J. Am. Chem. SOC.1993, 115, 1797-1804
We propose that oxidative addition to the 17-electron radical occurs to form, transiently, a 19-electron product which rapidly loses CO, eq 17. The product CpW(CO)z(Sn(n-Bu3)(H) is also CpW(CO),
+ (~-Bu)~SIIH
-
Scbeme I CpW(CO)3 R'
CpW(CO)3(Sn(n-Bu)3)
1797
+ RX
+ [CpW(CO)3]2
+
-
+ R' (18) R+ + CpW(CO)3- + CpW(CO)3 CpW(CO)3X
(19) is large (high [RX]), a high concentration of [R'] is produced. Consequently, enough R' proceeds by reaction 19 (even though a 17-electron radical and may abstract H from ( ~ - B U ) & H . ~ ~ radical coupling is simultaneously enhanced) to make loss of the This reaction occurs after the rate-determining step; therefore, dimer [CpW(CO)3]zsignificant. A similar scheme can be written no information about it can be obtained from the kinetic data. with superoxide (free or coordinated to CPW(CO)~+)or CoChain Reactions. For some organic halides, such as CHI3, (II)(dmgH)zPPh3as the species which reduces [CpW(CO)p],z. Ph3CBr, and BrCHzCHzCN,as well as for B r c ~ ( d m g H ) ~ P P h ~ , Further studies of the mechanisms of these photoinitiated chain (n-Bu)&11, and Oz, high concentrations gave postflash loss of reactions are in progress. dimer absorbance, shown for O2in Figure 6b. The concentration of oxidant required to observe dimer loss rather than radical Acknowledgment. We thank Professor R. J. Angelici and his recombination (dimer recovery) was [ox] > 105/kx. Our obstudents for a gift of [CpW(CO)3]z. This work was supported servations are consistent with the onset of a chain reaction in which by the US. Department of Energy, Office of Basic Energy organic and organometallic radicals are chain carriers. A possible Sciences, Chemical Sciences Division, under Contract W-7405Eng-82. mechanism is shown in Scheme I. When the rate of reaction 18 (H)
fast
CpW(CO)2(Sn(n-Bu)3)(H)+ C O (17)
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Ligand Conformational Changes Affecting 5T2 'Al Intersystem Crossing in a Ferrous Complex James K. McCusker,' Hans Toftlund,' Arnold L. Rheing~ld,~ and David N. Hendrickson*.' Contribution from the Department of Chemistry-0506, University of California at San Diego, La Jolla, California 92093-0506, Department of Chemistry, University of Odense, DK-5230 Odense M,Denmark, and Department of Chemistry, University of Delaware, Newark, Delaware 1971 6. Received July 27, 1992
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Abstract: Results are presented from a variable-temperature solution-phase laser photolysis study of the 'T2 'A, intersystem crossing in [Fe(t-tpchxn)](C104)2,where the hexadentate ligand t-tpchxn is tranr-l,2-bis(bis(2-pyridylmethyl)amino)cyclohexane. The complex [Fe(t-tp~hxn)](ClO~)~.H~O-CH30H crystallizes in the space group pZ1212L,which at 299 K has a unit cell with a = 9.565(4) A, b = 13.178(5) A, c = 27.276(23) A, and Z = 4. Refinement with 1905 observed [F> 3.0a(F)] reflections gave R = 0.0924 and R, = 0.1 147. The structure indicates that the complex has spontaneously resolved into optically pure 'Al band (Apump = 440 nm), relaxation profiles were determined at 420 nm for crystals. After excitation into a 'MLCT a CH30H solution of [Fe(t-tpchxn)](C104)2in the range 191-280 K. In the range 190-250 K, the compound exhibits biphasic relaxation kinetics, whereas a single-exponential model was adequate from 260 to 280 K. From plots of In(k) versus 1/T for each of the two relaxation processes in the 190-250 K range, activation parameters and frequency factors were found to be 964 f 23 cm-l and (3.7 f 0.5) X lo9s-', respectively, for the T~ proms and 2370 60 cm-'and ( 5 2) X 10l2sd, respectively, in CH30H stands in contrast for the 72 process (T]< 72). The observation of two relaxation processes for [Fe(t-tp~hxn)](CIO~)~ to the data reported previously for the same complex in either DMF or CH3CN,where in both solvents only a single-exponential relaxation profile is seen. Both of the relaxation processes observed for the methanol solution are assigned as 'T2 'A, intersystem crossing processes. It is suggested that [Fe(t-tpchxn)12+undergoes a solvent-induced conformational change of the cyclohexyl ring, giving rise to two different high-spin forms of the complex having different spin-state interconversion dynamics.
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*
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Introduction A spin-crossover complex has a low-energy excited electronic state which can be thermally populated4 The factors which affect the rate of intersystem crossing between the low-spin ('A,) and high-spin ( T 2 ) states of ferrous spin-crossover complexes are being studied for a variety of reasons.5 Apart from fundamental interest ( I ) University of California at San Diego. (2) University of Odense. (3) University of Delaware. (4) (a) Toftlund, H. Coord. Chem. Rev. 1989,94,67. (b) KBnig, E. Prog. Inorg. Chem. 1987, 35, 527-622. (c) Giitlich, P. Strucr. Bonding (Berlin) 1981, 44, 83. (d) Goodwin, H. A. Coord. Chem. Rev. 1976, 18, 293. (e) Scheidt, W. R.; Reed, C. A. Chem. Rev. 1981,81, 543. (0 Konig, E.; Ritter, G.;Kulshreshtha, S . K. Chem. Reu. 1985, 85, 219. (9) Bacci, M. Coord. Chem. Rev. 1988, 86. 245. (h) Giitlich, P. In Chemical Mdssbauer Spectroscopy; Herber, R. H., Ed.; Plenum Press: New York, 1984. (i) Maeda, Y.; Takashima, Y . Commenrs Inorg. Chem. 1988, 7, 41. Q) Giitlich, P.; Hauser. A. Coord. Chem. Rev. 1990, 97, 1-22. (5) Beattie, J. K. Adu. Inorg. Chem. 1988, 32, 1-53.
0002-7863/93/ 1515-1797$04.00/0
in the dynamics of intersystem crossing processes, transition metal sites in metalloenzymes catalyze the reaction of paramagnetic O2 with diamagnetic organic substrates. In fact, spin-state changes occurring a t metal sites in proteins can be rate controlling in the functioning of certain metalloenzymes, e.g., mammalian P450.6 Finally, intersystem crossing rates in Fe" complexes are being studied in order to understand the LIESST (light-induced excited-spin-state trapping) effect discovered by Gatlich et aL7 In (6) Fisher, M. T.; Sligar, S. G. Biochemisrry 1987, 26, 4797-4803. (b) Backes, W. L.; Sligar, S. G.;Schenkman, J. B. Biochemistry 1982, 21, 1324-13330. (c) Tamburini, P. P.; Gibson, G.G.;Backes, W.L.;Sligar, S. G.; Schenkman, J. B. Biochemistry 1984, 23, 4526-4533. (7) (a) Decurtins, S.; Giitlich, P.; Kohler, C. P.; Spiering, H.; Hauser, A. Chem. Phys. Lerr. 1984, 105, 1. (b) Decurtins, S.; Giltlich, P.; Kohler, C. P.; Spiering, H. J. Chem. Soc., Chem. Commun. 1985, 430. (c) Decurtins, S.; Giitlich, P.; Hasselbach, K. M.;Hauser, A.; Spiering, H. Inorg. Chem. 1985, 24, 2174. (d) Poganuich, P.;Decurtins, S.; Giitlich, P. J . Am. Chem. Soc. 1990, 112, 3270-3278. (e) Kahn, 0.;Launay, J. P. Chemtronics 1988, 3, 140-151.
0 1993 American Chemical Society
McCusker
1798 J . Am. Chem. SOC..Vol. 115, NO.5, 1993 this phenomenon, solid samples of Fell spin-crossover complexes maintained at low temperatures can be optically driven back and forth between the stable 'Al form and the metastable 5T2form. McGarvey and L a w t h e d were the first to employ the laserflash photolysis technique to directly measure the rate a t which spin-crossover complexes interconvert between the 5T2and IAl states. Several other laser-flash studies f o l l ~ w e d . ~The temperature dependence of the ST2 'Al relaxation has been measured for several Fe" spin-crossover complexes both in solution and in the solid state.'O In the latter case, the temperature dependence has been measured from 300 to 4.2 K. At temperatures below 150 K, several complexes exhibit a temperatureindependent rate of 5T2 'Al interconversion which has been taken as evidence of tunneling.'O Very recently we employed' variable-temperature laser photolysis to study the 5T2 'Al intersystem crossing dynamics of polypyridyl Fe" complexes such as [Fe(terpy),] (C104)2 and [Fe(tpen)](C104),, where terpy is terpyridine and tpen is the following hexadentate ligand:
l2000
,
et
al.
I
-
-
250
350
450
550
Wavelength (nm)
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Figure 1. Electronic absorption spectrum of [Fe(?-tpch~n)](ClO~)~ in MeOH solution in the region of the IMLCT 'Al transition.
Table I. Crystallographic Data for [Fe(?-tpchxn)](C104)z-H20.CH30H
The ethylenediamine moiety in tpen was changed to various diamine units to examine whether a torsional distortion imparted by the tpen-type hexadentate ligand affected the kinetics of the 5T2 !Al intersystem crossing. Fitting of the temperature-dependence data measured for several tpen-type Fell complexes in solution to classical, semiclassical, and quantum-mechanical expressions for interconversion between two weakly coupled states indicated that the kinetics cannot be described by coupling to the metal-ligand stretching mode in a single conf@rational coordinate model. Rather, the data are consistent with coupling to lowfrequency ( 1 ps are not reliable; see text for further details. 'For T 1 260 K, a single exponential was sufficient to model the observed decay. 'Based on data from biexponential fits for T I 250 K.
*
Arrhenius
4
I
I
I
4.0
45
50
55
correlation 0.994 0.997 0.992 0.997
I / T ( K ) x 1000
Figure 7. Arrhenius plot of relaxation data for [Fe(r-tp~hxn)](CIO,)~ collected in CHIOH at a concentration of 2.51 X lo4 M. The two data sets are for iI (A)and r2 (m). The solid line represents a best-fit linear least-squares regression of the data; see Table 111 for details.
be consistent with the positive value for AV observed by McGarvey et a1.,I2 since activation from a chair to a twisted boat will involve a small increase in molecular volume. The overall process is envisioned to occur in the following way. The [Fe(t-tpchxn)12' complex in a C H 3 0 Hmedium is fvst excited to the 'MLCT excited state. From our previous studies,22 it is known that this complex, and for that matter all low-spin Fe" complexes studied, intersystem-crosses to the 5T2state in less than -700 fs. It is also believed22that it takes only 2-3 ps to vibrationally cool in the 5T2state. Thus, in the short time (-700 fs) 5T2interthat it takes for the complex to make the 'MLCT system crossing, the C H 3 0 H solvent structure influences the conformation of the [Fe(t-tpchxn)12' complex. One form of the complex develops in less than -700 fs which has the cyclohexylene ring in a chair conformation. The other form of the complex also develops in less than -700 fs and probably has the cyclohexylene ring in another conformation. The two different forms of the complex then thermalize and undergo the 5T2 'A, relaxation at different rates. If molecular motion accompanying the spin change were to proceed along a predominantly torsional coordinate, then it would be anticipated that a conformational change in the cyclohexylene ring structure in [Fe(r-tpchxn)]*+ accompanies the 5T2 ' A l intersystem crossing. It is difficult to predict which form of the molecule would produce a greater barrier to spin-state relaxation,
-
-
-
Le., which of 71 and 72 is due to a given conformation. Since the X-ray structure indicates a chair configuration, a change to a twisted-boat form would introduce an additional barrier to ground-state recovery that would increase the observed activation energy for the 5T2 'AI process; this could be assigned as T ~ In the case of 7 2 , a chair form that is somehow distinct from the ground-state structure would require the formation of a twisted-boat intermediate and would likely cause a substantial increase in the activation energy for the 5T2 'A, relaxation. Regardless of these details, we wish to point out that any kind of a distinct conformational change along a torsional coordinate would fit well with the model developed in our previous study" whereby geometric preferences of the ligand can induce the system to proceed further along the reaction coordinate than is minimally necessary to achieve a spin change. This gives rise to more dynamical mixing of the S = 0 and S = 2 spin manifolds and a larger intrinsic rate for spin-state interconversion. In this context, the frequency factor of ( 5 f 2) X 10I2s-I for [Fe(t-tp~hxn)](ClO,)~ is of interest, since it is a full 2 orders of magnitude larger than that found for [Fe(tptn)](C104)2 despite the lower energy of the 5T2state in [ Fe(r-tpch~n)](CIO~)~. Both of these scenarios place the role of the solvent in the forefront since they require stabilization of a particular conformation of the molecule as the system relaxes in the high-spin state. Although this is certainly consistent with the available data, we have very little information on the nature of solvation and solvent dynamics in these types of systems. As a result, it is difficult to comment on whether or not solvent-induced conformational changes of the type we are suggesting are at all reasonable. It is curious to note, however, that the frequency factor for the high-energy process in [ Fe(t-tpchxn)] is approaching a value where solvent dynamics may play a critical role.23
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-
.
J . Am. Chem. SOC.1993, 115, 1804-1816
1804
Acknowledgment. We are grateful for funding from NSF Grant CHE-9115286 and N I H Grant HL-13652. Supplementary Material Available: Tables of crystallographic data, bond distances and angles, anisotropic displacement coef-
ficients, calculated hydrogen atom coordinates, magnetic susceptibility data, and positional parameters for [Fe(t-tpchxn)](C104)2.H20-CH30H (10 pages); a listing of observed and calculated structure factors (9 pages). Ordering information is given on any current masthead page.
High-Spin Molecules: [ MnI2Ol2(02CR),6(H20)J Roberta Sessoli,' Hui-Lien Tsai,*AM R. Schake,k Sheyi W a t ~ g John , ~ ~ B. Vincent? Kirsten F ~ l t i n g Dante , ~ ~ Gatteschi,**'George Chri~tou,**~~ and David N. Hendrickson*J Contribution from the Department of Chemistry-0506, University of California at San Diego, La Jolla, California 92093-0506, Dipartimento di Chimica, Universitd di Firenze, 501 44 Firenze, Italy, and Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, Indiana 47405. Received September 18, 1992
Abstract: The syntheses and electrochemical and magnetochemical properties of [Mn,2012(02CPh)16(H20)4] (3), its solvate 3PhCOOH.CH2Cl2, and [Mn12012(02CMe)L6(H20)4]-MeCOOH.3H20 (4) are reported. Complex 3 can be prepared either by reaction of M ~ ( O A C ) ~ . ~ H benzoic ~ O , acid, and NBun4Mn04in pyridine or by reaction of PhCOOH with complex 4 slurried in CH2C12. Complex 3 crystallizes in the triclinic space group Pi,which at -146 OC has a = 27.072(19) A, b = 17.046(11) A, c = 14.254(8) A, a = 98.39(3)O, B = 98.44(4)', 7 = 89.27(4) A, and Z = 2. The structure was refined with 4814 observed
[ F > 3.Ou(F)] reflections to give R = 9.54 and R, = 10.07. [Mn12012(02CPh)r6(H20)4] (3) consists of a central [MdV404]*+ cubane held within a nonplanar ring of eight Mn"' atoms by eight p3-@- ions. Peripheral ligation is provided by 16 p2-02CPhand four terminal H 2 0 groups, where the four H 2 0 ligands are located on two Mn atoms. Four redox waves are seen in the cyclic voltammogram of complex 3 in CH2CI2: two reversible waves [an oxidation wave at 0.79 V (vs ferrocene/ferrocenium) and a reduction wave at 0.1 1 VI and two irreversible waves at -0.23 and -0.77 V. Complex 4 exhibits the same four redox couples in MeCN. Variable-temperature DC magnetic susceptibility data measured at 10.0 kG are presented for polycrystalline samples of complex 3 and the solvate 3.PhCOOHCH2C12. At 320 K, pell/molecule is 12 p~ and increases to a maximum of -20-21 pB at 10 K, whereupon pcrr/moleculedecreases rapidly at low temperatures. It is concluded that these complexes exhibit appreciable magnetic anisotropy. Even at fields as low as 1 kG the polycrystallites have to be restrained from torquing by embedding the polycrystalline sample in parafilm. Complexes 3 and 3PhCOOH-CH2C12exhibit somewhat different perr/moleculeversus temperature curves. Magnetization measurements at 20.0, 30.0,40.0, and 50.0 kG in the 2-4 K range are used to determine that in these fields complexes 3 and 3PhCOOHCHzC12 have S = 10 and S = 9 ground states, respectively. A relatively large zerdield splitting is in evidence, and this was confirmed by high-field EPR experiments with a C 0 2 far-infrared laser. AC susceptibility data in zero applied field are given for complexes 3 and 4 in the 4-25 K range. It is concluded that complex 3 has a S = 9 ground state at zero field, whereas complex 4 has a S = 10 ground state at zero field. The most interesting observation for complexes 3 and 4 derives from the out-of-phase (imaginary) component of the AC susceptibility, xM". Both of these complexes exhibit a nomro xM", which when measured at various frequencies shows a maximum at different temperatures. These two complexes are the only molecular solids known to exhibit a nonzero xM" in the paramagnetic phase. The results of theoretical calculations of the ordering of spin states in a M n ~ V M n ~complex, ** assuming reasonable values for the exchange parameters characterizing the different pairwise interactions, are presented to rationalize the S = 8-10 ground states.
-
-
Introduction
Considerable effort has been directed at understanding magnetic exchange interactions occurring in polynuclear transition-metal complexe~.~The nature (antiferromagnetic or ferromagnetic) and magnitude of a magnetic exchange interaction between two metal ions are reasonably well understood in terms of the energetics and overlap of 'magnetic The success in understanding magnetic exchange interactions in polynuclear complexes has prompted efforts in the last few years to see if these complexes can be used as molecular building blocks for materials exhibiting interesting properties. In general, the concept of molecular-based materials is being pursued in many directions. Instead of using solids consisting of extended lattices such as in oxides, the goal (1) Universitl di Firenze. (2) University of California at San Diego. (3) (a) Department of Chemistry, Indiana University. (b) Molecular Structure Center, Indiana University. (4) (a) Magneto-Structural Correlation in Exchange-Coupled Systems; Willett, R. D., Gatteschi, D., Kahn, 0.. Eds.; NATO AS1 Series C, 140; D. Reidel Publishing Co.: Dordrecht, The Netherlands, 1985. (b) Magnetic Molecular Materials; Gatteschi, D., Kahn, O., Miller, J. S.,Palacio, F., Eds.; NATO AS1 Series E, 198; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991.
is to make up a solid lattice of molecular building blocks. With certain building blocks arranged properly in a solid lattice, it may be possible to prepare a material that exhibits interesting properties. Examples of this molecular-based materials approach can be found in the active research on organic materials with nonlinear optical propertiesS and the studies on organic conductors and superconductors.6 Miller, Epstein, and co-workers' have prepared organometallic ferromagnets where metallocene cations (D') and organic anions are assembled (A?), each with a single unpaired electron (S = in alternating stacks. The pairwise (D+-.A-) magnetic exchange interactions in each stack are important, but it is the exchange (5) (a) Nonlinear Optical Properries of Organic Molecules and Crystals; Chemla, D. S., Zyss, J., Eds.; Academic Press: Orlando, 1987; Vols. 1 and 2. (b) Williams, J. M. Angew. Chem., Int. Ed. Engl. 1984, 23, 690. (c) Materials for Nonlinear Optics, Chemical Perspectives; Marder, S. R., Sohn, J. E., Stucky, G. D., Eds.; ACS Symposium Series 455; American Chemical Society: Washington, DC, 1991. (6) (a) Bredas, J. L.; Street, G. B. Acc. Chem. Res. 1985, 18, 309. (b) Wudl, F. Acc. Chem. Res. 1984, 17, 227. (c) Torrance, J. B.Acc. Chem. Res. 1979, 12, 79. (d) Williams, J. M. Prog. Inorg. Chem. 1985, 33, 183. (7) (a) Miller, J. S.; Epstein, A. J.; Reiff, W. M. Chem. Rev. 1988, 88, 201 and referencts therein. (b) Miller, J. S.; Epstein, A. J.; Reiff, W. M.Acc. Chem. Res. 1988, 21, 114 and references therein.
0002-786319311515- 1804%04.00/0 0 1993 American Chemical Society
