Synthesis of a Stable Compound with Fivefold Bonding Between Two Chromium(I) Centers

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Science  04 Nov 2005:
Vol. 310, Issue 5749, pp. 844-847
DOI: 10.1126/science.1116789


Although in principle transition metals can form bonds with six shared electron pairs, only quadruply bonded compounds can be isolated as stable species at room temperature. Here we show that the reduction of {Cr(μ-Cl)Ar′}2 [where Ar′ indicates C6H3-2,6(C6H3-2,6-Pri2)2 and Pr indicates isopropyl] with a slight excess of potassium graphite has produced a stable compound with fivefold chromium-chromium (Cr–Cr) bonding. The very air- and moisture-sensitive dark red crystals of Ar′CrCrAr′ were isolated with greater than 40% yield. X-ray diffraction revealed a Cr–Cr bond length of 1.8351(4) angstroms (where the number in parentheses indicates the standard deviation) and a planar transbent core geometry. These data, the structure's temperature-independent paramagnetism, and computational studies support the sharing of five electron pairs in five bonding molecular orbitals between two 3d5 chromium(I) ions.

A quadruple bond between metal centers consisting of σ, 2π, and δ orbital overlaps was shown to be present in salts containing the [Re2Cl8]2– ion in 1964 (1). Since then, a rich chemistry has developed around this class of transition-metal compounds (2), whose bond order exceeds the previously known limit of three for compounds of the p-block elements. Beginning in the mid-1970s, theoretical and spectroscopic investigations of diatomic transition-metal species M2 (where M is either Cr or Mo) trapped in inert matrices at low temperatures indicated that sextuple bonds consisting of 2σ, 2π, and 2δ overlaps (derived from valence s and d atomic orbitals) could exist between these metals (314). However, such molecules have no stable existence at room temperature and so cannot be isolated for bulk manipulation.

If ligands are used to stabilize multiply bonded metal centers, their binding reduces the number of valence orbitals available to form metal-metal bonds. Thus, the number of ligands must be minimized, and the number of metal valence electrons that fill bonding orbitals must be maximized in order to achieve the highest bond order possible in an isolable compound. Moreover, the ligands must be sufficiently bulky to inhibit intermolecular reactions that yield clusters or polymers with lower bond orders. We have shown (15) that the sterically encumbering monovalent terphenyl ligand C6H3-2,6(C6H3-2,6-Pri2)2 (hereafter designated Ar′), where Pri is isopropyl, and related derivatives can stabilize many compounds with low coordination numbers or unusual bonding (16, 17). We now show that this ligand allows room-temperature isolation of the Ar′CrCrAr′ chromium dimer to occur. The structural, spectroscopic, and magnetic properties of this compound are consistent with a quintuple Cr–Cr bond formed by a fivefold overlap between the metal d orbitals (18).

The compound Ar′CrCrAr′ (compound 1) was isolated as dark red crystals from the reduction of {Ar′Cr(μ-Cl)}2 with KC8 (19). The crystals are thermally robust and decompose slowly above 200°C, but they are spontaneously flammable when exposed to air. X-ray crystallography of 1 (Fig. 1) (20) showed a structure characterized by a center of symmetry at the midpoint of the very short [1.8351(4) Å, where the number in parentheses indicates SD] Cr–Cr bond. Each Cr is bonded to the ipso carbon atom [distance Cr(1)–C(1) = 2.131(1) Å] of an Ar′ substituent. There is also a weaker interaction between each Cr ion [Cr(1)–C(7A) = 2.2943(9) Å] and the ipso carbon [C(7) or C(7A)] of a flanking ring of the terphenyl group attached to the other Cr. The core atoms, C(1)Cr(1)Cr(1A)C(1A), are coplanar, but they have a trans-bent structure with C2h local symmetry and a bending Cr(1A)Cr(1)C(1) angle of 102.78(3)°. Magnetic measurements revealed a temperature-independent paramagnetism of 0.000112(5) electromagnetic units (emu) per mol of Cr (21). The electronic absorption spectrum of 1 displays strong absorptions below 250 nm and a broad absorption at 488 nm, with an intensity (ϵ) of 3200 mol–1 L cm–1.

Fig. 1.

Thermal ellipsoid (30%) drawing of Ar′CrCrAr′ (compound 1). Hydrogen atoms are not shown. Selected bond distances and angles ° are as follows: Cr(1)–Cr(1A), 1.8351(4) Å; Cr(1)–C(1), 2.131(1) Å; Cr(1)–C(7A), 2.2943(9) Å; Cr(1)–C(8A), 2.479(1) Å; Cr(1)–Cr(12A), 2.414(1) Å; C(1)–C(2), 1.421(1) Å; C(1)–C(6), 1.423(2) Å; C(7)–C(8), 1.421(1) Å; C(7)–C(12), 1.424(1) Å; Cr(1A)–Cr(1)–C(1), 108.78(3)°; Cr(1A)–Cr(1)–C(7A), 94.13(3)°; C(1)–Cr(1)–C(7A), 163.00(4)°; Cr(1)–C(1)–C(2), 114.34(7)°; Cr(1)–C(1)–C(6), 131.74(7)°; and C(2)–C(1)–C(6), 113.91(9)°.

The metal-metal bonding in compound 1 arises from the interaction of two Cr(I) centers with d5 electron configurations. In a simplified molecular-orbital overlap diagram with the assumption of local C2h symmetry, five metal-metal bonding molecular orbitals can be visualized (Fig. 2) (22, 23). Also, two further metal-ligand orbital combinations, bonding and antibonding with respect to the metal-metal bond, are present. The bonding is actually more complex, because mixing of the orbitals with the same symmetry (i.e., 4s and 3dz2 or 3dx2y2) can occur. Nonetheless, σ (dz2 – dz2, Ag), 2π (dyz – dyz, dxz – dxz, Au, Bu), and 2δ (dx2y2 – dx2y2, dxy – dxy, Ag, Bg) Cr–Cr overlaps, in which electrons from each metal become paired to fill the five bonding orbitals, are possible (23).

Fig. 2. (Left)

Schematic drawing of simplified molecular orbital overlaps for M-M and M-C bonding. Fig. 3. (Right) Electron density surfaces and energies for the Cr–Cr bonding orbitals in Ar′CrCrAr′ (36).

This fivefold Cr–Cr interaction is supported by structural and magnetic data. The Cr–Cr distance is extremely short and is very close to the 1.828(2) Å bond found in the Cr(II) dimer, Cr2{C6H3-2-OMe-5-Me}4, which has the shortest reported metal-metal bond distance (24). In this Cr(II) compound and related species, the chelating nature of the ligand plays a key role in pushing the Cr centers close together, and it could be argued that the Ar′ ligand acts similarly in 1 through the secondary Cr—C interactions. However, we have also synthesized the related Ar′FeFeAr′ and Ar′CoCoAr′ dimers, which are structurally similar to 1 but have much longer Fe–Fe and Co–Co distances, ∼2.53 and 2.80 Å, respectively. Thus, the Ar′ ligand can accommodate M-M separations that vary by almost 1 Å. For this reason, the bridging shown by the Ar′ ligand in 1 is unlikely to be the cause of the short metal-metal distance. In other words, the very short Cr–Cr bond in 1 is mainly due to the interaction of the d5 Cr centers, rather than a constraining ligand geometry (25).

The temperature-independent weak paramagnetism of 1 is also consistent with strongly coupled d5-d5 bonding electrons. Temperature-independent paramagnetism has been observed for several other M-M–bonded transition-metal complexes (2629). Nonetheless, the possibility that the Cr–Cr multiple bond may be a combination of covalent bonding with antiferromagnetic coupling, which was recently calculated for the Cr2 dimer (14), should not be dismissed. The distinction between antiferromagnetic coupling and what constitutes a bond is not clearly defined; therefore, it would be of great interest to determine the contribution of the antiferromagnetic exchange coupling to the overall Cr–Cr bond energy. This exchange coupling is so strong in 1 between 2 and 300 K that, unfortunately, there is no increase in the susceptibility as the S > 0 states are populated; i.e., –2J, the antiferromagnetic exchange coupling, is so negative that only the S = 0 ground state is effectively populated at these temperatures. As a consequence, the susceptibility never begins to increase with increasing temperature, and it is difficult to determine –2J. The unpopulated S > 0 excited states yield a secondorder Zeeman contribution of 0.00112(5) emu/mol Cr to the molar magnetic susceptibility. This is the so-called “temperature-independent paramagnetism” (TIP), a contribution which must be added to the essentially zero contribution of the S = 0 ground state.

Further insight on the bonding in 1 may be obtained from computational data. However, calculations on multiply bonded transitionmetal species have often been difficult because of electron correlation problems (30, 31). Nonetheless, recent studies (8, 32, 33) have suggested that density functional theory (DFT) methods can compete successfully with high-level ab initio calculations. Both the trans-bent geometry and the quintuple-bond formulation are predicted by the simple, Lewis-like electron-pair sharing scheme of Landis and Weinhold for transition-metal complexes (34, 35). We carried out restricted DFT calculations (36) using hybrid and pure functionals to further analyze the Cr–Cr interaction. These theoretical approaches (37) yielded very similar results. Molecular orbitals were generated from single-point calculations by using the atomic coordinates provided by the x-ray structure. The metal-metal orbital surfaces (Fig. 3) support the view that there are five orbital interactions between the Cr(I) ions. The symmetries of the highest occupied molecular orbital (HOMO) and HOMO – 1, which differ in energy by 0.41 eV, correspond to δ bonds. The HOMO – 2 corresponds to Cr–Cr σ bonding and lies at ∼1.08 eV lower energy than HOMO – 1. HOMO – 3 and HOMO – 4 correspond to Cr–Cr π bonds and lie slightly (∼0.21 to 0.35 eV) below the σ-bonding level.

The calculated HOMO–lowest unoccupied molecular orbital (LUMO) energy gap (2.01 eV, 46.35 kcal mol–1), which may correspond to a δ-δ* transition, is at a somewhat lower energy than the 58.59 kcal mol–1 calculated from the 488-nm absorption maximum in the electronic spectrum. This discrepancy has precedent in σ2π4δ2 quadruply bonded M-M species, for which the experimental δ-δ* transition energies are usually higher than those calculated (2). Moreover, the putative δ-δ* transition lies at the higher energy end of the ∼450 to 1600-nm range observed for quadruply bonded compounds (2), which suggests that the δ bonds in 1 are as strong as those observed in the quadruply bonded compounds.

Supporting Online Material

Materials and Methods

Figs. S1 to S3

Tables S1 to S16

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