Bis(η5-tert-butylcyclopentadienyl)dichloroniobium(IV)

McCamley, Andrew; Miller, Tod J.; Clegg, William

doi:10.1107/s0108270193006961

Cited by 7 publications

(5 citation statements)

References 1 publication

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“…The H−Ti−B bond angle of only 36° was too small to formulate 2c as a Ti(IV) boryl hydride complex . Further, the Ti−B bond length of 2.267(6) Å was, again, comparable to or longer than those in metallocene boryl complexes of W, Ta, and Nb, despite the smaller size of Ti. ,,− The Ti−H bond distance of 1.61 Å is typical of bridging titanium hydrides . These data again suggest the presence of a partial bond order among titanium, hydrogen, and boron in complex 2c .…”

Section: Resultsmentioning

confidence: 81%

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Titanocene Borane σ-Complexes

1999

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The chemistry of titanocene bisborane complexes Cp2Ti(HBcat‘)2 (1a − g) (HBcat‘ = catecholborane or a substitued catecholborane) and monoborane complexes Cp2Ti(HBcat‘)(L) (2 − 4) (L = PMe3, PhSiH3, or PhCCPh) is reported. These complexes are unusual σ-complexes. The B−H bond in the catecholborane of 1 acts as a two-electron-donor ligand. The 4-tert-butyl version 1a was studied in depth and underwent ligand substitution reactions with PMe3, CO, PhSiH3, and PhCCPh. The products of the reaction of 1a with PMe3 and PhSiH3 are the novel monoborane σ-complexes Cp2Ti(HBcat‘)(PMe3) (2a; HBcat‘ = HBO2C6H3-4-t-Bu) and Cp2Ti(HBcat‘)(PhSiH3) (3; HBcat‘ = HBO2C6H3-4-t-Bu), in which the catecholborane remains a two-electron-donating ligand. Reaction with CO formed Cp2Ti(CO)2. Reaction with PhCCPh formed Cp2Ti(HBcat‘)(PhCCPh) (4; HBcat‘ = HBO2C6H3-4-t-Bu), which was observed in solution and reductively eliminated the vinyl boronate ester (Ph)(Bcat‘)CC(Ph)(H). The rates for the reactions of 1a with these substrates showed a first-order dependence on the concentration of 1a and a zero-order dependence on the concentrations of both the departing HBcat‘ and the incoming ligand. The substitution reaction proceeded at the same rate ((3.8 ± 0.3) × 10-4) regardless of the identity of the incoming ligand. The entropy of activation was +30 ± 5 eu. These data are consistent with a dissociative substitution mechanism for the reaction of 1a with these substrates. The ΔH ⧧ value of 25 ± 3 kcal mol-1 for these reactions provides an upper limit for the strength of the borane−metal interaction. Electronic effects on the reaction rate support a bonding model involving back-donation from titanium to the borane, and the unusual steric effects allow a proposal for the geometric changes that occur upon formation of the transition state.

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Section: Resultsmentioning

confidence: 81%

“…the smaller size of Ti. 41,42,[46][47][48][49][50][51][52] The Ti-H bond distance of 1.61 Å is typical of bridging titanium hydrides. 44 These data again suggest the presence of a partial bond order among titanium, hydrogen, and boron in complex 2c.…”

Section: Synthesis Ofmentioning

confidence: 99%

Titanocene Borane σ-Complexes

1999

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“…However, this greater stability of the [M(C 5 H 4 SiMe 3 ) 3 (L)] complexes is not reflected in their molecular structures since, quite unexpectedly, the M-N distances are systematically longer than in the [M(C 5 H 4 Bu t ) 3 (L)] counterparts. The slight lengthening of the M-N bond in the [M(C 5 H 4 SiMe 3 ) 3 (L)] complexes may be related to the closest proximity of the less sterically congested C 5 H 4 SiMe 3 rings to the metal centre, 38 which renders the approach of the Lewis base less easy. The crystal structures of the [M(C 5 H 4 R) 3 (L)] complexes provide a new and clear illustration of the concept of "longer but stronger bonds".…”

Section: Discussionmentioning

confidence: 99%

Lanthanide(iii)/actinide(iii) differentiation in coordination of azine molecules to tris(cyclopentadienyl) complexes of cerium and uranium

et al. 2004

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Reaction of azine molecules L with the trivalent metallocenes [M(C5H4R)3](M = Ce, U; R = But, SiMe3) in toluene gave the Lewis base adducts [M(C5H4R)3(L)](L = pyridine, 3-picoline, 3,5-lutidine, 3-chloropyridine, pyridazine, pyrimidine, pyrazine, 3,5-dimethylpyrazine and s-triazine), except in the cases of M = U and L = 3-chloropyridine, pyridazine, pyrazine and s-triazine where oxidation of U(III) was found to occur. In the pairs of analogous compounds of Ce(III) and U(III), i.e.[M(C5H4But)3(L)](L = pyridine, picoline) and [M(C5H4SiMe3)3(L)](L = pyridine, lutidine, pyrimidine and dimethylpyrazine), the M-N and average M-C distances are longer for M = Ce than for M = U; however, within a series of azine adducts of the same metallocene, no significant variation is noted in the M-N and average M-C distances. The equilibria between [M(C5H4R)3], L and [M(C5H4R)3(L)] were studied by 1H NMR spectroscopy. The stability constants of the uranium complexes, KUL, are greater than those of the cerium counterparts, KCeL. The values of KML are much greater for R = SiMe3 than for R = But and a linear correlation is found between the logarithms of KML and the hydrogen-bond basicity pKHB scale of the azines. Thermodynamic parameters indicate that the enthalpy-entropy compensation effect holds for these complexation reactions. Competition reactions of [Ce(C5H4R)3] and [U(C5H4R)3] with L show that the selectivity of L in favour of U(III) increases with the [small pi] donor character of the metallocene and is proportional to the pi accepting ability of the azine molecule, measured by its reduction potential.

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“…The distance Nb-Cl (246.37(7) pm) and angle ClNb-Cl ca. (85.38(3)°) are in the expected range [95][96][97][98][99][100]. Table 5 Selected bond lengths (pm) and angles (°) for 8, 9 and 12 Cent (1) (17) 124.18 C(10)-C(11)-C (12) 127.39 C(10A)-C(11A)-C(12A)…”

Section: Structural Studiesmentioning

confidence: 98%

“…4) reveals that the niobium atom has a distorted tetrahedral geometry with both C 5 rings bound to the metal in an g 5 mode. The centroids of the cyclopentadienyl rings form an angle with the niobium atom of 132.2°, typical for niobocene dichloride complexes [95][96][97][98][99][100]. The distance Nb-Cl (246.37(7) pm) and angle ClNb-Cl ca.…”

Section: Structural Studiesmentioning

confidence: 99%