Ultrashort Mn-Mn Bonds in Organometallic Complexes T. Alonso-Lanza (*),1 J. W. Gonz´alez,1 F. Aguilera-Granja,1;2 A. Ayuela1 1Centro de F´ısica de Materiales CFM-MPC CSIC-UPV/EHU, Donostia International Physics Center (DIPC), Departamento de F´ısica de Materiales, Fac. de Qu´ımicas, UPV-EHU, 20018 San Sebasti´an, Spain 2Instituto de F´ısica, Universidad Aut´onomade San Luis de Potos´ı, 78000 San Luis Potos´ı S.L.P., M´exico (Dated: August 2, 2017) Manganese metallocenes larger than the experimentally produced sandwiched MnBz2 compound are studied using several density functional theory methods. First, we show that the lowest energy structures have Mn clusters surrounded by benzene molecules, in so-called rice-ball structures. We then find a strikingly short bond length of 1.8 A˚ between pairs of Mn atoms, accompanied by magnetism depletion. The ultrashort bond lengths are related to Bz molecules caging a pair of Mn atoms, leading to a Mn-Mn triple bond. This effect is also found when replacing benzenes by other molecules such as borazine or cyclopentadiene. The stability of the Mn-Mn bond for Mn2Bz2 is further investigated using dissociation energy curves. For each spin configuration, the energy versus distance plot shows different spin minima with barriers, which must be overcome to synthesize larger Mn-Bz complexes. Introduction bonds are dimanganese decacarbonyl[26{28] Mn2(CO)10 and methylcyclopentadienyl manganese tricarbonyl (CH3C5H4)Mn(CO)3, which has been used to increase A number of organometallic compounds have been syn- the octane level of gasoline[29]. Experiments on TM-Bz thesized and used in a wide range of applications[1{ compounds have been performed, leading to their 5]. The synthesis of ferrocene[6{9] stimulated a synthesis in the gas phase and their characterization search for related metallocenes, such as cobaltocene[10], using spectroscopic methods [18, 30{32] and in magnetic nickelocene[11], rhodocene[12] and manganocene[13], in experiments[33, 34]. TM-Bz compounds are present in which a transition metal atom is located between two cy- many types of molecular clusters. Small Mn-Bz com- clopentadiene molecules. Similar compounds have been pounds, such as MnBz2, have been synthesized[18, 35]. proposed combining transition-metal atoms with benzene As for Mn adatoms in graphene, Mn sandwiched between molecules (TM-Bz compounds)[14]. The basic unit of Bz molecules prefers to be in a hollow position[36, 37]. such compound is a transition metal atom over a sin- Theoretical studies are required to understand larger gle Bz ring, known as a half-sandwich[15{17]. Such ba- complexes of inner Mn clusters surrounded by Bz sic units can be combined to form larger compounds molecules as to be produced experimentally. with either sandwich-like structures for early transition metals (Sc, Ti, and V) or rice-ball structures for late transition metals (Fe, Co, and Ni)[18]. TM-Bz com- pounds look promising for future applications. For in- stance, sandwich-like molecules have been proposed as conductors involving spin transport[19], either isolated bridging the tips of a Cu nanocontacts[19, 20] or pile up building infinite wires having Sc, Ti, V, Cr, and Mn[21]. Sandwiched molecules and infinite wires containing V, Nb, and Ta have been found to provide strong magnetic anisotropy [22]. We are interested in characterizing larger arXiv:1703.04903v3 [cond-mat.mtrl-sci] 1 Aug 2017 clusters with rice-ball structures, specifically in the study of Mn-Bz compounds, to gain knowledge on their still un- familiar structures and properties. The field of organometallics was recently boosted following a breakthrough in the synthesis of binu- clear metallocenes, so-called dimetallocenes, such as 5 decamethyldizincocene[23, 24] Zn2(η C5Me5)2) and FIG. 1: Scheme showing the ultrashort bond effect in Mn 5 (Zn2(η C5Me4Et)2). Compounds containing Mn with surrounded by benzene molecules. Note the large reduction many nuclei have plenty of uses in organometallics[25]. in Mn-Mn distance and the total quenching of the magnetic Some previously described molecules containing Mn-C moment. 2 Dimetallocenes and multimetallocenes have been isomer with 7 µB lies 0.5 eV higher in energy. All the largely investigated using density functional theory isomers are lying higher in energies than the structures (DFT) calculations. Different external molecules presented in Fig. 2. have been considered, such as Bz, cyclopentadiene, All the ground state molecules have strikingly short and even fullerenes[38]. Similarly, many different bond lengths between the Mn couples. The Mn2Bz2 central metal atoms have been studied, leading to molecule is the simplest system to show this effect with alkaline-earth dimetallocenes[39{42], transition metal a Mn-Mn distance of 1.8 A.˚ The ultrashort bond lengths dimetallocenes[43{45], heterodinuclear compounds[46, are unexpected because larger distances between Mn 47] and zinc isoelectronic elements, such as Cd and Hg atoms have been reported, such as 3.4 A˚ for the Mn [48]. Although multinuclear metallocenes of Co, Cu, and dimer[55{57], 2.93 A˚ for dimanganese decacarbonyl[26, Ni, and other transition metals have been described [43{ 28], 2.4{2.6 A˚ for manganese carbides[58, 59] and the 45, 49{54], the study of larger manganese benzene com- broad distance range of 2.25{2.95 A˚ for several Mn bulk pounds is still to be done. allotropes [57]. On the other hand, analogous short mul- We herein investigate organometallic compounds in tiple bonds have been shown theoretically and experi- which Bz molecules cage clusters with a few Mn atoms, mentally for other transition metals, such as in ultrashort MnnBzm. The benzene molecules strongly affect the Cr dimers (1.73 A)˚ within organic molecules [60]. bonding between manganese atoms, differently to other We focus on the locally unpolarized Mn2Bz2 molecule transition metals. Manganese benzene compounds are with a Mn-Mn distance of around 1.8 A.˚ The loss of mag- also interesting because they are in the middle between netism is a hallmark of the ultrashort bonds between Mn two trends, sandwich-like structures for early transition atoms capped with Bz molecules. The magnetic moments metals and rice-ball structures for late transition metals of irregular systems, such as Mn3Bz3 and Mn4Bz3, are al- [18]. We find that the clusters contain Mn-Mn multiple most depleted. In the unit consisting of a Mn atom over a (triple) bonds with ultrashort interatomic distances and benzene molecule, the magnetic moment decreases from depleted magnetism. We then focus on the electronic 5 µB to 3 µB [16]. However, the total elimination of the structure of the smaller Mn2Bz2 molecule to further im- magnetism for the Mn2Bz2 molecule is still surprising. prove our understanding of such molecules. The two Bz When combining two Mn2Bz2 units to form a Mn4Bz4 molecules covering the Mn2 molecule appear to stabilize molecule, the larger compound also has ultrashort bonds, a non-magnetic local minimum in the inner Mn dimer. and the total and local magnetic moments remain zero. We verify that these results also appear when replac- The Bz molecules around the Mn atoms form a dis- ing the caging benzenes with other molecules. Further- torted tetrahedron, tilted because of neighboring Bz more, ultrashort bonds are also found in larger Mn-Bz rings. There are two short bond lengths of 1.94 A˚ compounds, indicating that the effect is robust and that and four larger bond lengths of 2.56 A.˚ The short bond strong dimerization is induced in larger molecules. Be- lengths are between the opposite atom pairs, labeled 1-2 cause most of experiments work with charged clusters, we and 3-4 in Fig. 2. have checked that positively charged Mn-Bz clusters also The two cases Mn2Bz2 and Mn4Bz4 have ultrashort present ultrashort Mn-Mn bonds. Lastly we study the distances and total magnetism depletion. The ultrashort synthesis of the Mn2Bz2 molecule, as a first step towards Mn-Mn bonds also occurs for other molecules, such as larger Mn-Bz compounds, and we show that different Mn3Bz3 and Mn4Bz3. The Mn3Bz3 ground state geom- high local spin magnetic moment states create barriers etry depletes most of the magnetism, and gives an ultra- to such syntheses. short distance of 1.88 A˚ between Mn(1) and Mn(3). The third Mn atom, Mn(2), moves away and remains bonded mainly to Mn(1), with a bond length of 2.22 A.˚ The side I. RESULTS AND DISCUSSION position of Mn(2) bends the Bz molecule associated with Mn(1). The total spin magnetic moment is 1 µB, local- A. Metallization induced superdimerization ized mainly on Mn(2) with 1.53 µB. The Mn(1) atom has antiferromagnetic coupling with a magnetic moment The ground state geometries of the Mn2Bz2, Mn3Bz3, of -0.57 µB, and the Mn(3) local moment decreases to a Mn4Bz3, and Mn4Bz4 molecules are shown in Fig. 2. very small value of 0.28 µB. [86] The added Mn-Bz unit We also consider the roles of isomers for each cluster, distorts the Mn2Bz2 geometry, giving nonzero local mag- and study other geometries and magnetic solutions. We netic moments on the Mn atoms. The peculiar distorted first determine whether a sandwich or rice-ball structure structure is due to the large energy gain because of the ul- is preferred. The rice-ball geometry has lower energy for trashort Mn-Mn bond. The ultrashort bond phenomenon each of the four Mn-Bz clusters studied here. The results is therefore found partially for Mn3Bz3. The dimerized for the sandwich structures are shown in the Supplemen- Mn4Bz4 and Mn3Bz3 structures indicate that the ultra- tal Material.