Fully Solvated, Monomeric Re^II Complexes: Insights into the Chemistry of [Re(NCCH₃)₆]²⁺

Abstract

The oxidation of [Re( 6 -C10H8)2] + with Ag I in acetonitrile yields [Re(NCCH3)6] 2+ . This fully solvated Re II compound was characterized by spectroscopic methods and X-ray structure analyses. We show that [Re(NCCH3)6] 2+ acts as a precursor complex for a variety of substitution reactions. Treatment with monodentate triphenyl phosphine (PPh3) and bidentate 1,2bis(diphenylphosphino)ethane (dppe) yields the Re I complexes [trans-Re(PPh3)2(NCCH3)4] 2+ and [trans-Re(dppe)2(NCCH3)2] + , respectively. [trans-Re(dppe)2(NCCH3)2] + is oxidized under mild conditions by Ag I to its Re II analogue [trans-Re(dppe)2(NCCH3)2] 2+ . Reactions of [Re(NCCH3)6] 2+ with a halide mixture consisting of NaX and AgX (X = Cl, I) results in the formation of the corresponding Re III complexes [trans-ReX2(NCCH3)4] + . [trans-ReBr2(NCCH3)4] + can be obtained directly from [Re( 6 -C10H8)2] + by oxidation with FeBr3 in acetonitrile. The title compound is thus a convenient starting material for Re II and Re III complexes by simple solvent exchange, which are otherwise difficult to access.

One Electron Changes Everything: Synthesis, Characterization, and Reactivity Studies of [Re(NCCH3)6]3+

“…[ 3a ]­(BF 4 ) 2 is easier to reduce than its acetonitrile analogue (−0.56 V for [ 3a ]­(BF 4 ) 2 and −1.00 V for [ 2a ]­(PF 6 ) 2 ; in CH 3 CN (0.1 M NBu 4 (PF 6 )) vs Fc + /Fc), whereas oxidation becomes more challenging (+0.44 V for [ 3a ]­(BF 4 ) 2 and +0.25 V for [ 2a ]­(PF 6 ) 2 , Figure ). These electrochemical differences align with findings for other CH 3 CN/PhCN complexes. , As expected, the PhCN ligands make [ 3a ]­(BF 4 ) 2 much more lipophilic as compared to its acetonitrile analogue [ 2a ] 2+ , increasing the solubility of [ 3a ]­(BF 4 ) 2 in solvents such as CHCl 3 , CH 2 Cl 2 , or acetone. This better solubility might make [ 3a ]­(BF 4 ) 2 a more promising candidate for a fully solvated Re II precursor complex than its acetonitrile analogue [ 2a ]­(BF 4 ) 2 .…”

“…X-ray diffraction analysis of [ 3a ]­(BF 4 ) 2 revealed a slightly distorted octahedral geometry with N–Re–N angles ranging from 86.58(7)° to 93.48(7)° (Figure ). The Re–N bond lengths determined in [ 3a ]­(BF 4 ) 2 (2.033(2)–2.0509(18) Å) are slightly shorter than those found in [ 2a ]­(OTf) 2 (2.041(4)–2.056(3) Å) . Similar to those in [Ru­(NCPh) 6 ]­(BF 4 ) 2 , the phenyl moieties of the PhCN ligands in the trans position with respect to each other are not in the same plane …”

“…We demonstrated that the Re II precursor complex [Re­(NCCH 3 ) 6 ] 2+ ([ 2a ] 2+ ) is accessible by Ag I -mediated oxidation of [Re­(η 6 -C 10 H 8 ) 2 ] + in CH 3 CN. By DFT calculations, we have proposed that the actual oxidation step does not take place directly on [Re­(η 6 -C 10 H 8 ) 2 ] + but rather on a CH 3 CN adduct, for example, [Re­(η 6 -C 10 H 8 )­(NCCH 3 ) 3 ] + . By combining these two observations, we have developed a new procedure for [ 2a ] 2+ that circumvents the low-yield synthesis of [Re­(η 6 -C 10 H 8 ) 2 ] + .…”

“…Single-crystal X-ray diffraction analysis of [ 2b ]­(BF 4 ) 3 (Figure ) did not evidence any major differences compared to that of its reduced form [ 2a ]­(OTf) 2 despite the different oxidation states. The Re–N bond lengths range from 2.044(2) to 2.0506(15) Å (2.041(3)–2.059(3) Å for [ 2a ]­(OTf) 2 ) with N–Re–N bond angles from 88.26(6)° to 92.33(6)° (86.6(1)°–93.4(1)° for [ 2a ]­(OTf) 2 ) . [ 2b ]­(BF 4 ) 3 crystallized in the orthorhombic space group Pnma .…”

“…Hexa-aquo complexes are scarce for 4d and 5d transition-metal elements and have not yet been found for Re or Tc in particular. For these two elements there are examples for fully solvated complexes, though with CH 3 CN as the ligand, for example, [M 2 (NCCH 3 ) 10 ] 4+ or [M­(NCCH 3 ) 6 ] 2+ (M = Re, 99 Tc). − Recently, we have examined the potential of [Re­(NCCH 3 ) 6 ] 2+ as a precursor for the rare oxidation state +2, but one of the major drawbacks of substitution reactions with [Re­(NCCH 3 ) 6 ] 2+ was its unexpected inertness . Thus, we have wondered if changing the electron configuration (e.g., by oxidation) would have a significant effect on substitution rates as found in, for example, Fe II /Fe III hexa-aquo complexes.…”