Variable-Temperature Studies of Laser-Initiated 5T2 1A1 Intersystem Crossing in Spin-Cro
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- 作者：James K. McCusker ; Arnold L. Rheingold ; and David N. Hendrickson
- 刊名：Inorganic Chemistry
- 出版年：1996
- 出版时间：March 27, 1996
- 年：1996
- 卷：35
- 期：7
- 页码：2100 - 2112
- 全文大小：422K
- 年卷期：v.35,no.7(March 27, 1996)
- ISSN：1520-510X
文摘
Results are presented from a variable-temperature solution-phaselaser photolysis study of the 5T2 
ages/entities/rarr.gif">1A1 intersystemcrossing in a series of related complexes:[Fe(tpen)](ClO4)2,[Fe(tppn)](ClO4)2,[Fe(tptn)](ClO4)2,[Fe(t-tpchxn)](ClO4)2, and[Fe(dpa)2](ClO4)2.The hexadentate ligands are formed with four 2-pyridylmethyl armsattached toethylenediamine (tpen), 1,2-diaminopropylene (tppn),1,3-diaminopropylene (tptn), ortrans-1,2-diaminocyclohexane(t-tpchxn). The dpa ligand is a tridentate analogue ofthese ligands, namely, bis(2-pyridylmethyl)amine.Thecomplex[Fe(mtpen)](ClO4)2·2/3H2O,where mtpen is the same as tpen except one of the pyridyl rings hasa6-methyl substituent, crystallizes in the space groupC2/c, which at 173 K has a unit cell witha = 41.390(13) Å,b = 9.5239(16) Å, c = 24.016(6)Å, ages/gifchars/beta2.gif" BORDER=0 ALIGN="middle"> = 108.24(3)ages/entities/deg.gif">, and Z = 12. Refinementwith 2844 observed [F >5.0ages/gifchars/sigma.gif" BORDER=0 >(F)]reflections gave R = 0.075 and Rw= 0.076. The complex[Fe(tppn)](ClO4)2·2/3H2Ocrystallizes in the spacegroup P21/n, which at 296 K has aunit cell with a = 12.979(4) Å, b =12.624(4) Å, c = 19.475(6) Å, ages/gifchars/beta2.gif" BORDER=0 ALIGN="middle">=108.17(2)ages/entities/deg.gif">, and Z = 4. Refinement with 2357observed [F > 5.0ages/gifchars/sigma.gif" BORDER=0 >(F)] reflections gaveR = 0.1198 and Rw =0.1141. The mtpen complex is a high-spin FeII complexat all temperatures (4.2-400 K). The hydrated tpencomplex is a spin-crossover complex with the temperature where thereare 50% high-spin complexes (T1/2) isages/entities/sime.gif">385 K, the hydrated tppn complex is also spin-crossover with a higherT1/2 value, and the hydrated tptncomplexis low spin up to 400 K. The present crystallographic results,together with previously reported structural resultsfor the tpen complex at two temperatures, are used to show that theconversion from low spin to high spin leadsto an increase in the trigonal twist of these distorted octahedralcomplexes. The influence of this variation intrigonal twist on the rate of 5T2 ages/entities/rarr.gif">1A1 intersystem crossing is examined withvariable-temperature laser-flashphotolysis. Data collected for the tpen complex in MeOH in the190-294 K range give a linear Arrhenius plotwith an activation energy of Ea = 767 ± 22cm-1 and a preexponential term ofA = (1.35 ± 0.2) × 109s-1. Thetppn complex gives similar results of Ea = 771± 17 cm-1 and A = (1.45 ±0.2) × 109 s-1. At 294 Kthe rate(k-1) for the5T2 ages/entities/rarr.gif"> 1A1 intersystemcrossing is 2.87 × 107 s-1 forthe tpen complex, and 3.21 × 107s-1 for thetppn complex. On the other hand the tptn complex hask-1 = 6.25 × 108s-1 at 295 K as measured with apicosecond spectrometer, and together with nanosecond data measured inthe 186-210 K range gives an Arrheniusactivation energy of Ea = 777 ± 50cm-1 with A = (2.6 ± 0.8) ×1010 s-1. Thebis(tridentate) complex [Fe(dpa)2](ClO4)2 in MeOH isfound to give k-1 = 4.59 ×107 s-1 at 282 K and with the191-282 K data givesArrhenius values of Ea = 339 ± 13cm-1 and A = (2.5 ± 0.25) ×108 s-1. The terpyridinecomplex [Fe(terpy)2](ClO4)2 is found to havek-1 = 1.0 × 108s-1 in MeOH at 239 K, and the analysis of190-239 K data gives Ea= 532 ± 36 cm-1 and A = (2.4± 0.4) × 109 s-1.Previous studies have shown that the greater the trigonaltwist,the lower in energy is the 3T1 state whichfacilitates the spin-orbit interaction between the5T2 high-spin and1A1low-spin states. It is suggested that the trigonal twist is avibrational coordinate strongly coupled to the5T2 ages/entities/rarr.gif">1A1 intersystem crossing. It isadditionally shown that the data are consistent with a model whereinthe "intrinsic"rate of 5T2 ages/entities/rarr.gif"> 1A1intersystem crossing, as gauged by the preexponential term, is afunction of how far along thereaction coordinate a complex proceeds.