UC San Diego
Organometallic Photobase Generators and Computational Investigations of the Isomeric Preference of Unsaturated Metal Carbonyl and Metal Isocyanide Complexes /
- Author(s): George, Stephen Philip
- et al.
In order to develop a new class of transition-metal-based, two-stage photobase generators (PBGs), the synthesis, structural characterization, spectroscopic properties and base-generation ability of a series of [CpRu([Eta]6- benzylcarbamate)]X salts (X = [PF₆]⁻, [O₃SCF₃]⁻) are reported. Treatment of [CpRu(NCMe)₃]PF₆ with the N,N- dialkylcarbamate PBGs 2-nitrophenylmethyl N,N- dicyclohexylcarbamate (2NC), 4-nitrophenylmethyl N,N- dicyclohexylcarbamate (4NC) and 3,5-dimethoxyphenyl)methyl N,N-dicyclohexylcarbamate (DMC) results in [Eta]6- complexation of the arene fragment and formation of the corresponding [CpRu([Eta]6-benzylcarbamate)]PF₆ salt. The [PF₆]- salt, [CpRu([Eta]6-DMC)]PF₆, is converted to the triflate salt (triflate = [O₃SCF₃]⁻ = [OTf]⁻) by treatment with Me₃SiOTf. The molecular structures of the [CpRu([Eta]6-benzylcarbamate)]X salts were determined by X -ray diffraction and varied by the orientation of the carbmamate unit relative to the [Eta]6-Cp ligand. Experimental UV-vis data and Time-Dependent DFT calculations suggest that the [CpRu([Eta]6- benzylcarbamate)]⁺ cations possess high-energy excitations that potentially render these salts as useful two-stage PBGs. The photobase generation ability of the [CpRu([Eta]6 -benzylcarbamate)]X salts at 254 nm irradiation is assessed in both acetonitrile solution and polymethacrylate thin films. The irradiation studies demonstrate that [Eta]6-coordination of a benzylcarbamate PBG does not deactivate photolytic base generation. Also, relying on the isoloble relationship between isocyanides (CNR) and CO, our group has used the m-terphenyl isocyanide CNArMes² (Mes = C₆H₃-2,4,6-Me₃) to isolate a mononuclear [Co(CNArMes²)₄]⁺ species analogous to [Co(CO)₄]⁺. Characterization of this diamagnetic compound shows a square planar geometry about the Co center, disagreeing with previous work pertaining to the cobalt carbonyl cationic species. In order to more fully understand the nature of our complex compound with respect to the carbonyl species a thorough computational analysis has been conducted. The current work shows that the calculated ground state geometries of both the cobalt carbonyl and model cobalt isocyanide (CNCH₃, CNC₆H--2,6- Me₂) cations are dependent on the functional chosen. The commonly used B3LYP functional calculates the C2v triplet state to be the ground state for all three species. However, when other functionals (BLYP, O3LYP, OLYP and G96LYP) are used, the D4th singlet ground state is more often found to be the ground state. The nature of the B3LYP functional has been explored by adjusting the amount of Hartree Fock exchange mixed into the functional to gauge the impact on the calculated stretching frequencies of the CO and CN fragments. In addition, a series of DFT calculations were performed on the complex Co(N=CtBu₂)₄ in order to understand the solid state squashed tetrahedral geometry about the cobalt center. These calculations corroborate that the adopted geometry maximizes ketimide- to-cobalt [Pi]-donation which accounts for significantly more of the orbital contribution to the total energy than does [Pi]-backbonding. Herein is also reported the computational support for the coordinated alkane, n-hexane forming the cobalt complex ([mu]²-([Eta]²-H,C- (CH₃(CH₂)₄CH₃)[Co(SiMe₃)(CNArMes²)₃]2·(n-C₆H₁₄·2(C₆H₆). Both DFT and AIM analysis support the [Eta]²-H,C [signma]- type interaction seen in the solid state complex. AIM calculations show a bond critical point with electron density at the low-end of the range of accepted values for typical agostic intereactions. AIM analysis also suggests that the weak van der Waals forces play a significant role in stabilizing coordinat