Synthesis , molecular and electronic structure of an incomplete cuboidal Re 3 S 4 cluster with an unusual quadruplet ground state

a Nikolaev Institute of Inorganic Chemistry SB RAS, Ak. Lavrentieva Av., 3, 630090 Novosibirsk, Russian Federation. Fax: +7 (383) 330 9489; Tel: +7 (383) 316 5845; E-mail: konch@niic.nsc.ru b Novosibirsk State University, Pirogova St., 2, 630090 Novosibirsk, Russian Federation c International Tomography Centre SB RAS, Institutskaya St., 3a, 630090 Novosibirsk, Russian Federation. d Departament de Química Física i Analítica, Universitat Jaume I, Av. Sos Baynat s/n, 12071, Castelló, Spain. Fax: (+34) 964728066; Tel.: (+34) 964728086; E-mail: Rosa.Llusar@qfa.uji.es e Instituto de Ciencia Molecular (ICMol), Parque Científico, Universidad de Valencia, 46980 Paterna (Valencia), Spain. f Departamento de Química Física, Universidad de Zaragoza, Spain. g Unidad Asociada BIFI-Instituto de Química Física Rocasolano (CSIC)

Triangular clusters with incomplete cuboidal M 3 (m 3 -Q)(m-Q) 3 cores (Q = S, Se) are basic units in the chemistry of the heaviest group six transition metal cluster chalcogenides. 1 With a single reported exception, 2 M 3 Q 4 (M = Mo, W) clusters are electron precise with six CSE for the formation of three metalmetal bonds that results in stable diamagnetic complexes. 3These M 3 Q 4 compounds are building blocks for the formation of heterobimetallic M 3 Q 4 M 0 species with variable CSE populations ranging from 13 to 17 electrons.The catalytic potential of these cubane-type clusters has been recently reviewed. 4nlike numerous examples of molybdenum and tungsten M 3 Q 4 clusters, their Re analogues remain much less investigated. 5In 1990 the first synthesis of a family of Re(V) trinuclear cluster chalcogenides of formula [Re 3 Q 7 X 6 ]X (Q = S, Se; X = Cl, Br) was reported. 6In these systems with six CSE and three metalmetal bonds, the rhenium atoms define an equilateral triangle with a capping chalcogen and three bridging dichalcogenides.The cluster core in [Re 3 Q 7 X 6 ]X shares structural and electronic features with the M 3 (m 3 -Q)(m-Q 2 ) 3 unit found in the polymeric {M 3 Q 7 X 4/2 X 2 } N (M = Mo, W) phases, widely used as starting materials for the preparation of molybdenum and tungsten M 3 Q 4 cuboidal complexes upon treatment with phosphanes.The extension of this synthetic strategy to rhenium using [Re 3 S 7 X 6 ]X and monophosphanes as precursors was widely explored by Saito et al. in the nineties. 7Re(V) in the starting material is partially or totally reduced to Re(IV) to yield monocapped (with one m 3 -S ligand) [Re 3 S 4 Cl 6 (PEt 3 ) 3 ] À with 8 CSE or bicapped (with two m 3 -S) [Re 3 S 4 Cl 5 (PEt 3 ) 3 ] and [Re 3 S 4 Br 4 (PEt 3 ) 4 ] with 9 and 10 CSE, respectively.Subtle changes in reaction conditions produce cluster species with different molecular and electronic structures.The use of PPh 3 instead of PEt 3 produces a mixture of co-crystallized bicapped clusters with 9 and 10 CSE. 8 Heterometallic Re 3 Q 4 M 0 cubanes (M 0 = Co, Ni and Cu) with terminal phosphanes are also known. 9otivated by the chemistry developed on molybdenum and tungsten [M 3 Q 4 (diphosphane) 3 X 3 ] + cuboidal complexes, 1c, [10][11][12] we decided to investigate the reactivity of [Re 3 S 7 Br 6 ]Br towards diphosphanes.Refluxing of a mixture of [Re 3 S 7 Br 6 ]Br and dppe in acetonitrile leads to [Re 3 S 4 (dppe) 3 Br 3 ]Br ([1]Br) with 9 CSE.zDuring the reaction Re(V) is reduced to Re(IV) by means of dppe.Diphosphane also serves as a sulfur acceptor, converting m-S 2 ligands into m-S.The structure of [1]BrÁ3MeCN (Fig. 1) was established by single crystal X-ray diffraction experimentsy and it shares structural features with its molybdenum and tungsten analogues.The metal atoms in Re 3 S 4 define an almost equilateral triangle (average Re-Re distance 2.780[9] A ˚) with one capping m 3 -S atom and three bridging m-S.The Re 3 S 4 unit can also be regarded as an incomplete cube in which the metal and sulfur atoms occupy adjacent vertex with a missing rhenium atom.The Re-(m-S) distances that are roughly trans to a Re-P bond are noticeably (by 0.05 A ˚) longer than those trans to the Re-Br bond.The two types of Re-P distances also differ, the one trans to m 3 -S ligand being ca.0.03 A ˚shorter (see Table 1).The specific coordination of the diphosphane ligands results in cubane-type sulfido clusters with backbone chirality.
The mixed-halide [Re 3 S 4 (dppe) 3 Br 1.6 Cl 1.4 ]BrÁ4.5CH 2 Cl 2 ([2]BrÁ4.5CH 2 Cl 2 ) cluster is isolated when dichloromethane is used as a reaction solvent, even at room temperature, or upon crystallization of [1]Br from the dichloromethane/ether mixture.Halogen composition of 2 + was determined by X-ray diffraction and confirmed by ES mass spectrometry.A similar situation has This journal is c The Royal Society of Chemistry 2012 also been observed in their homologous molybdenum and tungsten cluster complexes but only at high temperatures. 11This observation suggests that 1 + can activate C-Cl bonds at room temperature.Halogen substitution doesn't have a significant effect on the cluster unit geometry (see Table 1).
In spite of the structural similarities existing between the Re 3 S 4 cluster unit and the molybdenum and tungsten M 3 Q 4 cores, their electronic count differs by three CSE.According to the Cotton-Haas scheme, six CSE enter the low energy 1a 1 and 1e metal cluster orbitals which in the case of Mo and W clusters results in electron precise species with three M-M bonds. 13For rhenium, the three extra electrons would occupy 2a 1 and 2e orbitals which are predicted to be approximately M-M nonbonding.This non-bonding character is in agreement with the fact that Re-Re distances in 1 + and 2 + differ only slightly from Mo-Mo bond lengths in Mo 3 S 4 clusters with 6 CSE (Table 1).However, discrepancies exist regarding the energy ordering and character of these last orbitals which are certainly influenced by the nature of the ligands. 3,14To get a deeper insight into the electronic structure of the Re 3 S 4 core we decided to investigate the magnetic properties of 1 + in combination with DFT calculations.
Magnetic susceptibility measurements of [1]BrÁ3MeCN show an almost constant m eff value (Fig. S1, ESIw) in the 30-300 K temperature range, as expected for a nearly perfect paramagnetic system.Unexpectedly, at 300 K m eff equals 3.87 BM in agreement with the presence of three unpaired electrons per formula unit.This fact makes clusters with Re 3 S 4 cores attractive building blocks for the construction of highspin arrays.Due to its paramagnetic nature [1]Br does not show any signals in the 31 P{ 1 H} NMR-spectrum.
A DFT study at the B3LYP level has been carried out using [Re 3 S 4 (H 2 PCH 2 CH 2 PH 2 ) 3 Br 3 ] + as a molecular model of 1 + .The experimental geometry is well reproduced by gas-phase geometry optimizations (Table S4, ESIw).The cluster cation presents a quadruplet ground state lying 4.61 kcal mol À1 below the doublet state.Fig. 2 shows the orbital energy ordering in the SOMO-LUMO region together with an isocontour plot of the unpaired electron spin density for 1 + .Within the C 3 symmetry of the cluster, the three extra CSE occupy almost degenerate metal-metal non-bonding orbitals of symmetry a and e to afford an unusual high spin configuration for the incomplete cuboidal Re 3 S 4 cluster complex.The molecular orbital overlap population between rhenium atoms for the three SOMO has been calculated (Table S5, ESIw), yielding values of 0.006, À0.024 and À0.011.These values are close to zero proving the non-bonding M-M character of these orbitals.Unpaired electrons are basically located on the metal atoms (65%) and to a lower extent on the bridging sulfides (35%).
The calculated electronic structure for 1 + was confirmed by variable temperature EPR studies.A single line with parallel and perpendicular components at ca. 5200 and 12 900 G (corresponding to g = 4.70 and 1.90, respectively) emerges below 100 K (Fig. 3), suggesting that the three unpaired electrons are delocalized over the three Re centres at high temperatures but they progressively become localized as the temperature decreases below 100 K.   Electron density localization below 100 K is confirmed by the presence of hyperfine coupling of ca.375 G clearly observed in the perpendicular component as a group of six lines.This coupling arises from the coupling of the electron spin with the nuclear spin I = 5/2 of the 185 Re and 187 Re isotopes (with natural abundances of 37.4 and 62.6%, respectively).This result agrees with the MO calculations that locate most of the spin density on the Re d x 2 Ày 2 orbital.The thermal dependence of the EPR signal shows an almost paramagnetic behaviour from ca. 100 to 30 K and a decrease of m eff below ca.30 K in agreement with the behaviour observed in the SQUID magnetic measurements.
Thus it has been shown that the [Re 3 (m 3 -S)(m-S) 3 (dppe) 3 X 3 ] + clusters can be prepared by the reaction of [Re 3 S 7 Br 6 ]Br with dppe.The quadruplet ground state of these compounds makes them promising building blocks for heterospin magnetic arrays.

Fig. 1
Fig. 1 Molecular structure of 1 + cluster cation depicting thermal ellipsoids at the 50% probability level.Hydrogen atoms are omitted for clarity.

Fig. 2
Fig. 2 Schematic MO diagram and isocontour plot of unpaired electron spin density for 1 + calculated at the DFT level (cut-off value 0.02).