DFT Studies for Selected Catalytically Related Systems and Biological Assemblies Based on First-Row Transition Metals and Main Group Elements

Abstract

Mechanisms for selected catalytically relevant systems involving first-row transition metals and main-group elements, as well as metal-based amino acid assemblies, were studied through density functional theory (DFT) calculations using Gaussian 09 and 16. These works serve to provide insight towards the observed structure, reactivity patterns and catalytic activities of the selected systems, which would help elucidate the role of first-row transition metals or main-group species in the context of catalytically or biologically relevant systems. This thesis focused particularly on four main systems, including (1) gas-phase hydrated monocationic Mn clusters, (2) Cu-based amino acid assemblies, (3) solution-phase disilane-mediated catalytic CO₂ hydroboration, and (4) solution-phase diborene-mediated catalytic CO₂ hydroboration.

Hydrated monocationic Mn clusters were first discussed in Chapter Three and Four. In Chapter Three, the identity of the MS-observed hydrated clusters [Mn, n H₂O]⁺ were comprehensively explored for clusters sizes from n = 1 to n = 12 at unrestricted BHandHLYP/6-311++G(d,p) level of theory. Energetics and structure of the two main plausible forms of the cluster, including the non-inserted Mn(I) hydrated clusters [Mn^I(H₂O)_n]⁺ and the inserted Mn(III) hydride-hydroxide form [HMn^IIIOH(H₂O)_n-1]⁺, were explored in both quintet and septet states. The septet Mn(I) form was shown to be thermodynamically favourable at smaller clusters sizes with n ≤ 8, while the quintet Mn(III) decrease in energy as cluster size increase and overtakes as global minima at cluster sizes n > 8, which suggests the Mn(III) form to be the dominant form of the clusters in previous MS experiments. Further studies in the reactivity with N₂O for both Mn(I) and Mn(III) forms of clusters revealed that Mn(I) clusters were capable of undergoing both binding and largely exothermic reduction of the Mn-bound N₂O, while Mn(III) could only bind to N₂O relatively weakly. The predominance of the N₂O-inert Mn(III) form was consistent with the MS-observed inertness of the hydrated monocationic Mn clusters. In Chapter Four, the reactivity pattern of the Mn(III) hydride [HMn^IIIOH(H₂O)_n-1]⁺ was further explored using ethene and propene as reaction partners, which demonstrates the hydrogen atom transfer (HAT) ability of the cluster. Using [HMn^IIIOH(H₂O)₃]⁺ (i.e. n = 4) as a model reactant, it has been shown that HAT towards C=C bonds could take place easily in an essentially barrierless reaction. Further addition of the second alkene moiety was also possible with a barrier of 48 kJ mol^-1. These results were also consistent with MS experiments, which revealed the uptake of up to two propene molecules by the [HMn^IIIOH(H₂O)₃]⁺ cluster.

The assembly and structure of biologically relevant Cu-based amino acid clusters were presented in Chapter Five. The structure of charged Cu-based isoleucine complexes were first determined systematically using a bottom-up approach using B3LYP-based DFT calculations from monomers up to dodecamers. Cu(II)-based neutral dimers, [Cu^II(Ile-H)₂], particularly the η²-N,O bidentate coordinated cis and trans dimers complexes, were found to be the fundamental building blocks of the studied assemblies. Generally, neutral dimers were found to reside around cationic charge carriers including H⁺, Cu⁺, Cu²⁺, and the Cu(II)-based monocationic monomer [Cu^II(Ile-H)]⁺. In smaller complexes, the thermodynamically more favorable trans dimers were commonly found among the global minimum structures. However, as the cluster size increases, the cis dimer blocks emerged as more favourable building blocks due to their stronger binding towards cationic species and hydrogen bonding ability. The collision cross section were evaluated using the exact hard sphere scattering (EHSS) model and compared against the experimental data from ion mobility experiments. The CCS data from experimental and theoretical studies showed qualitative agreement with each other, which is consistent with a near-isotropic growth of the Cu-isoleucine assemblies as the number of isoleucine residues increase. The size-dependent CCS difference between Cu-isoleucine, Cu-alloisoleucine and Cu-leucine assemblies has also been reproduced. The source of CCS difference arose from side chain orientation differences, which stemmed from either inherent connectivity difference in the case of structurally isomeric in isoleucine-leucine pair, or their relative spatial arrangements as in the isoleucine-alloisoleucine pair.

Finally, the discussion of solution-phase main-group based catalyst was included in Chapters Six and Seven, in which the disilane- and diborene-mediated CO₂ hydroboration with pinacolborane (HBpin) were explored respectively. In Chapter Six, the catalytic pathways for CO₂ hydroboration involving the intact disilane and its dissociated borylsilylene product were explored. The intact disilane, which was sterically bulky, was found to be relatively ineffective in catalyzing the HBpin-based hydroboration of CO₂. However, the dissociation of the Si–Si bond could lead to a Si=B bond-containing borylsilylene species, which was capable of performing the catalysis. In Chapter Seven, the catalytic role of the B=B bond in a diborene-mediated CO₂ hydroboration was explored. An alternative pathway, which involved the stable CO₂-diborene adduct as the active catalytic species, was found. In particular, the CO₂-diborene adduct serve as a Lewis base catalyst to activate the reducing agent, HBpin, and facilitates the hydride transfer process from HBpin to CO₂ and its reduction products, converting the starting CO₂ to its final methoxy reduction product, MeOBpin. Both disilane- and diborene-mediated suggests that highly reactive main group species could potentially function as precatalysts instead of the actual catalytic species and therefore would require detailed studies in order to confirm the catalytic mechanism, which would be crucial to future optimizations in catalytic design.

Date of Award	28 Jul 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chi Kit Andy SIU (Supervisor)