Brunker Tim J, Green Jennifer C, O'Hare Dermot
Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, UK.
Inorg Chem. 2003 Jul 14;42(14):4366-81. doi: 10.1021/ic030073u.
The electronic and magnetic properties of a series of mixed-sandwich complexes MCp(R)Tp (Cp(R) = Cp or Cp; Tp = hydrotris(pyrazolyl)borate; M = V, Cr, Fe, Co or Ni) have been studied and compared to their homoleptic analogues, MCp(R)(2) and MTp(2). Solid-state magnetic susceptibility measurements and EPR spectroscopic data indicate that complexes with d(3), d(6), and d(8) configurations are similar electronically to their metallocene analogues, except for FeCpTp, which displays a spin equilibrium (S = 0 if S = 2) in solution which was investigated by variable- temperature NMR spectroscopy. The d(2) complex VCpTp displays magnetic behavior consistent with an orbitally nondegenerate ground state. The d(4) species CrCpTp has a high-spin (S = 2) ground state. The d(7) species CoCpTp is high spin (S = 3/2) whereas its Cp analogue and NiCpTp are both low-spin (S = 1/2) species. The optical spectra of the d(3), d(6), and d(8) complexes were assigned in a fashion similar to the analogous metallocenes and ligand-field parameters (delta(1) = delta-sigma gap, delta(2) = sigma-pi gap for d-orbitals in axial symmetry) calculated. The analysis shows that for 15-electron species the total ligand-field splitting, delta(TOT), is larger than for their metallocene analogues, whereas for the 18-electron case Delta(TOT) is smaller and for 20-electron systems delta(TOT) is approximately the same. In all cases delta(2) is substantially reduced compared to the metallocenes, and in the majority of cases delta(1) is markedly larger. DFT calculations were performed to investigate further the nature of the ligand environment on the frontier orbitals in these complexes. Orbital energies and compositions were calculated and compared for a series of homoleptic and mixed-sandwich complexes of Ni(II) and across the 1st transition series for MCp(R)Tp species. The ability of Tp (vs Cp) to act as a delta-donor (with respect to the principal molecular axis) imparts significant ligand antibonding character to the delta-orbitals and results in decreased epsilon(pi)-epsilon(delta) values compared to the metallocenes and an increased tendency toward high-spin complexes in the mixed-sandwich complexes. Structure calculations were performed for CrCpTp, VCpTp, and CoCpTp which show substantial distortions from ideal axial symmetry in their crystal structures. The origins of these distortions were confirmed to arise from unequal occupancy of near-degenerate delta- and pi-levels.
研究了一系列混合夹心配合物MCp(R)Tp（Cp(R) = Cp或Cp*；Tp = 氢三(吡唑基)硼酸酯；M = V、Cr、Fe、Co或Ni）的电子和磁性性质，并将其与其同配体类似物MCp(R)₂和MTp₂进行了比较。固态磁化率测量和电子顺磁共振光谱数据表明，具有d³、d⁶和d⁸构型的配合物在电子性质上与其茂金属类似物相似，但FeCpTp除外，它在溶液中表现出自旋平衡（如果S = 2，则S = 0），通过变温核磁共振光谱对其进行了研究。d²配合物[VCpTp]⁺表现出与轨道非简并基态一致的磁性行为。d⁴物种CrCpTp具有高自旋（S = 2）基态。d⁷物种CoCpTp是高自旋（S = 3/2），而其Cp类似物和[NiCpTp]⁺都是低自旋（S = 1/2）物种。以类似于类似茂金属的方式对d³、d⁶和d⁸配合物的光谱进行了归属，并计算了配体场参数（δ₁ = δ - σ能隙，δ₂ = 轴向对称中d轨道的σ - π能隙）。分析表明，对于15电子物种，总配体场分裂δ(TOT)大于其茂金属类似物，而对于18电子情况，Δ(TOT)较小，对于20电子体系，δ(TOT)大致相同。在所有情况下，与茂金属相比，δ₂都大幅减小，并且在大多数情况下，δ₁明显更大。进行了密度泛函理论计算，以进一步研究这些配合物中前沿轨道上配体环境的性质。计算并比较了一系列Ni(II)的同配体和混合夹心配合物以及MCp(R)Tp物种在第一过渡系中的轨道能量和组成。Tp（相对于Cp）作为δ供体（相对于主分子轴）的能力赋予δ轨道显著的配体反键特性，导致与茂金属相比，ε(π) - ε(δ)值降低，并且混合夹心配合物中高自旋配合物的倾向增加。对CrCpTp、[VCpTp]⁺和CoCpTp进行了结构计算，结果表明它们在晶体结构中与理想轴向对称存在显著畸变。这些畸变的起源被证实是由于近简并的δ和π能级的占据不均。
