Rhenium is different: CO tetramerization induced by a divalent lanthanide complex in rhenium carbonyls

The reduction of M2(CO)10 (M = Mn, Re) with different divalent lanthanide (Ln = Sm, Yb) compounds was investigated. Depending on the steric demand of the ligand, either unusual CO tetramerization or formation of a new Re carbonyl anion occurred in the case of Re. Theoretical calculations were performed for a better understanding of the nature of bonding in the newly formed species.

The coupling of carbon monoxide in the presence of hydrogen to form short hydrocarbon chains is performed on an industrial scale in the Fischer-Tropsch process.Thus, carbon monoxide is a key C 1 feedstock for the industrial production of hydrocarbons.The primary products of the Fischer-Tropsch synthesis are alkanes, alkenes, and to a minor extent alcohols, which are all valuable starting materials in the chemical industry. 1The product selectivity in the Fischer-Tropsch process can be varied over a wide range.Key parameters are the catalyst formulation and the reaction conditions.The chain growth probability is influenced by the catalyst that is selected for the hydrogenation process. 2 Industrially, mostly Fe or Co catalysts have been employed in this reaction. 3For a better understanding of the Fischer-Tropsch process, numerous metal complexes, which promote the coupling of CO have been published. 4For example, CO dimerization has been observed using Th-H, 4d Zr-H, 4e Ce-H, 4f or Mg-H/Ca-H 4g,h catalysts.However, the number of complexes forming higher coupling products (C3, C4, and C6) is The reductive behaviour of 5d transition-metal (TM) carbonyl complexes is far less explored as compared with their lighter analogues.In 2017, Deacon and co-workers trapped the elusive [W 2 (CO) 10 ] 2À anion featuring an unsupported W-W bond by the reduction of W(CO) 6 with a divalent samarium mesooctaethylcalix [4]pyrrolide. 13Recently, single-electron reduced rhenium carbonyl complexes have shown electrocatalytic reduction of CO 2 to CO. 14 Herein, we report the reduction of Re 2 (CO) 10 with divalent lanthanide complexes, which leads to a tetramerization of CO to give a Fischer-type rhenacycle.We also studied the effect of the ligands on the reduction ability of the lanthanide complexes and compared the reactivity of Re 2 (CO) 10 to that of Mn 2 (CO) 10 .
Reaction of [(Cp*) 2 Sm II (thf The Re-Re bond in 1 is slightly shorter than the Re-Re single bond in Re 2 (CO) 10 (2.934(3) vs. 3.041(11) Å, respectively) but theoretical investigation indicates only a weak interaction. 17he Re1-C1 (2.188(6) Å) and Re1-C4 (2.164(7) Å) bond distances are in the typical range of conjugated rhenacyclic Fischer-type carbenes. 18In principle, two different forms, either a double Fischer-carbene (Scheme 1, 1a) or a metallacyclopentadiene (Scheme 1, 1b), can describe the 5-membered rhenacycle.The short C2-C3 bond distance in the central C 4 O 4 fragment (1.406( 9) Å (C2-C3) vs. 1.445( 9) Å (C1-C2) and 1.460( 9) Å (C3-C4)) suggests the predominance of 1a. 18Thus, we consider compound 1 as a cyclic double Fischer-carbene type complex.Interestingly, formation of the 5-membered Fischer-type rhenacycle in 1 occurred via an unprecedented tetramerization of CO ligands, presumably by reductive C-C coupling.It should be noted that a total of 12 CO units are present in the [(m-O 4 C 4 )(m-Z 2 -CO) 2 (m-Z 1 -CO)(CO) 5 Re 2 ] 4À moiety, more than in the Re 2 (CO) 10 starting material.Since no external source of CO is present, the formation of 1 implies that more than one equivalent of Re 2 (CO) 10 reacts with four equivalents of [(Cp*) 2 Sm II (thf) 2 ].To the best of our knowledge, CO tetramerization has never been observed with rhenium carbonyls.Albeit two reports describe a similar tetramerization of CO ligands, both involve reactions between trimethylsilylhalide (halide = Br, or I) or [{(Me 3 Si) 2 N}BBr 2 ] and [Na 2 Fe(CO) 4 ]. 12 Furthermore, the resulting products did not feature Fischer-carbene type character but metallacyclopentadiene type character (Scheme 1, 1b).The reactivity observed is in sharp contrast to all the reports on Ln-TM carbonyl complexes discussed earlier. 13,19ecently, we have shown that using sterically demanding ligands in the coordination sphere of divalent lanthanides, different activations of main-group elements and complexes can be achieved. 20We therefore investigated the reduction of TM carbonyls with [(DippForm) 2 Sm II (thf as red coloured crystals in 62% yield.The solid-state IR spectrum of 2 showed characteristic n CO stretches.The low-frequency stretch at 1804 cm À1 is consistent with the occurrence of bridging isocarbonyls.14a The solid-state structure (Fig. 2) confirmed the identity of 2, which is formed through the transfer of 2e À from two molecules of [(DippForm) 2 Sm II (thf) 2 ] to one molecule of Re 2 (CO) 10 in single-electron transfer (SET) steps, accompanied by the loss of two CO ligands.In complex 2, the [Re 2 (CO) 8 ] 2À anion is entrapped between two [(DippForm) 2 Sm III (thf)] + moieties.To the best of our knowledge, the [Re 2 (CO) 8 ] 2À anion has never been reported to date.Its lighter analogue, [Mn 2 (CO) 8 ] 2À , has recently been isolated by reduction of Mn 2 (CO) 10 with silylene, however, the authors failed to isolate [Re 2 (CO) 8 ] 2À under similar conditions. 22he isolation of the [(m-CO) 4 (CO) 4 Re 2 ] 2À fragment in compound 2 can be traced back to the high reductive nature of divalent samarium and the sterically demanding nature of the [(Dipp-Form) 2 Sm III (thf)] + moieties.In the [Re 2 (CO) 8 ] 2À anion, each rhenium atom is coordinated by the carbon donor of two terminal and two bridging CO ligands.The Re-C51 (m-Z 2 -CO) bond lengths, 2.049(3) and 2.206(3) Å, are longer than the other Re-C bonds due to the Z 2 -type bridging mode with two Re atoms.The Re-Re 0 bond length (2.689(3) Å) is substantially shorter than the Re-Re single bond in Re 2 (CO) 10 (3.041(11) Å), 17 suggesting a double bond character between the two Re atoms at the first glance. 23lthough the [Re 2 (CO) 8 ] 2À anion fits with the 36 electron count only after considering Re-Re double bond, our theoretical calculations suggest a lower bonding order.
The bonding situation in compounds 1 and 2 was investigated by theoretical methods.In many cases the bonding description nicely succeeds by calculation of the shared electron numbers (SEN) according to the population analysis of Ahlrichs and Heinzmann. 24These numbers give a reliable measure of the covalent bond strength, especially even in the case of multi-centre bonding contributions.This effect is referred to as ''supported metal-metal bonding'' which might appear with the Re-Re  To confirm the findings of the population analyses in an orienting manner, we performed a different topological approach by means of the AIM (atoms in molecules) method introduced by Bader. 26The results of the AIM analysis are given in Fig. S10 (ESI †).Compound 2, unlike 1, consists of a bond path with a bond critical point between both rhenium atoms that undoubtedly confirms the presence of a Re-Re bond.The low value of the electron density r bcp and the high value for the ellipticity e (r bcp = 0.05 a.u., e = 1.31) calculated for this bond critical point of Re-Re indicate a rather weak bond with a high ''multiple bond contribution'' presumably due to the interaction with the supporting p-type Re-Re-C multicentre bonds.
The bonding properties of the rhenacycle in compound 1 are also interesting.The values for the SEN of the bonds C1-C2, C2-C3, C3-C4 of 1.514, 1.593, 1.518 correspond to strong single bonds.For pure single or double bonds, SEN values of about 1.4 or 2.28, 25 respectively, are expected.The SEN (Re-C) is calculated to be 0.95 which is of the same order of magnitude that is also found for the terminal Re-CO bond (1.276).Overall, the results support the view of a double Fischer carbene complex as shown in 1a (Scheme 1).
For comparison, we investigated the reactivity between Mn 2 (CO) 10  In the solid-state structure of 4 (Fig. 3), each Sm atom lies in a distorted octahedral geometry, coordinated by two amidinates, one thf ligand and the oxygen atom of the bridging isocarbonyl.The [Mn(CO) 5 ] À fragment is in a Rhenium is different: CO tetramerization induced by a divalent lanthanide complex in rhenium carbonyls † Ravi Yadav, Thomas Simler, Michael T. Gamer, Ralf Ko ¨ppe and Peter W. Roesky * The reduction of M 2 (CO) 10 (M = Mn, Re) with different divalent lanthanide (Ln = Sm, Yb) compounds was investigated.Depending on the steric demand of the ligand, either unusual CO tetramerization or formation of a new Re carbonyl anion occurred in the case of Re.Theoretical calculations were performed for a better understanding of the nature of bonding in the newly formed species.

Fig. 1 Scheme 2
Fig. 1 Molecular structure of 1. Hydrogen atoms are omitted for clarity.Bond lengths and angles are given in ESI.†

Fig. 3
Fig. 3 Cutout of the polymeric structure of 3 (left) and molecular structure of 4 (Ln = Sm) and 5 (Ln = Yb) (right).Hydrogen atoms are omitted for clarity.Bond lengths and angles are given in ESI.†