Synthesis and electronic structure determination of N-alkyl-substituted bis(imino)pyridine iron imides exhibiting spin crossover behavior.

Three new N-alkyl substituted bis(imino)pyridine iron imide complexes, ((iPr)PDI)FeNR ((iPr)PDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)-N═CMe)(2)C(5)H(3)N; R = 1-adamantyl ((1)Ad), cyclooctyl ((Cy)Oct), and 2-adamantyl ((2)Ad)) were synthesized by addition of the appropriate alkyl azide to the iron bis(dinitrogen) complex, ((iPr)PDI)Fe(N(2))(2). SQUID magnetic measurements on the isomeric iron imides, ((iPr)PDI)FeN(1)Ad and ((iPr)PDI)FeN(2)Ad, established spin crossover behavior with the latter example having a more complete spin transition in the experimentally accessible temperature range. X-ray diffraction on all three alkyl-substituted bis(imino)pyridine iron imides established essentially planar compounds with relatively short Fe-N(imide) bond lengths and two-electron reduction of the redox-active bis(imino)pyridine chelate. Zero- and applied-field Mössbauer spectroscopic measurements indicate diamagnetic ground states at cryogenic temperatures and established low isomer shifts consistent with highly covalent molecules. For ((iPr)PDI)FeN(2)Ad, Mössbauer spectroscopy also supports spin crossover behavior and allowed extraction of thermodynamic parameters for the S = 0 to S = 1 transition. X-ray absorption spectroscopy and computational studies were also performed to explore the electronic structure of the bis(imino)pyridine alkyl-substituted imides. An electronic structure description with a low spin ferric center (S = 1/2) antiferromagnetically coupled to an imidyl radical (S(imide) = 1/2) and a closed-shell, dianionic bis(imino)pyridine chelate (S(PDI) = 0) is favored for the S = 0 state. An iron-centered spin transition to an intermediate spin ferric ion (S(Fe) = 3/2) accounts for the S = 1 state observed at higher temperatures. Other possibilities based on the computational and experimental data are also evaluated and compared to the electronic structure of the bis(imino)pyridine iron N-aryl imide counterparts.

[1]  Steven D. Brown,et al.  Ground-state singlet L3Fe-(mu-N)-FeL3 and L3Fe(NR) complexes featuring pseudotetrahedral Fe(II) centers. , 2005, Journal of the American Chemical Society.

[2]  Patrick L. Holland,et al.  Selectivity and mechanism of hydrogen atom transfer by an isolable imidoiron(III) complex. , 2011, Journal of the American Chemical Society.

[3]  A. Verma,et al.  A Stable Terminal Imide on Iron , 2000 .

[4]  D. Case,et al.  Orbital interactions, electron delocalization and spin coupling in iron-sulfur clusters , 1995 .

[5]  T. Betley,et al.  Catalytic C-H bond amination from high-spin iron imido complexes. , 2011, Journal of the American Chemical Society.

[6]  J. Love,et al.  Transformation of Coordinated Dinitrogen by Reaction with Dihydrogen and Primary Silanes , 1997, Science.

[7]  A. P. Ginsberg.  Magnetic exchange in transition metal complexes. 12. Calculation of cluster exchange coupling constants with the X.alpha.-scattered wave method , 1980 .

[8]  Marco Häser,et al.  Auxiliary basis sets to approximate Coulomb potentials , 1995 .

[9]  P. Budzelaar,et al.  Ligand-centred reactivity in diiminepyridine complexes. , 2006, Dalton transactions.

[10]  P. Chirik,et al.  On the origin of dinitrogen hydrogenation promoted by [(η5- C5Me4H)2Zr]2(μ2, η2,η2-N2) , 2004 .

[11]  Steven D. Brown,et al.  Hydrogenolysis of [PhBP3]Fe⋮N-p-tolyl: Probing the Reactivity of an Iron Imide with H2 , 2004 .

[12]  Shubin Liu,et al.  Towards understanding performance differences between approximate density functionals for spin states of iron complexes. , 2006, The Journal of chemical physics.

[13]  Florian Weigend,et al.  Auxiliary basis sets for main row atoms and transition metals and their use to approximate Coulomb potentials , 1997 .

[14]  F. Neese,et al.  Electronic structure of bis(imino)pyridine iron dichloride, monochloride, and neutral ligand complexes: a combined structural, spectroscopic, and computational study. , 2006, Journal of the American Chemical Society.

[15]  P. Budzelaar,et al.  The Electronic Structure of (Diiminopyridine)cobalt(I) Complexes , 2004 .

[16]  T. Betley,et al.  C-H bond amination from a ferrous dipyrromethene complex. , 2009, Inorganic chemistry.

[17]  J. Perdew,et al.  Accurate and simple density functional for the electronic exchange energy: Generalized gradient approximation. , 1986, Physical review. B, Condensed matter.

[18]  P. Gütlich,et al.  Spin Crossover— An Overall Perspective , 2005 .

[19]  P. Holland,et al.  Three-coordinate terminal imidoiron(III) complexes: structure, spectroscopy, and mechanism of formation. , 2010, Inorganic chemistry.

[20]  J. Perdew,et al.  Density-functional approximation for the correlation energy of the inhomogeneous electron gas. , 1986, Physical review. B, Condensed matter.

[21]  P. Chirik,et al.  Hydrogenation and cleavage of dinitrogen to ammonia with a zirconium complex , 2004, Nature.

[22]  F. Neese,et al.  Comparison of two efficient approximate Hartee–Fock approaches , 2009 .

[23]  Patrick L. Holland,et al.  C–H activation by a terminal imidoiron(III) complex to form a cyclopentadienyliron(II) product , 2011 .

[24]  P. Chirik,et al.  Preparation and molecular and electronic structures of iron(0) dinitrogen and silane complexes and their application to catalytic hydrogenation and hydrosilation. , 2004, Journal of the American Chemical Society.

[25]  Frank Neese,et al.  Prediction and interpretation of the 57Fe isomer shift in Mössbauer spectra by density functional theory , 2002 .

[26]  Frank Neese,et al.  Performance of nonrelativistic and quasi-relativistic hybrid DFT for the prediction of electric and magnetic hyperfine parameters in 57Fe Mössbauer spectra. , 2005, Inorganic chemistry.

[27]  Patrick L. Holland,et al.  Catalytic nitrene transfer from an imidoiron(III) complex to form carbodiimides and isocyanates. , 2009, Chemical communications.

[28]  S. Seki,et al.  Phonon coupled cooperative low-spin 1A1high-spin 5T2 transition in [Fe(phen)2(NCS)2] and [Fe(phen)2(NCSe)2] crystals , 1974 .

[29]  F. Neese,et al.  Efficient, approximate and parallel Hartree–Fock and hybrid DFT calculations. A ‘chain-of-spheres’ algorithm for the Hartree–Fock exchange , 2009 .

[30]  F. Neese,et al.  Calibration of modern density functional theory methods for the prediction of 57Fe Mössbauer isomer shifts: meta-GGA and double-hybrid functionals. , 2009, Inorganic chemistry.

[31]  Steven D. Brown,et al.  A low-spin d5 iron imide: nitrene capture by low-coordinate iron(I) provides the 4-coordinate Fe(III) complex [PhB(CH2PPh2)3]Fe=N-p-tolyl. , 2003, Journal of the American Chemical Society.

[32]  Patrick L. Holland,et al.  Coordination-number dependence of reactivity in an imidoiron(III) complex. , 2006, Angewandte Chemie.

[33]  Salvador G. Alvarez,et al.  A Practical Procedure for the Synthesis of Alkyl Azides at Ambient Temperature in Dimethyl Sulfoxide in High Purity and Yield , 1997 .

[34]  Connie C. Lu,et al.  An electron-transfer series of high-valent chromium complexes with redox non-innocent, non-heme ligands. , 2008, Angewandte Chemie.

[35]  Frank Neese,et al.  An improvement of the resolution of the identity approximation for the formation of the Coulomb matrix , 2003, J. Comput. Chem..

[36]  Christine M. Thomas,et al.  Characterization of the terminal iron(IV) imides [[PhBP(t)(Bu)2(pz')]Fe(IV)NAd]+. , 2006, Journal of the American Chemical Society.

[37]  F. Weigend,et al.  Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. , 2005, Physical chemistry chemical physics : PCCP.

[38]  Frank Neese,et al.  Definition of corresponding orbitals and the diradical character in broken symmetry DFT calculations on spin coupled systems , 2004 .

[39]  A. Becke.  Density-functional thermochemistry. III. The role of exact exchange , 1993 .

[40]  P. Holland,et al.  A bridging hexazene (RNNNNNNR) ligand from reductive coupling of azides. , 2008, Journal of the American Chemical Society.

[41]  Haobin Wang,et al.  Thermodynamics of hydrogen atom transfer to a high-valent iron imido complex. , 2008, Journal of the American Chemical Society.

[42]  Hans W. Horn,et al.  Fully optimized contracted Gaussian basis sets for atoms Li to Kr , 1992 .

[43]  Karl Wieghardt,et al.  Radical Ligands Confer Nobility on Base-Metal Catalysts , 2010, Science.

[44]  F. Neese,et al.  Electronic structure and spectroscopy of "superoxidized" iron centers in model systems: theoretical and experimental trends. , 2008, Physical chemistry chemical physics : PCCP.

[45]  A. Schäfer,et al.  Fully optimized contracted Gaussian basis sets of triple zeta valence quality for atoms Li to Kr , 1994 .

[46]  P. Chirik,et al.  Synthesis and hydrogenation of bis(imino)pyridine iron imides. , 2006, Journal of the American Chemical Society.

[47]  J. Fettinger,et al.  Reaction of a sterically encumbered iron(I) aryl/arene with organoazides: formation of an iron(V) bis(imide). , 2008, Chemical communications.

[48]  Ruiqin Q. Zhang,et al.  Effective simulation of biological systems: Choice of density functional and basis set for heme-containing complexes , 2007 .

[49]  F. Neese,et al.  Iron(II) complexes with redox-active tetrazene (RNNNNR) ligands. , 2009, Inorganic chemistry.

[50]  M. Mehn,et al.  Mid- to high-valent imido and nitrido complexes of iron. , 2006, Journal of inorganic biochemistry.

[51]  P. Chirik,et al.  Kinetics and mechanism of N2 hydrogenation in bis(cyclopentadienyl) zirconium complexes and dinitrogen functionalization by 1,2-addition of a saturated C-H bond. , 2005, Journal of the American Chemical Society.

[52]  Philipp Gütlich,et al.  Spin crossover in iron(II)-complexes , 1981 .

[53]  J. Berry.  TERMINAL NITRIDO AND IMIDO COMPLEXES OF THE LATE TRANSITION METALS , 2009 .

[54]  F. Neese,et al.  Accurate modeling of spin-state energetics in spin-crossover systems with modern density functional theory. , 2010, Inorganic chemistry.

[55]  F. Neese,et al.  Spectroscopic and computational evaluation of the structure of the high-spin Fe(IV)-oxo intermediates in taurine: alpha-ketoglutarate dioxygenase from Escherichia coli and its His99Ala ligand variant. , 2007, Journal of the American Chemical Society.

[56]  F. Neese,et al.  The unusual electronic structure of dinitrosyl iron complexes. , 2010, Journal of the American Chemical Society.

[57]  Marco Häser,et al.  Auxiliary basis sets to approximate Coulomb potentials (Chem. Phys. Letters 240 (1995) 283-290) , 1995 .

[58]  P. Chirik,et al.  Synthesis of bis(imino)pyridine iron amide and ammonia compounds from an N-H transfer agent. , 2009, Inorganic chemistry.

[59]  J. Peters,et al.  Terminal iron dinitrogen and iron imide complexes supported by a tris(phosphino)borane ligand. , 2011, Angewandte Chemie.

[60]  Axel D. Becke,et al.  Density Functional Calculations of Molecular Bond Energies , 1986 .

[61]  D. Powell,et al.  Preparation of iron amido complexes via putative Fe(IV) imido intermediates. , 2005, Journal of the American Chemical Society.

[62]  F. Neese,et al.  Theoretical bioinorganic chemistry: the electronic structure makes a difference. , 2007, Current opinion in chemical biology.

[63]  Parr,et al.  Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. , 1988, Physical review. B, Condensed matter.

[64]  P. Müller,et al.  Catalytic N-N coupling of aryl azides to yield azoarenes via trigonal bipyramid iron-nitrene intermediates. , 2010, Journal of the American Chemical Society.