Quantitative Determination of the Singlet-Triplet Energy Gap in Pseudo-Octahedral Complexes of W(IV): Measurement of the Relative .pi.-Acidities of N-Heterocyclic Ligands

Kriley, Charles E.; Fanwick, Phillip E.; Rothwell, Ian P.

doi:10.1021/ja00091a029

Cited by 25 publications

(24 citation statements)

References 2 publications

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“…In compound 1 there is significant π-back-bonding from the metal center (metallacyclopropane in the Dewar−Chatt−Duncanson model) resulting in a large 2 J ( 1 H− 1 H) coupling constant approaching that typical of diastereotopic sp 3 -methylene protons (12−15 Hz) and a reduced 1 J ( 13 C− 1 H) coupling constant compared to that found in free olefins (Table ). Replacement of the thf (a straightforward σ-donor) by two pyridine ligands (potential π-acceptors) can be seen to decrease the degree of π-back-bonding between the niobium metal center and the vinyl group as judged by these two parameters. This effect becomes dramatically apparent in complex 3 (Table ) which contains two strongly π-accepting isocyanide ligands which can compete with the chelated π-vinyl group for the electron density on the metal center.…”

Section: Resultsmentioning

confidence: 99%

Intramolecular Dehydrogenation of Alkyl Groups at Niobium Aryloxide Centers: Bonding and Reactivity of the Ensuing Niobacyclopropane Ring

Felter

Potyen

et al. 1996

Organometallics

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The niobium dichloride [Nb(OC6H3Pri 2-2,6)3Cl2] (OC6H3Pri 2-2,6 = 2,6-diisopropylphenoxide) undergoes reduction (2 Na per Nb)) in tetrahydrofuran (thf) solvent to produce the dark green complex [Nb(OC6H3Pri-η2-CMeCH2)(OC6H3Pri 2-2,6)2(thf)] (1). The solid-state structure of 1 shows the α-methylvinyl group to be strongly η2-bound to the niobium metal center. The dehydrogenation of the ortho-isopropyl group of an aryloxide group to generate 1 is argued to proceed via CH bond activation within a d2-Nb(III) aryloxide followed by β-hydrogen abstraction/elimination of H2. The thf ligand in 1 can be substituted by pyridine (py) and ButNC to yield the adducts [Nb(OC6H3Pri-η2-CMeCH2)(OC6H3Pri 2-2,6)2(L)2] (L = py, 2; L = ButNC, 3). The solid-state structure of 3 shows both isocyanide ligands to be terminally bound with no interaction with the η2-olefin fragment. The spectroscopic properties of these three adducts give insight into the degree of π-back-bonding between the metal and the chelated olefin fragment. The metallacyclopropane ring in 1 will undergo ring expansion (coupling of the olefin) with a variety of unsaturated substrates. The addition of Ph2CO leads to a 2-oxaniobacyclopentane ring in 4 while PhC⋮CPh produces a niobacyclopent-3-ene derivative, [Nb(OC6H3Pri-CMeCH2CPhCPh)(OC6H3Pri 2-2,6)2] (5), which was structurally characterized. In the case of addition of PhC⋮CH to 1 the initial, kinetic product 6a, possibly containing a 2-phenylniobacyclopent-2-ene ring, is thermally isomerized to the 3-phenylniobacyclopent-2-ene compound 6b. With the protic reagents HOC6H3Pri 2-2,6 and PhNH2 the compounds [Nb(OC6H3PriCMe2)(OC6H3Pri 2-2,6)3] (8) and [Nb(OC6H3PriCMe2)(OC6H3Pri 2-2,6)2(NHPh)] (9) are produced. The solid-state structure of 8 confirmed the presence of the cyclometalated aryloxide formed via protonation of the methylene group in 1.

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Section: Resultsmentioning

confidence: 99%

Intramolecular Dehydrogenation of Alkyl Groups at Niobium Aryloxide Centers: Bonding and Reactivity of the Ensuing Niobacyclopropane Ring

Felter

Potyen

et al. 1996

Organometallics

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“…According to the literature, the temperature-dependent chemical shifts for a spin singlet/triplet equilibrium system are related to fundamental molecular properties as shown in eq 2. [33][34][35] The constants in eq 2 have their usual meanings, while δ dia is the non-contact-shifted chemical shift, A is the hyperfine electronic coupling constant, and E is the singlet-triplet energy gap. [36][37][38][39][40] Using the Curve Fit program, 41 a nonlinear least-squares fitting (quasi-Newtonian method) of the data to eq 2 allows the determination of the parameters δ dia , A, and E. Two of the resonances were sufficiently sharp to allow accurate measurements and independently yielded an identical energy gap within the experimental error [2.24(11) and 2.3(7) kcal/mol], as expected.…”

Section: Resultsmentioning

confidence: 99%

“…This phenomenon is consistent with a singlet ground state (as suggested by the X-ray structure) and a thermally populated triplet excited state. According to the literature, the temperature-dependent chemical shifts for a spin singlet/triplet equilibrium system are related to fundamental molecular properties as shown in eq 2. − …”

Section: Resultsmentioning

confidence: 99%

Section: Syntheticmentioning

confidence: 99%

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Synthesis and Properties of Cyclopentadienylniobium(III) Complexes. A Magneto−Structural Correlation for 16-Electron Four-Legged Piano Stool Complexes

et al. 1996

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Sodium reduction of (ring)NbCl 4 in the presence of the appropriate phosphine ligand affords the 16-electron Nb(III) complexes (ring)NbCl 2 L 2 (ring ) C 5 H 5 , L 2 ) dppe or ring ) C 5 H 4 Me, L ) PEt 3 ). While the previously reported Cp*NbCl 2 (PMe 3 ) 2 complex behaves as a Curie-Weiss spin triplet paramagnet, compound (C 5 H 4 Me)NbCl 2 (PEt 3 ) 2 has a spin singlet ground state and a thermally populated excited triplet state (2.3 kcal/mol higher in energy) and (C 5 H 5 )NbCl 2 (dppe) is essentially diamagnetic. The X-ray structure of (C 5 H 4 Me)NbCl 2 (PEt 3 ) 2 shows a four-legged piano stool geometry, with Cl-Nb-Cl and P-Nb-P angles correlating with the experimentally determined singlet ground state. Theoretical calculations with geometry optimization at the MP2 level on both 1 A′ and 3 A′′ states for the (C 5 H 5 )NbCl 2 -(PH 3 ) 2 model system confirm the strong dependence of the angular parameters on the spin state.

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“…This is similar to values reported by Rothwell and coworkers for a large series of W(IV) complexes that exhibited spin crossover in solution. 16 For these cyclometallated 2,6-diphenylphenol compounds, W(OC 6 H 3 Ph-C 6 H 4 -) 2 L 2 , the energy difference varied from 358-1205 cm −1 , where L was a variety of different pyridine derivatives. Similarly, Schrock and co-workers have reported singlet-triplet spin crossover for the Mo(IV) species [(Me 3 SiNCH 2 CH 2 ) 3 N]MoNMe 2 .…”

Section: Synthesis and Characterization Of Compoundsmentioning

confidence: 99%

Single-site N–N bond cleavage by Mo(iv): possible mechanisms of hydrazido(1–) to nitrido conversion

2013

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Mo(NMe(2))(4) and the tridentate, dipyrrolyl ligand H(2)dpma(mes) were found to form 5-coordinate Mo(NMe(2))(2)(dpma(mes)) (1), which exhibits spin-crossover behaviour in solution. The complex is a ground state singlet with a barrier of 1150 cm(-1) for production of the triplet in d(8)-toluene. The complex reacts with 1,1-disubstituted hydrazines or O-benzylhydroxylamine to produce nitrido MoN(NMe(2))(dpma(mes)). The mechanism of the 1,1-dimethylhydrazine reaction with 1 was examined along with the mechanism of substitution of NMe(2) with H(2)NNMe(2) in a diamagnetic zirconium analogue. The proposed mechanism involves production of a hydrazido(1-) intermediate, Mo(NMe(2))(NHNMe(2))(dpma(mes)), which undergoes an α,β-proton shift and N-N bond cleavage with metal oxidation to form the nitrido. The rate law for the reaction was found to be -d[1]/dt = k(obs)[1][hydrazine] by initial rate experiments and examination of the full reaction profile. This conversion from hydrazido(1-) to nitrido is somewhat analogous to the proposed mechanism for O-O bond cleavage in some peroxidases.

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Quantitative Determination of the Singlet-Triplet Energy Gap in Pseudo-Octahedral Complexes of W(IV): Measurement of the Relative .pi.-Acidities of N-Heterocyclic Ligands

Cited by 25 publications

References 2 publications

Intramolecular Dehydrogenation of Alkyl Groups at Niobium Aryloxide Centers: Bonding and Reactivity of the Ensuing Niobacyclopropane Ring

Intramolecular Dehydrogenation of Alkyl Groups at Niobium Aryloxide Centers: Bonding and Reactivity of the Ensuing Niobacyclopropane Ring

Synthesis and Properties of Cyclopentadienylniobium(III) Complexes. A Magneto−Structural Correlation for 16-Electron Four-Legged Piano Stool Complexes

Single-site N–N bond cleavage by Mo(iv): possible mechanisms of hydrazido(1–) to nitrido conversion

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