Computational Study of General Relationships in Chemistry: C−C, C−N, and B−B Coupling, B−B Catenation, and 2D Metal Borides
Elements in the periodic table are categorized into four major blocks, main group (MG) elements, transition metals (TM), lanthanides (Ln), and actinides (An), which differ dramatically. Recent studies revealed some interesting relationships among them, such as structural similarities of An-metallacycle with TM-metallacycle, low-valent MG-complexes mimicking TM-chemistry and boron mimicking carbon chemistry. The present thesis discusses the possible relationships among these areas of chemistry by studying different coupling reactions computationally, mainly using density functional theory Chapter 2 analyzes acetylide coupling reaction across selected elements from main group (MG) elements, transition metals (TM), lanthanides (Ln), and actinides (An). Here, we discuss the geometric and electronic structure of di-acetylide bridged bimetallic complexes of formula [L]M(μ-CCR)2M[L] and their C-C coupled product [L]M(μ-RC4R)M[L], where M = MG, TM, Ln and An, considering all the available examples in the Cambridge Structural Database (CSD) and show how the central (μ-CCR)2 and (μ-RC4R) units reorganize as M traverses across the periodic table. In this context, transition metal and actinide complexes are similar, while lanthanide and main group complexes show similarity. The ground state electronic configuration and metal oxidation state control these striking differences. Replacement of μ-C4R2 unit by μ-C2N2R2 unit having two extra electrons is an effective strategy to make a cross connection between two sets. At the end, we show the possible extension of the generalization from acetylide C−C coupling to isocyanide, the valence isomer of acetylide, C−C coupling. In chapter 3, we discuss reductive coupling of isocyanide and CO mediated by Cr−Cr quintuple bonded complex and B−B multiple bonded complexes. The ground state electronic configurations of different products are discussed using electron counting scheme. A detailed mechanistic study shows that CO coupling reaction mediated by Cr−Cr quintuple bond follows only singlet potential energy surface whereas the isoelectronic isocyanide coupling reaction involves many intersystem crossings. The difference in donor-acceptor capability of isocyanide and CO ligands controls the variations in reactivities and thus the product distributions. Similarly, reactions of B−B multiple bonded complexes with CO and isocyanides are also controlled by donor-acceptor capabilities of ligands and the C−C coupling takes place by changing the oxidation state of the boron centers from I to II, in contrast to the classical main group mediated reactions where stable oxidation states are preserved. This part of the main-group chemistry, dominated by donor-acceptor bonding interaction, is more likely to follow transition metal behavior. In chapter 4, we utilize donor-acceptor bonding model to design gem-diborene and metallacycloboryne. The donation of two electrons from the metal fragment, ZrCp2, to the in-plane π-bonding orbital of the central B−B unit forms an effective strategy to stabilize B−B triple bond in a metallaborocycle. We also show that strong σ-donating and chelating bis-phosphine ligands (Me2P(CH2)nPMe2), which stabilize donor-acceptor bonding interaction in gem-diborene L2B-BBr2 (10), would be a good choice along the synthetic path toward cyclic boryne. A comparison of the energetics of cyclic boryne system with the linear boryne shows that cyclization does not create any angle strain. In the last chapter, we focus on B−B coupling of borylene (BR), isolobal to CO, to build a larger boron unit on a transition metal template. The first part of the chapter discusses a known reaction, B-B coupling and B−B catenation reactions starting from a metal-bis(borylene) complex Fe(CO)3[B(Dur)B(N(SiMe3)2)] [Dur = 2,3,5,6-tetramethylphenyl]. Our study highlights that the borylene fragment has an inherent tendency to undergo coupling. This coupling phenomenon is utilized to predict M2B10H10 and M2@B10H8 complexes (where M = Mn and Fe) in the second part of the chapter. Electronic structure analysis of Mn2B10H10 shows that metal d-orbitals provide stability to the unique interlocked boron wheel structure with Möbius aromaticity. Two additional electrons in Fe2@B10H10 stabilize the twisted boron analog of annulene. Removal of 2H from Mn2B10H10 and Fe2@B10H10 leads to planar structures, Mn2@B10H8, and Fe2@B10H8, respectively. The presence of 10π electrons in M2@B10H8 maps it with naphthalene having Hückel π-aromaticity. Condensation of naphthalene to graphene in two dimensions gives a hint to build different metal boride monolayers, FeB5 and Fe2B5, considering Fe2B10 as the building block, thus bridging molecular boron chemistry with solid-state. One of the predicted monolayers, β-Fe2B5 is the global minimum in the planar arrangement from USPEX crystal structure search algorithm. Our electronic structure analysis further shows that the stabilization mechanism in the molecular building block remains unaltered in the solid-state.