0.73(5) mm s - l ) . The line-broadened doublet has an isomer shift of 0.84 and A EQ of 3.60(4) mm s -~ , the doublet with the narrow line-width (F1/2=0.22(1) mm s -~) has an isomer shift of 0.78(1) and AEQ of 4.55 mm s -~. The best leastsquares fit of the Lorentzian curves to the data points was achieved by constraining the relative intensities of the doublets to a ratio of 2:1. The former parameters are attributed to the three-coordinate terminal iron atoms and the latter to the central distorted tetrahedrally coordinated iron. The isomer shift values are consistent with all the iron atoms being high-spin iron(lI). Both of the quadrupole splitting values can be described as 'unusually' large and that associated with the central iron atom is extremely large and equals that previously reported as the largest (see above). The origin of the extremely large quadrupole splitting is unclear. High-spin iron(II) complexes generally show large quadrupole splittings due to a valence contribution, arising from the additional electron over the half-filled d-shell, to the total electric field gradient. The maximum valence contribution gives A EQ = 4 mm s -~. Larger quadrupole splittings may be attained by, in addition
\
/
~
Si\ N --Fe
\/ _Si
/
\/ S-/ \
Fe
"-.,...........\. / S
\
~
......S......,..... S
"" F e - -
/ Si\ N
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Velocity (ram/s)
Fig. 2. Zero-field, solid state M6ssbauer spectrum at 77 K of i • [Fe3(SC6H2 Pr3-2,4,6)4{N(S1Me3)2}2] and, top, spectrum fitted by two overlapping quadrupole split doublets.
to the valence contribution, a significant lattice contribution to the electric field gradient. In conclusion, 'unusually' large quadrupole splittings are no longer that unusual. Three-, four-, five-, six-and eight-coordinate high-spin iron(II) complexes can give M6ssbauer spectra with AEQ >__ 3.5 mm s -~. Here, in one compound, large A E Q v a l u e s were observed for both distorted trigonal planar and distorted tetrahedral geometries about iron. The general inadequacy of using quadrupole splitting as a guide to geometry in high-spin iron(II) complexes will be discussed in more detail elsewhere [15].
/\ References
Fig. 1. Molecular structure of the trinuclear high-spin iron(I1) i complex [Fe3(SC 6 H 2 Pr3-2,4,6)4{N(S1Me3)z}2 ].
[1] K.K. Andersson, T.A. Kent, J.D. Lipscomb, A.B. Hooper and E. Munck, J. Biol. Chem. 259 (1984) 6833.
136
DJ. Evans/Chemical Physics Letters 255 (1996) 134-136
[2] J. Silver and B. Luckas, Inorg. Chim. Acta 80 (1983) 107. [3] J. Silver, B. Luckas and G. AI-Jaff, Inorg. Chim. Acta 91 (1984) 125. [4] H. Narsi, J. Fischer, R. Weiss, E. Bill and A. Trautwein, J. Am. Chem. Soc. 109 (1987) 2549. [5] B.A. Shaevitz, G. Lang and C.A. Reed, lnorg. Chem. 27 (1988) 4607. [6] D. Mandon, F. Ott-Woelfel, J. Fischer, R. Weiss, E. Bill and A.X. Trautwein, Inorg. Chem. 29 (1990) 2442. [7] E.L. Bominaar, X.-Q. Ding, A. Gismelseed, E. Bill, H. Winkler, A.X. Trautwein, H. Nasri, J. Fischer and R. Weiss, Inorg. Chem. 31 (1992) 1845. [8] B.W. Fitzimmons, S.E. AI-Mukhtar, L.F. Larkworthy and R.R. Patel, J. Chem. Soc. Dalton Trans. (1975) 1969.
[9] D.V. Naik and G.J. Palenik, Chem. Phys. Letters 24 (1974) 260. [10] A. Hills, D.L. Hughes, M. Jimenez-Tenorio, G.J. Leigh, A. Houlton and J. Silver, J. Chem. Soc. Chem. Commun. (1989) 1774. [11] D.L. Hughes, M. Jimenez-Tenorio, G.J. Leigh, A. Houlton and J.Silver, J. Chem. Soc. Dalton Trans. (1992) 2033. [12] E. K6nig, G. Ritter, E. Linder and I.P. Lorenz, Chem. Phys. Letters 13 (1972) 70. [13] P. Singh, A. Clearfield and I. Bernal, J. Coord. Chem. 1 (1971) 29. [14] F.M. MacDonnell, K. Ruhlandt-Senge, J.J. Ellison, R.H. Holm and P.P. Power, Inorg. Chem. 34 (1995) 1815. [15] D.J. Evans, manuscript in preparation.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 255 (1996) 134-136
An extremely large 57Fe MSssbauer quadrupole splitting for a distorted tetrahedral high-spin iron(II) complex David J. Evans Nitrogen Fixation Laboratory, John lnnes Centre, Norwich Research Park, Colney, Norwich, NR4 7UH, UK
Received 26 February 1996; in final form 29 March 1996
Abstract The zero-field, solid state, 57Fe M~3ssbauer spectrum of the trinuclear high-spin iron(II) complex [Fe3(SC6H2~Pr3 2,4,6)4{N(SiMe3)2} 2] has been recorded at 77 K. 'Unusually' large quadrupole splitting values (> 3.5 mm s -~) are observed for both the distorted trigonal planar (3.60 mm s-J) and distorted tetrahedral iron atoms (4.55 m m s ]) of the complex. The quadrupole splitting of the distorted tetrahedral iron atom is extremely large and equals that previously reported as the largest. 'Unusually' large quadrupole splittings for high-spin iron(If) are not that unusual and, in fact, are found for several coordination numbers and geometries. 1. Introduction High-spin iron(II) complexes with 'unusually' large 57Fe MSssbauer quadrupole splittings (AEQ > 3.5 mm s -1) have been reported and include the five-coordinate high-spin ferrous heme-like moiety found in N i t r o s o m o n a s europeae [1 ], five-coordinate high-spin ferrous porphyrins with an anionic oxygen or nitrogen donor or an halide ion as an axial ligand of iron [2-7], five-coordinate bis(dithiocarbamate) iron(II) complexes [8], six-coordinate polymeric iron(II) thiosemicarbazide [9], and iron(II) complexes with polythioether ligands [10,11]. In most of the five-coordinate complexes the geometry about the iron is square pyramidal, however, it has been noted that a large quadrupole splitting cannot be used to differentiate between high-spin square pyramidal and square planar coordination [11]. The largest AEQ reported [12] to date (4.54 mm s - t at 77 K) is for tetrakis(1,8-naphthyridine)iron(II) perchlorate, which has been shown by X-ray crystallography [13] to be eight-coordinate.
2. Experimental i
[Fe3(SC6H2Pr3-2,4,6)4{N(SIMe3)~}2] was prepared by the published procedure [14]. M~Sssbauer spectra were recorded on an ES-Technology MS105 spectrometer with a 925 MBq 57Co source in a rhodium matrix. Spectra were referenced against a 25 p~m natural iron foil at 298 K.
3. Results and discussion The trinuclear high-spin iron(II) complex i [Fe3(SC6H 2 Pr3-2,4,6)4{N(SiMe3)2} 2] has recently been prepared and its structure determined by X-ray crystallography (Fig. 1) [14]. The central iron atom has distorted tetrahedral geometry, coordinated by four thiolate ligands. The two terminal iron atoms, coordinated by two bridging thiolate and one amide ligand, have distorted trigonal planar geometry. There is no crystallographic evidence for any other interactions between, for example, the substituted aromatic
0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved PII S0009-2614(96)00366- 1
D.J. Evans/Chemical Physics Letters 255 (1996) 134-136
ring of bridging thiolate and the central iron atom which would increase the coordination number of, and change the geometry about, this central iron atom. The solid state 57Fe M6ssbauer spectrum of the trinuclear complex recorded at 77 K, in zero field, is shown in Fig. 2. It is clear that the spectrum is, as expected, not a simple quadrupole split doublet but arises from the overlap of two quadrupole split doublets, one of which has a very large line-width (Iv'l/2 = 0.73(5) mm s - l ) . The line-broadened doublet has an isomer shift of 0.84 and A EQ of 3.60(4) mm s -~ , the doublet with the narrow line-width (F1/2=0.22(1) mm s -~) has an isomer shift of 0.78(1) and AEQ of 4.55 mm s -~. The best leastsquares fit of the Lorentzian curves to the data points was achieved by constraining the relative intensities of the doublets to a ratio of 2:1. The former parameters are attributed to the three-coordinate terminal iron atoms and the latter to the central distorted tetrahedrally coordinated iron. The isomer shift values are consistent with all the iron atoms being high-spin iron(lI). Both of the quadrupole splitting values can be described as 'unusually' large and that associated with the central iron atom is extremely large and equals that previously reported as the largest (see above). The origin of the extremely large quadrupole splitting is unclear. High-spin iron(II) complexes generally show large quadrupole splittings due to a valence contribution, arising from the additional electron over the half-filled d-shell, to the total electric field gradient. The maximum valence contribution gives A EQ = 4 mm s -~. Larger quadrupole splittings may be attained by, in addition
\
/
~
Si\ N --Fe
\/ _Si
/
\/ S-/ \
Fe
"-.,...........\. / S
\
~
......S......,..... S
"" F e - -
/ Si\ N
\_. ~i--
135
!/
4
•~
~..,..:.-.?... ~!..! :,,,
.....
•
. ......
:,.... :-"
........... .
,
0
.-,
.;.
""
,=3.
:
.,:-.. -.,,:: ...%: " !i-,: ...':-. , -•-.
"!
-::
",
."
. -
:..:.:.-.
•
'
:.,
..,.
".
1
4
'
..
.
2
..
...,
%
-
.'~
L
,_,,S
I
I
l
-4
-3
-2
I
-I
I
I
I
[
I
I
0
I
2
3
4
.5
Velocity (ram/s)
Fig. 2. Zero-field, solid state M6ssbauer spectrum at 77 K of i • [Fe3(SC6H2 Pr3-2,4,6)4{N(S1Me3)2}2] and, top, spectrum fitted by two overlapping quadrupole split doublets.
to the valence contribution, a significant lattice contribution to the electric field gradient. In conclusion, 'unusually' large quadrupole splittings are no longer that unusual. Three-, four-, five-, six-and eight-coordinate high-spin iron(II) complexes can give M6ssbauer spectra with AEQ >__ 3.5 mm s -~. Here, in one compound, large A E Q v a l u e s were observed for both distorted trigonal planar and distorted tetrahedral geometries about iron. The general inadequacy of using quadrupole splitting as a guide to geometry in high-spin iron(II) complexes will be discussed in more detail elsewhere [15].
/\ References
Fig. 1. Molecular structure of the trinuclear high-spin iron(I1) i complex [Fe3(SC 6 H 2 Pr3-2,4,6)4{N(S1Me3)z}2 ].
[1] K.K. Andersson, T.A. Kent, J.D. Lipscomb, A.B. Hooper and E. Munck, J. Biol. Chem. 259 (1984) 6833.
136
DJ. Evans/Chemical Physics Letters 255 (1996) 134-136
[2] J. Silver and B. Luckas, Inorg. Chim. Acta 80 (1983) 107. [3] J. Silver, B. Luckas and G. AI-Jaff, Inorg. Chim. Acta 91 (1984) 125. [4] H. Narsi, J. Fischer, R. Weiss, E. Bill and A. Trautwein, J. Am. Chem. Soc. 109 (1987) 2549. [5] B.A. Shaevitz, G. Lang and C.A. Reed, lnorg. Chem. 27 (1988) 4607. [6] D. Mandon, F. Ott-Woelfel, J. Fischer, R. Weiss, E. Bill and A.X. Trautwein, Inorg. Chem. 29 (1990) 2442. [7] E.L. Bominaar, X.-Q. Ding, A. Gismelseed, E. Bill, H. Winkler, A.X. Trautwein, H. Nasri, J. Fischer and R. Weiss, Inorg. Chem. 31 (1992) 1845. [8] B.W. Fitzimmons, S.E. AI-Mukhtar, L.F. Larkworthy and R.R. Patel, J. Chem. Soc. Dalton Trans. (1975) 1969.
[9] D.V. Naik and G.J. Palenik, Chem. Phys. Letters 24 (1974) 260. [10] A. Hills, D.L. Hughes, M. Jimenez-Tenorio, G.J. Leigh, A. Houlton and J. Silver, J. Chem. Soc. Chem. Commun. (1989) 1774. [11] D.L. Hughes, M. Jimenez-Tenorio, G.J. Leigh, A. Houlton and J.Silver, J. Chem. Soc. Dalton Trans. (1992) 2033. [12] E. K6nig, G. Ritter, E. Linder and I.P. Lorenz, Chem. Phys. Letters 13 (1972) 70. [13] P. Singh, A. Clearfield and I. Bernal, J. Coord. Chem. 1 (1971) 29. [14] F.M. MacDonnell, K. Ruhlandt-Senge, J.J. Ellison, R.H. Holm and P.P. Power, Inorg. Chem. 34 (1995) 1815. [15] D.J. Evans, manuscript in preparation.