Syntheses, Molecular Structures, and Vibrational Spectra of Chloropentacarbonylrhodium(III) and -iridium(III) Undecafluorodiantimonate(V), [Rh(CO)₅Cl][Sb₂F₁₁]₂and [Ir(CO)₅Cl][Sb₂F₁₁]₂:  An Experimental and Density Functional Study

Abstract

The reactions of either bis(mu-chloro)tetracarbonyldirhodium(I), [Rh(CO)2(mu-Cl)]2, or chlorotricarbonyliridium(I), [Ir(CO)3Cl]n, in the conjugate Brønsted-Lewis superacid HF-SbF5 and in a CO atmosphere, produce [Rh(CO)5Cl][Sb2F11]2 or [Ir(CO)5Cl][Sb2F11]2, respectively. In these oxidative carbonylation reactions, antimony(V) fluoride functions as an oxidizing agent. The reduced product is identified as 6SbF3.5SbF5. [Rh(CO)5Cl][Sb2F11]2 is obtained in the form of single crystals. Crystal data: monoclinic, space group P2(1) (No. 4); a = 9.721(1), b = 12.602(1), c = 10.538(1) A; beta = 106.51(1) degrees; V = 1237.7(2) A3; Z = 2; T = 300 K; R1 [I > 3 sigma (I)] = 0.0367, wR2 = 0.0739. Single crystals of [Ir(CO)5Cl][Sb2F11]2 are produced in small amounts from a solution of mer-Ir(CO)3(SO3F)3 in magic acid, HSO3F-SbF5. The possible source of chlorine will be discussed. Crystal data for [Ir-(CO)5Cl][Sb2F11]2: monoclinic, space group P2(1) (No. 4); a = 9.686(2), b = 12.585(2), c = 10.499(2) A; beta = 106.59(2) degrees; V = 1226.5(4) A3; Z = 2; T = 294 K; R1[I > 3 sigma (I)] = 0.032, Rw = 0.031. The bond lengths and bond angles are nearly identical in the two isostructural salts; however, the cell volume of [Ir(CO)5Cl][Sb2F11]2 is slightly smaller than that of [Rh(CO)5Cl][Sb2F11]2. The cations (point group C4v) feature unusually long M-C bonds (M = Rh, Ir) and correspondingly short CO bonds, as well as high CO stretching wavenumbers and high CO stretching force constants. The [Sb2F11]- anions are not symmetry related, and their dihedral and bridge angles differ slightly in both salts. There are significant interionic contacts in [Ir(CO)5Cl][Sb2F11]2 exclusively of the C-F type (about 2 for each C atom of the five carbonyl groups) resulting in extended structures. The vibrational spectra for both [M(CO)5Cl]2+ cations (M = Rh, Ir) are assigned with the help of density functional calculations, which also provide intensities for IR and Raman bands. While [Rh(CO)5Cl]2+ is the first cationic carbonyl derivative of Rh(III), the vibrational and structural parameters for [Ir(CO)5Cl]2+ are compared to data for [Ir(CO)6]3+ and mer-Ir(CO)3(SO3F)3.

Predominantly σ‐Bonded Metal Carbonyl Cations (σ‐Carbonyls): New Synthetic, Structural, and Bonding Concepts in Metal Carbonyl Chemistry

“…Here the use of nonoxidizing Lewis acids like MCl 3 , M=Al, Ga is advantageous (see Table 1). The deliberate use of SbF 5 in oxidative carbonylations for example, of [M(CO) 6 ] M=Mo, W12 results in the formation of a reduced byproduct (SbF 3 or the adduct 6 SbF 3 ⋅ 5 SbF 5 22), which needs to be separated from the product and results in reduced yields. In oxidative carbonylations of chloride containing precursors (e.g., [Ir(CO) 3 Cl]23), the MCl interaction becomes stronger on oxidation. Hence chloride abstraction is no longer feasible and, in the example cited above, [Ir(CO) 5 Cl] 2+ is obtained rather than [Ir(CO) 6 ] 3+ 23…”

“…In oxidative carbonylations of chloride containing precursors (e.g., [Ir(CO) 3 Cl]23), the MCl interaction becomes stronger on oxidation. Hence chloride abstraction is no longer feasible and, in the example cited above, [Ir(CO) 5 Cl] 2+ is obtained rather than [Ir(CO) 6 ] 3+ 23…”

“…As a result a substantial number of crystal structures are obtained (see Table 1). 12, 13, 21, 23 In the syntheses of [M(CO) 6 ] 2+ salts, M=Fe, Ru, Os,11–13 both [Sb 2 F 11 ] − and [SbF 6 ] − are possible counterions, on account of equilibria such as that given in Equation (1) 24 …”

“…Among the salts in Table 1, additional isostructural pairs are listed, formed by 4d and 5d metals: [M(CO) 4 ][Sb 2 F 11 ] 2 , M=Pd, Pt;30 [M(CO) 5 Cl][Sb 2 F 11 ] 2 , M=Rh, Ir;23 [M(CO) 6 ][Sb 2 F 11 ] 2 , M=Ru, Os12 and [M(CO) 6 ][SbF 6 ] 2 , M=Ru, Os,12, 13. In these complexes the unit cell volumes V , of the 5d metal salts are very slightly smaller (by about 0.3 to 0.8 %) than those of the corresponding salts formed by 4d metals,12, 13 on account of relativistic effects 40…”

Heterocations in Superacid Systems

“…1015 Strauss and co-workers 1011 have isolated the carborane anion salt [Eq. 1008,1016 The unusually long MÀC and short CÀO bonds and high CO stretching frequencies are characteristic features observed for the corresponding homoleptic metal carbonyl cations as well. The CÀRhÀC angle in the Rh(CO) 2 moiety is $90 deduced from the equal intensities of the two n(CO) bands.…”