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Inverted Ligand Fields What Is the Deal with Copper(III)?

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Inverted Ligand Fields What Is the Deal with Copper(III)? Inverted Ligand Fields What is The Deal With Copper(III)? ΨM ΨL ΨM ΨL Ψ L ΨM MacMillan Group Meeting June 15th, 2020 Cesar Nicolas Prieto Kullmer Outline of Talk Bonding Models of Coordination Complexes and Their Pitfalls — What are Inverted Ligand Fields? Early Experimental and Computational Evidence for Inverted Ligand Fields Copper (III) Complexes — Theoretical and Experimental Analysis of their Electronic Structure The Implications of Inverted Ligand Fields for Organometallic Complex Reactivity Outline of Talk Bonding Models of Coordination Complexes and Their Pitfalls — What are Inverted Ligand Fields? Early Experimental and Computational Evidence for Inverted Ligand Fields Copper (III) Complexes — Theoretical and Experimental Analysis of their Electronic Structure The Implications of Inverted Ligand Fields for Organometallic Complex Reactivity Crystal Field Theory d—orbitals: dxy dxz dyz dz2 dx2—y2 2 2 dx —y 2 2 2 2 E dz dx —y dz dxz dyz dxy Δo d d d-orbitals in xz yz dxy spherical field 2 2 2 dz dx —y 2 2 d d dxy dx —y xz yz dxz dyz dxy 2 dz L— L L free ion d-orbitals L— — L L L L n+ L n+ Mn+ Mn+ L Mn+ L Mn+ L L M L M L L L— — L L L L L L L L— - relies only ligand—metal electronic repulsion fails to predict certain ligand effects (i.e. OH vs. H2O) cannot account for neutral ligands only considers d—orbitals Houscroft, C. E.; Sharpe, A. G. Inorganic Chemistry, 2nd ed.; Pearson Education Ltd: London, 2005. Ligand Field Theory factors in ALL metal orbitals and ALL ligand orbitals 2t1u t1u 2a1g a1g 2eg Δo atomic orbitals symmetry-adapted linear eg of ligands combinations (SALCs) t2g 1t2g eg returns same splitting of d-orbitals as CFT t1u 1t1u rationalizes neutral ligand bonding and complex stability a1g 1e M g L assumption: ligand orbitals considered lower in energy 1a1g complete MO diagram Houscroft, C. E.; Sharpe, A. G. Inorganic Chemistry, 2nd ed.; Pearson Education Ltd: London, 2005. MO Shape and Composition Ψanb Ψanb = c1Ψ1 - c2Ψ2 MO composition c1 = c2 reflects individual Ψ Ψ AO contribution H1 H2 Ψ Ψbon = c1Ψ1 + c2Ψ2 bon non—polarized cC > cO C O Ψanb ΨC eletrons of MO reside mostly on main constributor ΨO c < c C O C O Ψbon polarized Houscroft, C. E.; Sharpe, A. G. Inorganic Chemistry, 2nd ed.; Pearson Education Ltd: London, 2005. Oxidation State Formalism According to the IUPAC: OS of an atom is the charge of this atom after ionic approximation of its heteronuclear bonds. Why? …underlying principle is that the ionic sign in an AB molecule is deduced from the electron allegiance in a LCAO-MO model: The bond’s electrons are assigned to its main atomic contributor. eg* eg* any tendgiven to MO: assign Ψ = oxidationc1Ψ1 + c2 Ψstate2 + …. main basedcontributor on electronegativity = largest coeffiecient t2g* Cl 2— Cl Cl 4+ — Ir Ir + 6 Cl t2g Cl Cl Cl t ESR hyperfine coupling w/ chlorine observed 2g e— hole 3% of time on any chlorine (18% total) IrCl6 σ only Cl π donation t2g What if combined ligand contributions outweigh metal? IUPAC. Compendium of Chemical Terminology, 2nd ed.; Blackwell Scientific Publications: Oxford, 1997. Owens, J.; Stevens, K. W. H. Nature 1953, 71, 836. Jørgensen, K. Coord. Chem. Rev. 1966, 1, 164-178. What is an inverted ligand field? cM > cL cM < cL Ψanb = cMΨM - cLΨL ΨM ΨL MO composition reflects individual cM = cL AO contribution ΨM ΨL Ψ L ΨM Ψbon = cMΨM + cLΨL cM < cL cM > cL classical covalent inverted inverted ligand fields show abnormnal MO orderings and compositions HOMO and LUMO are ligand-based rather than metal-based electrons do not reside on atoms as one would expect from oxdiation state formalism DiMucci, I. M; et al. J. Am. Chem. Soc. 2019, 141, 18508—18520. What is an inverted ligand field? Normal splitting Inverted splitting Ir4+ + 6 Cl— eg t2g Oh t2g eg Cl 2— Cl Cl Ir Cl Cl t2 e Cl Td e t2 b1g b2g Ir2+ + 5 Cl— + 1 Cl+ a1g eg Ir0+ + 4 Cl— + 2 Cl+ D4h eg Ir-2 + 3 Cl— + 3 Cl+ a1g etc. b2g b1g A Theoretical Inverted Octahedral Ligand Field 2t1u 2t1u t1u 2a1g 2a1g a1g t1u a1g 2eg 2eg eg t1u eg t2g a1g 1t2g eg 1t eg 1u t1u t2g 1t2g 1t1u a1g 1eg 1eg 1a1g 1a1g M = 6 e— normal MO diagram L = 12 e— M = 10 e— inverted MO diagram L = 8 e— only some MOs switch their order the metal—centered MOs change their identity Approaches to Ligand Field Inversion 2t lower metal-based orbitals 1u t1u charged metal center 2a1g a1g more electronegative metal 2eg eg t2g 1t2g raise ligand-based orbitals eg t1u 1t1u electron-releasing susbtituents a1g 1eg more electropositive elements M 1a1g L Hoffmann, R.; et. al. Chem. Rev. 2016, 116, 8173—8192. Outline of Talk Bonding Models of Coordination Complexes and Their Pitfalls — What are Inverted Ligand Fields? Early Experimental and Computational Evidence for Inverted Ligand Fields Copper (III) Complexes — Theoretical and Experimental Analysis of their Electronic Structure The Implications of Inverted Ligand Fields for Organometallic Complex Reactivity Early Evidence — KAgF4 Solid containing formally Ag(III): Ag—F bonding (—6.5 to —9.0 eV): dominant Ag d—orbital contribution Ag—F anti bonding (—0.7 eV): excess F 2p—orbital contribution … classical picture used for ionic inorganic compounds … an ionic 4 I III — formulation of KAgF as K Ag (F )4, must be modified in this case… Ag(d) F(p) Groachala, W.; Hoffmann, R. Angew. Chem. Int. Ed. 2001, 40, 2742—2781. + Early Evidence — [Ni(SnPh3)(np3)] N PPh2 Ph2P Ni PPh2 SnPh3 [Ni(np3)] known to exist independtly as stable complex donor—acceptor relationship between Ni and Sn inverted (σ-SnPh3 ~ 2eV higher than 2a1) 10 + metal is best described as d w/ SnPh3 ligand σ-noninnocence: ambiguity on which atom is acting as the donor Mealli, C.; Ghilardi, C. A.; Orlandini, A. Coord. Chem. Rev. 1992, 120, 361—387. Early Evidence — Vitamin B12 mimetics Me H H N N σ3 Co σ3 N N H H NH2 σ3 Cobaloxime σ2 σ2 part of e pz g σ2 σ3 σ1 σ1 σ1 σCH3 dz2 σ2 σNH2 Reduction of Co—N distance increases z2 energy σ1 Modulation of Co—N distance affects propensity metal to undergo inner-sphere e— transfer Modulation of Co—N distance affects propensity metal to undergo inner-sphere e— transfer Intermediate Co—N distance shows avoided crossing 2 σ1 — NH2, z bonding at longer Co—C distances (allows for Co—C homolysis) 2 σ2 — pz, CH3 bonding +NH2, z antibonding Intermediate Co—N distance shows avoided crossing 2 at longer Co—C distances —> allows for Co—C homolysis σ3 — pz, CH3 antibonding +NH2, z antibonding Mealli, C.; Sabat, M.; Marzilli, L. G. J. Am. Chem. Soc. 1987, 109, 1593—1594. Computational Investigation of d10 3t2 t2 2a1 Sub a1 M Sub Sub 2t2 Sub t2 e 1e Roald Hoffmann t2 a1 Using EH3 (E = N, P, As, Sb, Bi,) M = Ni, Pd, Pt: 1a1 M L Inherent instability (decay into ML2 + 2L) limits combinations to local minima 1t2 Donor ability of ligand does not correlate w/ electropositivity (inert electron pair effect) Varying metal did not return clear result due to realitites of atomic orbital energies 3t2 — 2+ Using L = X , PH3 M = Zn : 2a1 t2 2— [ZnF4] [Zn(PH3)4](BF4)2 a1 2t2 orbital %Zn %F energy(eV) %Zn %L energy(eV) t2 a1 2t2 0.87 16 84 —11.03 8 92 1a1 1a1 —1.70 19 81 —14.13 46 54 t2 1e e —2.86 92 8 —17.73 99 1 1e 1t2 —3.14 82 18 —17.79 96 4 M L 1t2 Hoffmann, R.; et. al. Chem. Rev. 2016, 116, 8173—8192. Inversion with a true transition metal —0.67 + [Rh(AlMe)4] bending MeAl AlMe formally Rh(I) MeAl AlMe 0.52 effectively Rh(—I) eg (xz, yz) rehybdridizes to form orbitals with significant lone-pair character trigonal prismatic structure highly electropositive ECR3 ligands strong π acceptors synthetically accessible Can we see a smiliar effect in homoleptic hexacoordinate AlMe complexes? Mo(AlMe)6 also optimizes into a trigonal prismatic structure Hoffmann, R.; et. al. Chem. Rev. 2016, 116, 8173—8192. Inversion with a true transition metal Mo(AlMe)6 Pd(AlMe)4 formally Mo(0) formally Pd(0) effectively Mo(—IV) effectively Pd(0) inverted orbital π—acidity of ligands splitting, ligand- crucial to stability centered FMOs tetrahedral structure as expected for d10 two e— more than the rhodium species removal of e— from Pd to Rh occur from ligand over metal inverted from that found in classical trigonal prismatic complexes (a′1 < e′ < e″) Hoffmann, R.; et. al. Chem. Rev. 2016, 116, 8173—8192. Outline of Talk Bonding Models of Coordination Complexes and Their Pitfalls — What are Inverted Ligand Fields? Early Experimental and Computational Evidence for Inverted Ligand Fields Copper (III) Complexes — Theoretical and Experimental Analysis of their Electronic Structure The Implications of Inverted Ligand Fields for Organometallic Complex Reactivity The Snyder Hypothesis — CF3 CF3 D putative intermediate in 2d Cu trifluoromethylations CF3 CF3 Snyder presumed that upon oxidation of 3— [Cu(CF3)4] a ligand is oxidized MO diagram implies that only four bonding electrons hold this complex together + CF3 both HOMO and LUMO ligand-based Snyder’s description: CF3 Cu CF3 — CF3 DFT calculations reveal: bonding is principally ionic and accompanied by two 5c/2e— bonds atomic charges: Cu (+0.71), C (+0.74), F (-0.39) depletion delocalized across all CF ligands d population: dxy, dxz, dyz, dz2 1.96—1.98, dx2-y2 1.77 3 — + copper is best described as a Cu(I) species CF3 groups carry 25.4 electrons (3CF3 + CF3 ) Snyder, J.
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