Upload
alexandria-konkol
View
55
Download
0
Embed Size (px)
Citation preview
Synthesis)and)Characteriza0on)of)Thermochromic)Metal)Complexes)with)a)Near)Room)
Temperature)Spin)Crossover))
Alexandria)Konkol)and)Jesse)W.)Karr*)
Department)of)Chemistry)and)Biochemistry)
Siena)College,)Loudonville,)NY)12211)
Discussion)and)Future)Work!
Thermochromic!complexes!have!a!color!change!associated!with!the!shi5!from!high!spin7to7low!spin!caused!by!temperature!change,!thus!crea<ng!a!physical!indica<on!of! the! spin! crossover.! It! appears! that! both! the! solvent! and! counter! ion! iden<ty!effect! the! thermodynamic! parameters! for! changing! the! spin! state! of! the![Fe(pybimH)3]
2+! complex! synthesized.! ! These! iron(II)! complexes! exhibi<ng! a! SCO!behavior! are! easily! synthesized! and! characterized! making! them! ideal! for!integra<on!into!an!undergraduate!laboratory!curriculum.!
Future! analysis! of! the! complexes! will! include! inves<ga<ng! the! temperature!dependence! on! the!magne<c! suscep<bility! of! these! complexes! using!mangne<c!resonance!spectroscopy!(i.e.!the!Evans!NMR!or!EPR!spectroscopy)!!
Acknowledgements!• Dr.!John!Caradonna!and!Ellen!Matson!of!!Boston!University!
• Dr.!John!Miecznikowski!of!Fairfield!University!
• Dr.!Jodi!O’Donnell!of!Siena!College!
• Siena!College!Department!of!Chemistry!and!Biochemistry!for!use!of!it’s!facili<es!
References!• Cambi,!L.;!Cagnasso,!A.,!A"#Accad.#Naz.#Lincei#1931,!13,!809713;!Cambi,!L.;!Szegö,!L.,!
Ber.Dtsch.#Chem.#Ges.#1931,!64,!259178.!• Baker,!W.!A.;!Bobonich,!H.!M.,!Inorg.#Chem.#1964,!3,!1184788.!• Hauser,!A.,!Top.#Curr.#Chem.#2004,)234,!1557198.!• Létard,!J.;!Guionneau,!P.;!Coux7Capes,!L.,!Top.#Curr.#Chem.#2004,!235,!221749.!• Telfer,!S.!G.;!Bocquet,!B.;!Williams,!A.!F.,!Inorg.#Chem.#2001,!40,!4818720;!Chabonnière,!
L.!J.;!Williams,!A.!F.;!Piguet,!C.;!Bernardinelli,!G.;!Rivara7Minten,!E.,!Chem.#Eur.#J.#1998,!4,!485793.!
• Wolsey,!W.!C.,!J.#Chem.#Educ.#1973,!50,!A33577.!• Bain,!G.!A.;!Berry,!J.!F.,!J.#Chem.#Educ.#2008,!85,!5327536.!• Sams,!J.R.;!Tsin,!T.B.,!5T2g⟶1A1g!Spin!crossover!in!tris[27(27pyridyl)benzimidazole]iron(II)!
complexes!!Inorganic#Chemistry.#1976,!15,!154471550.!
Determina0on)of)Thermodynamic)Parameters)
of)the)SCO)Process)• The!KSCO!was!defined!as!the!ra<o!of!A466/A494!=!εls[LS]/εhs[HS]!
• This!allowed!for!the!use!of!a!van’t!Hoff!plot!to!determine!ΔH,!ΔS,!and!TSCO!for!the!SCO!process.!
van’t) Hoff) plot) of) [Fe(pybimH)3](PF6)2) in)
methanol.)The!KSCO,! found!using!data!from!the!absorp<on!spectra,!was!plooed!against!the!inverse!temperature)
Abstract!It!is!widely!known!that!the!valance!electrons!of!a!compound!are!responsible!for!its!observed! color.! Thermochromic! metal! compounds! will! change! color! with!temperature! because! of! a! change! in! the! occupancy! of! their! valence! molecular!orbitals.!Tradi<onal! inorganic! texts! teach!that! for! transi<on!metal!complexes! the!distribu<on!of!valence!electrons! is!the!result!of!the!metal’s!oxida<on!state!along!with! the! nature! of! its! coordina<ng! ligands;! implying! that! this! electronic!arrangement! is! a! sta<c! phenomenon.! However,! according! to! the! Maxwell7Boltzmann!theory!the!electronic!distribu<on!will!also!be!temperature!dependent,!meaning!that!for!a!given!metal!complex!the!distribu<on!of!valence!electrons,!and!resul<ng! color,!will! also!depend!on! temperature.! This! changing!of! the!electronic!distribu<on!is!referred!to!as!a!spin!crossover!(SCO)!process.!Here!we!present!the!synthes is! and! character i za<on! of! a! smal l! l ibrary! of! t r i s [27 (27pyridyl)benzimidazole]iron(II),! [Fe(pybimH)3]
2+,! complexes! which! exhibit!thermochromic! proper<es! due! to! having! a! near! room! temperature! SCO! point.!Using! absorp<on! spectroscopy! we! examined! the! thermodynamic! parameters! of!this!spin!crossover!for!the!various![Fe(pybimH)3]
2+!salts.!Also!we!show!preliminary!data!to!suggest!this!spin!crossover!process!is!solvent!dependent.!Ul<mately!these!compounds!can!be!used!as!an!introductory!laboratory!experiment!demonstra<ng!the!SCO!process.!
Introduc0on!• Octahedral!metal!complexes!have!electrons!constrained!to!MOs!that!have!approximately!the!same!energy!as!the!atomic!d7orbitals!!
• Energy!difference!between!the!t2g!and!eg*!is!referred!to!as!ligand!field!strength!(Δo)!
• Low!spin!is!produced!by!strong!field!ligands,!conversely!high!spin!complexes!are!produced!by!weak!field!ligands.!!
• External!perturba<on!resul<ng!in!a!change!in!magnitude!of!Δo!can!cause!a!metal!ion!to!change!spin!state!
!Octahedral)Ligand)Field)Spli`ng)
Adapted#from#Hauser,#2004.#
• SCO!complexes!exist!in!a!ground!state!and!have!a!accessible!excited!spin!state.!
• Excita<ons!to!this!higher!energy!spin!states!can!thermally,!photoly<cally,!or!pressure!induced.!
• Electrons!in!these!complexes!can!be!excited!from!the!HOMO!to!the!LUMO!giving!rise!to!a!spectroscopic!signal.!
• This!spectroscopic!signal!provides!a!useful!way!of!monitoring!the!SCO!process.!
sT)=)0,)Low)Spin)
G.S.)Term)Symbol:)1A1g)
t2g)
eg*)
Δo)>)Πc)
t2g)
eg*)
sT)=)2,)High)Spin)
G.S.)Term)Symbol:)5T2g)
Δo)<)Πc)
Δo)
Synthe0c)Approach!• Approximately!three!molar!equivalents!of!pybimH!were!combined!with!one!molar!equivalent!of!iron!chloride!in!a!minimal!amount!of!methanol.!
• The!reddish!solu<on!was!then!added!to!an!aqueous!solu<on!of!the!desired!ammonium!counter!ion!salt!causing!the![Fe(pybimH)3]X2!complex!to!precipitate.!
• The!red!crystals!were!isolated!via!vacuum!filtra<on.!
FeCl2+!3!(pybimH)!!!!!!!!!!!!!!!!!!Fe(pybimH)3X2!!
27(27pyridyl)benzimidazole!
Fe(II)!+!3!
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
300 350 400 450 500 550 600 650 700
Abs
oban
ce (A
BS)
Wavelength (nm)
Temperature) dependent) absorp0on) of)[Fe(pybimH)3](PF6)2!nH2O) in) Methanol.) A!methanol! solu<on! with! a! concentra<on! of!1mg/mL! was! analyzed! using! the! UV7Vis!between! temperatures! of! 10°765°C! at! 5°!increments.!
Absorp0on)Spectroscopy)of)[Fe(pybimH)3](PF6)2)
Temperature) dependent) absorp0on) of)[Fe(pybimH)3](PF6)2!nH2O)in)Acetonitrile.!An! ace ton i t r i l e! so lu<on! w i th! a!concentra<on! of! 1mg/mL! was! analyzed!using!the!UV7Vis!between!temperature!of!10°750°C!at!10°!increments.!
[Fe(pybimH)3](PF6)2) in) methanol) at)
low,) high,) and) room) temperature.)
Aliquots! of! the! same! 1mg/mL![Fe(pybimH)3](PF6)2! complex! at!793°C,! 25°C,! and! 65°C! (from! le5! to!right,!respec<vely).)
NH4X!
• As! the! complex! is! heated,! there! is! a! blue! shi5! in! the! visible! peak! from! 494nm! to!466nm.!!