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Article

High-Spin Diimine Complexes of Iron(II) Reject Binding of Carbon Monoxide: Theoretical Analysis of Thermodynamic Factors Inhibiting or Favoring Spin-Crossover

Supporting Info

Ned J. Hardman, Xinggao Fang, Brian L. Scott, Robert J. Wright, Richard L. Martin,* and Gregory J. Kubas*

Chemistry Division and Theoretical Division, Los Alamos National Laboratory, MS-J514, Los Alamos, New Mexico 87545

Inorg. Chem., 2005, 44 (23), pp 8306–8316

DOI: 10.1021/ic050966h

Publication Date (Web): October 5, 2005

To whom correspondence should be addressed. E-mail: kubas@lanl.gov (G.J.K.); rlmartin@lanl.gov (R.L.M.).

Abstract

A new series of Fe(II) complexes, FeCl₂{N(R)C(Me)C(Me)N(R)}, containing diimine ligands with hemilabile sidearms R (R = CH₂(CH₂)₂NMe₂, 1, CH₂(CH₂)₂OMe, 2, CH₂(CH₂)₂SMe), 3) were synthesized. The crystal structure of 1 showed 6-coordination where both amine arms were attached, whereas 2 was a 5-coordinate 16e species with one methoxy arm dangling free. Extensive attempts were made to bind CO to these species to synthesize precursors for dihydrogen complexes but were unsuccessful. Reaction of 1 with 1 or 2 equiv of AgOTf under CO atmosphere resulted in isolation of only a 6-coordinate bis(triflate)-containing product [Fe{N(R)C(Me)C(Me)N(R)}(OTf)₂] (R = CH₂(CH₂)₂NMe₂), 5. Reaction of 5-coordinate 2 with AgSbF₆ under CO did not give a CO adduct but afforded instead a dicationic dinuclear complex [Fe{N(R)C(Me)C(Me)N(R)}(μ-Cl)]₂[SbF₆]₂ (R = CH₂(CH₂)₂OMe), 4, containing a weakly bound SbF₆. Thus coordination of hard-donor anions to iron was favored over CO binding. The unexpected rejection of binding of CO is rationalized by the iron being in a high-spin state in this system and energetically incapable of spin crossover to a low-spin state. Theoretical calculations on CO interaction with Fe(II) centers in spin states S = 0, 1, and 2 for both the 16e complexes and their CO adducts aid further understanding of this problem. They show that interaction of CO with a high-spin 5-coordinate Fe model diimine complex is essentially thermoneutral but is exergonic by about 48 kcal/mol to a comparable but low-spin diphosphine fragment. Spin crossover is thus disfavored thermodynamically rather than kinetically (e.g. a “spin block” effect); i.e., the ligand field strengths of the primarily N-donor groups are apparently insufficient to give a low-spin CO adduct.