Absolute Metal−Ligand σ Bond Enthalpies in Group 4 Metallocenes. A Thermochemical, Structural, Photoelectron Spectroscopic, and ab Initio Quantum Chemical Investigation

Abstract

Absolute metal-ligand σ bond enthalpies have been determined for a series of titanocene, zirconocene, and hafnocene halides and dimethyls by iodinolytic titration calorimetry. Absolute metal-iodine bond disruption enthalpies were measured by iodination of the monomeric trivalent group 4 metallocenes Cp tt 2 TiI, (Me 5 C 5 ) 2 -TiI, Cp tt 2 ZrI, and Cp tt 2 HfI (Cp tt ) η 5 -1,3-di-tert-butylcyclopentadienyl). Iodinolysis of Cp tt 2 ZrMe 2 and Cp tt 2 HfMe 2 in turn yields absolute Zr-Me and Hf-Me bond enthalpies. Derived values (kcal/mol) are D[Cp tt 2 Ti(I)-I] ) 40.6(5); D[(Me 5 C 5 ) 2 Ti(I)-I] ) 52.3(6); D[Cp tt 2 Zr(I)-I] ) 58.0(5); D[Cp tt 2 Hf(I)-I] ) 61.2(4); D h [Cp tt 2 Zr-Me 2 ] ) 43(1); and D h [Cp tt 2 Hf-Me 2 ] ) 47.6(9). That D[Cp tt 2 Zr(I)-(I)] ≈ D(I 3 Zr-I) and D[(Me 5 C 5 ) 2 -Ti(I)-I] ≈ D(I 3 Ti-I), while D[Cp tt 2 Ti(I)-I] ≈ D(I 3 Ti-I) -12 kcal/mol, argues for more reliable transferability of D(M IV -I) in sterically less congested metallocenes. The molecular structures of Cp 2 tt ZrI 2 , Cp 2 tt ZrI, andCp 2 ttHfI were determined by X-ray diffraction. In Cp tt 2 ZrI 2 , the Zr ligation is pseudotetrahedral, and the ring tert-butyl groups "straddle" the Zr-I bonds to minimize steric interactions. The geometry about Zr in Cp tt 2 ZrI is pseudotrigonal, with contracted Zr-ring centroid and Zr-I distances versus Cp tt 2 ZrI 2 , primarily reflecting substantially diminished ligand-ligand repulsive nonbonded interactions in the latter. Cp tt 2 HfI is isomorphous with Cp tt 2 ZrI, and the slightly different metrical parameters are in accord with Hf vs Zr ionic radii. The significant differences in interligand repulsive interactions in the trivalent versus tetravalent complexes are confirmed by van der Waals calculations. High-resolution UV PE spectra combined with ab initio relativistic effective core potential calculations provide details of electronic structure. Absolute ionization energy values indicate that iodine behaves as both a strong σ and π donor. Trends in the large Cp 2 MX n structural database can be understood in terms of the interplay between electronic and molecular structure factors, which are highly sensitive to the substitution patterns of the cyclopentadienyl ligands and, in particular, to competing σ vs π M-X bonding.

Synthesis of Titanium and Zirconium Complexes with 2‐­Pyridonate and 2, 6‐Pyridinedithiolate Ligands

“…Within the equatorial plane, the T(1)–Zr–T(2) angle of 132.84(4)° deviates from the theoretical value (120°) most likely due to the steric needs generated by the presence of the bulky tert ‐butyl groups. The bond lengths Zr–T(1) and Zr–T(2) are 2.2450(11) and 2.2391(10) Å respectively, which are comparable to those observed in other zirconium(IV) complexes such as [Cp tt 2 Zr(SH)(OSO 2 CF 3 )] (2.237(4) and 2.239(5) Å)11b or [Cp tt 2 ZrI 2 ] (2.248 and 2.251 Å) 21. The Zr–N bond length, 2.1642(19) Å, is similar to that found in zirconium complexes such as [( t Bu‐NH‐ o ‐C 6 H 4 ) 2 Zr(NMe 2 )Cl] (2.102(3) and 2.098(3) Å),22 [HC{SiMe 2 N(2‐FC 6 H 4 )} 3 Zr(S 2 C)Fe(CO) 2 Cp] (2.060(8)–2.124(8) Å),23a or [Cp 2 Zr(dipicolinato)] (2.199(2) Å] 23b.…”

Selectivity of C–H Activation and Competition between C–H and C–F Bond Activation at Fluorocarbons

“…Determination of absolute values of metal-ligand bond enthalpies is very difficult and some simplifications are often necessary. [84][85][86] There are three important aspects to the thermochemistry of hydrofluorocarbons that are needed for a general understanding of the thermodynamics of hydrofluorocarbon activation. One is the nature and magnitude of the interaction of the whole hydrofluorocarbon with the transition metal complex (Section 3.1), the second concerns the energetics of C-H oxidative addition, including the interaction of hydrofluorocarbyl fragment with the metal fragment after bond activation (Section 3.2) and the third concerns the corresponding energetics of C-F oxidative addition (Section 3.3).…”

“…As mentioned above, determination of the absolute energies of the bonds involving the metal is exceptionally difficult. 84,86 In addition, the chemist is mostly interested in the energetics of the overall transformation.…”

Trends in Covalency for d- and f-Element Metallocene Dichlorides Identified Using Chlorine K-Edge X-ray Absorption Spectroscopy and Time-Dependent Density Functional Theory

“…In addition, the % CI character in the 3a l orbital in 5 is also similar to what was observed in the analogous molecular orbitals of la l symmetry of the d-block metallocenes, but slightly less than the 9 % CI 3p character associated with (CsMes)2HfCI2' Overall, these covalency trends are similar to tlle trends revealed by previously reported measurements that used gas-phase photoelectron spectroscopy to probe the occupied orbitals of metallocene complexes. [56][57][58] …”