Fig. 3 Schematic drawing of the tiling of the ab plane of INB crystals with black and white dots (for up and down polar INB chains, respectively), showing a polar single crystal (a), a racemic (macro)twin (b), a faulted paracrystal with many twin boundaries (c) and a fully disordered Fddd (microtwinned) phase (d)

No abstract

The addition of [Re2H(CO)9]- to the electronically unsaturated complex [Re2(μ-H)2(CO)8] rapidly and selectively gives the anion [Re4H(μ-H)2(CO)17]- (2), containing an open chain tetranuclear metal skeleton, as revealed by a single-crystal X-ray analysis of its [NEt4]+ salt. In the solid state the three metal−metal interactions display a staggered−eclipsed−staggered conformation, while in solution 1H and 13C NMR spectra have shown conformational freedom around the three Re−Re interactions and a dynamic process exchanging the two hydrides bound to the terminal H2Re(CO)4 moiety, as well as the carbonyls trans to them (E a = 48(1) kJ/mol). A windshield-wiper motion of the H2Re(CO)4 fragment around the two trans diaxial carbonyls, analogous to that previously observed in the related anions [Re3H(μ-H)(CO)13]- and [Re2H2(μ-H)(CO)8]-, is likely responsible for this exchange. The tetrametallic skeleton of the anion 2 in solution easily undergoes fragmentation to trinuclear species. Under CO atmosphere the clean formation of [ReH(CO)5] and [Re3H(μ-H)(CO)13]- has been recognized. The anion 2 is formed (even if in lower yields) also by reaction of [Re2H2(μ-H)(CO)8]- with “Re2(CO)9(THF)”, obtained by treatment of [Re2(CO)10] with Me3NO in THF. A 13C NMR investigation has clarified that such “Re2(CO)9(THF)” reagent is indeed a mixture of three eq-[Re2(CO)9L] species, containing THF, H2O, and, in a minor amount, NMe3, as labile L ligands. The reaction of the same eq-[Re2(CO)9L] species with [Re2H(CO)9]- affords in good yields the tetranuclear cluster anion [Re4(μ-H)(CO)18}]- (3). The single-crystal X-ray analysis of [NEt4]3 has revealed also in this case a Re4 chain, with an all-staggered conformation, of idealized C 2 symmetry. The low-temperature 13C NMR spectrum of the carbonyls has shown a higher symmetry in solution, suggesting conformational freedom around all of the Re−Re interactions.

The addition of [M(CO) 5 ] -anions (M ) Re, Mn) to the electronically unsaturated [Re 2 (µ-H) 2 (CO) 8 ] complex rapidly and selectively gives the trinuclear anions [ReM(CO) 9 {(µ-H)ReH-(CO) 4 }] -(M ) Re, 2; M ) Mn, 3). The single-crystal X-ray analysis of [NEt 4 ]2 has revealed a fully staggered L-shaped structure. The anion 2 has been obtained with good selectivity also by reacting (i) [Re 2 (CO) 9 (THF)] with [H 2 Re(CO) 4 ] -and (ii) [HRe 2 (CO) 9 ] -with [HRe-(CO) 5 ]. The last reaction can be reversed upon treatment with CO. The reaction of the trinuclear open cluster [Re 2 (CO) 9 {(µ-H)Re(CO) 5 }] with [H 2 Re(CO) 4 ] -affords 2 as well, evenif not quantitatively. 1 H and 13 C NMR spectra of 2 show conformational freedom around both the Re-Re interactions and a dynamic process exchanging the two hydrides and the carbonyls trans to them (E a ) 67(2) kJ/mol). This last is attributable to a windshield-wiper motion of the H 2 Re(CO) 4 fragment around the two trans diaxial carbonyls. The exchange of the hydrides with comparable ∆G q has been observed also for 3, suggesting that the same type of motion is occurring. 13 C-NMR studies of the related [Re 2 (CO) 9 {(µ-H)Re(CO) 5 }] complex have shown the facile mobility of the bridging hydride between the two metal-metal interactions (at variance with the anion 2), resulting in a "dynamic" C 2v symmetry of the molecule in solution. Upon heating, the anion 2 looses CO and gives irreversibly the previously known triangular cluster anion [Re 3 (µ-H) 2 (CO) 12 ] -. The addition of a strong acid (CF 3 SO 3 H) results in fragmentation of the trinuclear skeleton of 2, affording [HRe(CO) 5 ] and [Re 2 (µ-H) 2 (CO) 8 ].

Protonation of the anion [Re(2)H(CO)(9)](-) (1) with a strong acid at 193 K affords the neutral complex [Re(2)H(2)(CO)(9)] (2), that in THF above 253 K irreversibly loses H(2) to give [Re(2)(CO)(9)(THF)], previously obtained by room-temperature protonation of 1. Treatment of 2 with NEt(4)OH restores the starting anion 1. Variable temperature (1)H and (13)C NMR spectra as well as T(1) measurements agree with the formulation of 2 as a classical [Re(2)H(mu-H)(CO)(9)] complex, in which two dynamic processes takes place. The "windshield-wiper motion" observed in several related complexes equalizes the two carbonyls trans to the hydrides (E(a) = 44(1) kJ mol(-)(1)), while another much faster process equalizes bridging and terminal hydrides already at 172 K. The variable temperature behavior of the (1)H transverse relaxation times revealed also proton exchange between 2, water, and the parent anion 1 (due to the acidity of 2), but such a process is too slow to account for the fast hydrides exchange in 2. The nature of the latter process has been investigated both experimentally and theoretically. Kinetic data, obtained by the analysis of the variable temperature (1)H spectra (E(a) = 24.5(5) kJ mol(-1)), revealed a small normal kinetic isotope effect (ca. 1.5). The (2)H chemical shift of the fully deuterated isotopomer 2-d(2) was found isochronous with 2, thus ruling out the presence of a significant concentration of a nonclassical [Re(2)(eta(2)-H(2))(CO)(9)] tautomer, in fast exchange with the classical dihydride. Density functional theory (DFT) calculations, carried out at the B3LYP level, confirmed the formulation of [Re(2)H(2)(CO)(9)] as a classical complex. However, when DFT was used to obtain a detailed description of the dynamic behavior of 2 in solution, a new type of hydride fast exchange emerged, involving the nonclassical tautomer as a relatively high energy (12.7 kJ mol(-1)) intermediate. Isotopic perturbation of the equilibrium by partial deuteration of 2 indicated the preference of deuterium for the bridging sites, with Delta H degrees = -475(4) J mol(-1) and Delta S degrees = -0.80(2) J K(-1) mol(-1). The same preference was observed in the anion [Re(2)H(mu-H)Cl(CO)(8)](-).