New hydrido complexes of the type [M2Cp2(μ-H)(μ-PRR‘)(CO)4] (M = Mo, W) have been prepared through the thermal reaction of [Mo2Cp2(CO)6] with HPCy2, H2PCy, or HPEt2 or the thermal reaction of [W2Cp2(CO)4] with HPR2 (R = Cy, Et, Ph). In contrast, UV irradiation of [M2Cp2(CO)6] and HPRR‘ leads with good yield to the bis(phosphido) complexes [M2Cp2(μ-PRR‘)2(μ-CO)] (R = R‘ = Cy, Et, Ph; R = Cy, R‘ = H). Related complexes having different phosphido groups, [M2Cp2(μ-PR2)(μ-PR‘R‘ ‘)(μ-CO)] (R = Cy, tBu, Ph; R‘ = Cy, tBu, Et; R‘ ‘ = Cy, tBu, Et, H), can be prepared in high yield through the photochemical reaction of [M2Cp2(μ-PR2)(μ-H)(CO)4] and HPR‘R‘ ‘ or [M2Cp2(μ-H)(μ-PR‘R‘ ‘)(CO)4] and HPR2. All triply bonded compounds react easily with carbon monoxide at room temperature or under moderate heating to finally yield the corresponding trans-dicarbonyl complexes [M2Cp2(μ-PRR‘)2(CO)2] or [M2Cp2(μ-PR2)(μ-PR‘R‘ ‘)(CO)2]. Some of the intermediates in these carbonylation reactions have been identified, including the cis-dicarbonyl complex [Mo2Cp2(μ-PPh2)(μ-PtBu2)(CO)2] and the tricarbonyl complex [Mo2Cp2(μ-PEt2)2(CO)3]. The structures of the new complexes are analyzed on the basis of the corresponding IR and NMR (1H, 31P, 13C) data, and the reaction pathways operative in these highly efficient syntheses of bis(phosphido) complexes is discussed on the basis of the available data and some cross-experiments.

The unsaturated complexes [W2Cp2(mu-PR2)(mu-PR'2)(CO)2] (Cp = eta5-C5H5; R = R' = Ph, Et; R = Et, R' = Ph) react with HBF4.OEt2 at 243 K in dichloromethane solution to give the corresponding complexes [W2Cp2(H)(mu-PR2)(mu-PR'2)(CO)2]BF4, which contain a terminal hydride ligand. The latter rearrange at room temperature to give [W2Cp2(mu-H)(mu-PR2)(mu-PR'2)(CO)2]BF4, which display a bridging hydride and carbonyl ligands arranged parallel to each other (W-W = 2.7589(8) A when R = R' = Ph). This explains why the removal of a proton from the latter gives first the unstable isomer cis-[W2Cp2(mu-PPh2)2(CO)2]. The molybdenum complex [Mo2Cp2(mu-PPh2)2(CO)2] behaves similarly, and thus the thermally unstable new complexes [Mo2Cp2(H)(mu-PPh2)2(CO)2]BF4 and cis-[Mo2Cp2(mu-PPh2)2(CO)2] could be characterized. In contrast, related dimolybdenum complexes having electron-rich phosphide ligands behave differently. Thus, the complexes [Mo2Cp2(mu-PR2)2(CO)2] (R = Cy, Et) react with HBF4.OEt2 to give first the agostic type phosphine-bridged complexes [Mo2Cp2(mu-PR2)(mu-kappa2-HPR2)(CO)2]BF4 (Mo-Mo = 2.748(4) A for R = Cy). These complexes experience intramolecular exchange of the agostic H atom between the two inequivalent P positions and at room-temperature reach a proton-catalyzed equilibrium with their hydride-bridged tautomers [ratio agostic/hydride = 10 (R = Cy), 30 (R = Et)]. The mixed-phosphide complex [Mo2Cp2(mu-PCy2)(mu-PPh2)(CO)2] behaves similarly, except that protonation now occurs specifically at the dicyclohexylphosphide ligand [ratio agostic/hydride = 0.5]. The reaction of the agostic complex [Mo2Cp2(mu-PCy2)(mu-kappa2-HPCy2)(CO)2]BF4 with CN(t)Bu gave mono- or disubstituted hydride derivatives [Mo2Cp2(mu-H)(mu-PCy2)2(CO)2-x(CNtBu)x]BF4 (Mo-Mo = 2.7901(7) A for x = 1). The photochemical removal of a CO ligand from the agostic complex also gives a hydride derivative, the triply bonded complex [Mo2Cp2(H)(mu-PCy2)2(CO)]BF4 (Mo-Mo = 2.537(2) A). Protonation of [Mo2Cp2(mu-PCy2)2(mu-CO)] gives the hydroxycarbyne derivative [Mo2Cp2(mu-COH)(mu-PCy2)2]BF4, which does not transform into its hydride isomer.

The phosphinidene-bridged complex [Mo2Cp2(μ-PR*)(CO)4] (R = 2,4,6- C6H2 tBu3) experiences an intramolecular C−H bond cleavage from a tBu group to give the phosphide-hydride derivative [Mo2Cp2(μ-H){μ-P(CH2CMe2)C6H2 tBu2}(CO)4] in refluxing diglyme (ca. 438 K) or under exposure to near-UV−visible light. In contrast, its exposure to UV light yields two different dicarbonyl derivatives depending on the reaction conditions, either the triply bonded [Mo2Cp2(μ-PR*)(μ-CO)2] (Mo−Mo = 2.5322(3) Å) or its isomer [Mo2Cp2(μ-κ 1:κ 1,η 6-PR*)(CO)2], in which the phosphinidene ligand bridges asymmetrically the metal centers while binding its aryl group to one of the molybdenum atoms in a η6-fashion. The latter complex experiences a proton-catalyzed tautomerization to yield the cyclopentadienylidene−phosphinidene derivative [Mo2Cp(μ-κ 1:κ 1,η 5-PC5H4)(η 6-R*H)(CO)2]. Carbonylation of the η 6-phosphinidene complex proceeds stepwise through the η 4-tricarbonyl complex [Mo2Cp2(μ-κ 1:κ 1,η 4-PR*)(CO)3] and then to the starting tetracarbonyl compound, whereas its reaction with CNtBu yields only the η 4-complex [Mo2Cp2(μ-κ 1:κ 1,η 4-PR*)(CNtBu)(CO)2], which was characterized through an X-ray study. The η 4-tricarbonyl species reacts with CNtBu in tetrahydrofuran to give the metal−metal bonded derivative [Mo2Cp2(μ-PR*)(CNtBu)(CO)3]. In petroleum ether, however, this reaction yields the bis(isocyanide) derivative [Mo2Cp2(μ-PR*)(CNtBu)2(CO)3], which has an asymmetric trigonal phosphinidene bridge and no metal−metal bond. All the above results can be explained by assuming the operation of two primary processes in the photolysis of [Mo2Cp2(μ-PR*)(CO)4], one of them involving a valence tautomerization of the phosphinidene ligand, from the trigonal (four-electron donor) to the pyramidal (two-electron donor) coordination mode. The carbonylation reaction of the η 6-complex is accelerated by the presence of CuCl, due to the formation of the trimetal species [CuMo2(Cl)Cp2(μ-κ 1:κ 1:κ 1,η 6-PR*)(CO)2] and [CuMo2(Cl)Cp2(μ-κ 1:κ 1:κ 1,η 4-PR*)(CO)3]. The latter complexes were also characterized by single-crystal X-ray studies.

Carboxylases are biocatalysts that capture and convert carbon dioxide (CO2) under mild conditions and atmospheric concentrations at a scale of more than 400 Gt annually. However, how these enzymes bind and control the gaseous CO2molecule during catalysis is only poorly understood. One of the most efficient classes of carboxylating enzymes are enoyl-CoA carboxylases/reductases (Ecrs), which outcompete the plant enzyme RuBisCO in catalytic efficiency and fidelity by more than an order of magnitude. Here we investigated the interactions of CO2within the active site of Ecr fromKitasatospora setae. Combining experimental biochemistry, protein crystallography, and advanced computer simulations we show that 4 amino acids, N81, F170, E171, and H365, are required to create a highly efficient CO2-fixing enzyme. Together, these 4 residues anchor and position the CO2molecule for the attack by a reactive enolate created during the catalytic cycle. Notably, a highly ordered water molecule plays an important role in an active site that is otherwise carefully shielded from water, which is detrimental to CO2fixation. Altogether, our study reveals unprecedented molecular details of selective CO2binding and C–C-bond formation during the catalytic cycle of nature’s most efficient CO2-fixing enzyme. This knowledge provides the basis for the future development of catalytic frameworks for the capture and conversion of CO2in biology and chemistry.