### Preparation of Composites Based on [M(Salen)] and Carbon Nanomaterials
#### 2.2.2 Features of the Formation of [M(Salen)]/Carbon Nanotubes and [M(Salen)]/Graphene Composites  
Currently, preparing composites based on [M(Salen)] complexes and CNTs or graphene remains a challenging and unsolved problem. This is primarily attributed to the ultra-smooth surfaces of both CNTs and graphene, which exhibit few broken bonds or structural defects, with the exception of the edges of graphene or the tips of nanotubes [97; 98]. Without pretreatment (functionalization), CNT or graphene surfaces tend to bind easily due to strong van der Waals interactions between individual nanostructures, resulting in the formation of poorly dispersed bundles or agglomerates. As a result, poor interfacial interaction between the components of the composite is observed, which seriously impairs its characteristics. To solve this problem, there are methods based on noncovalent (due to electrostatic interactions or physical adsorption) and covalent functionalization (with the formation of stable chemical bonds) of the CNT or graphene surface. Noncovalent functionalization techniques can improve the dispersion quality of carbon nanomaterials  
Figure 4: Possible types of graphene and representation of the process of obtaining reduced graphene oxide by oxidative exfoliation of graphite. (**a**) Types of graphene and the main oxygen-containing functional groups on its edges; (**b**) Scheme for obtaining reduced GO from graphite by chemical treatment. in solution. However, they do not address the challenges of interphase interactions at composite interfaces. Covalent functionalization is a more sought-after way of forming composites and is usually obtained by direct reaction of the metal complex with the surface groups of the carbon material or by a spacer (a chain of atoms linking a functional group on the surface of the carbon material and the metal complex) pregrafted onto the carbon material or onto the [M(Salen)] complex (Figure 5) [57]. For example, the composites [Mn(Salen)]/MWCNTs [100] and [Co(Salen)]/SWCNTs [101] were formed by mixing and reacting both components under ultrasonic conditions without pretreatment of MWCNTs. These composites were then used as a coating for a glass-carbon electrode. In [102; 103], a [Ni(Salen)]/MWCNTs composite was prepared by first treating MWCNTs with HNO\({}_{3}\) followed by SOCl\({}_{2}\) and then mixing the complex with functionalized MWCNTs in CHCl\({}_{3}\) (Figure 6). This material has been used to oxidize primary and secondary alcohols and phenol. In other studies [104; 105], [M(Salen)]/MWCNTs (M = Cu, Ni, Co) composites were synthesized via amino-functionalized with 3-aminopropylritehoxysilane (APTES) or ammonium benzoate. A comparable technique was employed for synthesizing amino-functionalized graphene oxide and preparing [M(Salen)]/GO (M = Mn, Cu, Co, Fe and VO) composites (Figure 6) [106; 107; 108; 109; 110; 111; 112]. The preparing composites underwent testing in conditions of catalytic epoxidation of styrene and unfunctionalized olefins as well as lignin oxidation [106; 107; 108]. The literature provides several key methods for forming composites of MWCNTs (SWCNTs), graphene, and electrically conductive polymers based on [Ni(Salen)] complexes by electrochemical deposition. One approach is to prepare a conductive electrode with a carbon layer that serves as a substrate for the formation of a polymer, such as poly-[Ni(Salen)] [5; 7; 42; 113; 114], poly-[Ni(CH\({}_{3}\)-Salen)] [8; 114], poly-[Ni(Salphen)] [8; 42], poly-[Ni(CH\({}_{3}\)-Salphen)] [8], or poly-[Ni(Saldmp)] [42], by using a solution consisting of an electrolyte and a monomer and applying potentiodynamic, conventional, or pulsed  
Figure 5: Representation of various groups covalently grafted to the surface of CNTs and graphene by amino-functionalized, chlorination, through spacer attachment, or through direct reaction of the ligand with the surface groups of the carbon support. potentiostatic methods.