Electron-Reservoir Multiple Bond in RhC^– for Dinitrogen Cleavage

Abstract

Multiple bonds involving the main-group element represent a prominent class of interactions whose formation and cleavage involve substantial electron redistribution, yet its functional role in redox processes remains unexplored. Herein, we identify a triple-bond character in the RhC^– anion, which was generated in the gas phase and characterized spectroscopically by mass spectrometry and photoelectron spectroscopy. Concurrently, the RhC^– species is demonstrated to exhibit dinitrogen cleavage capability. Quantum chemical calculations reveal that the resulting products feature a pronounced weakening of the Rh–C interaction along with concomitant C–N bond formation. Electronic structure analysis further indicates that the Rh–C triple bond acts as an electron reservoir, and nitrogen reduction proceeds primarily through cleavage of the Rh–C bond, which releases stored electrons. This work establishes the functional application of metal–carbon multiple bonds in dinitrogen cleavage, demonstrating their unique capacity to couple structural dynamics with redox chemistry and thereby establishing a transformative mechanism for designing electron-reservoir catalysts in dinitrogen activation and beyond.

Introduction

Chemical bond represents one of the most fundamental concepts in chemistry, with multiple bonds between atoms constituting a cornerstone of the field. The discovery of the rhenium–rhenium quadruple bond by Cotton and co-workers marked a pivotal point in inorganic chemistry, , stimulating extensive research to elucidate the characteristics of multiple metal–metal bonds. − The highest experimentally confirmed bond order in a stable compound is the Cr–Cr quintuple bond, with theoretical studies predicting that W₂ and Mo₂ can attain the maximum covalent bond multiplicity, a formal sextuple bond. Subsequently, considerable attention has also been drawn to multiple bonds between heteronuclear metals, including transition metals, p-block and s-block main-group metals, − as exemplified by Zhou’s proposal that the Be–Fe bond in the BeFe­(CO)₄ ^– complex comprises a covalent two-electron σ bond and two weak Be←Fe­(CO)₄-conjugated π interactions. While multiple bonding in main-group elements has traditionally been limited to triple bonds, the proposal of quadruple bonding in C₂ and its isoelectronic analogues (CN⁺, BN, and CB^–) by Shaik and colleagues has reignited interest in higher bond orders, despite ongoing debate. −

Unfilled d- or f-orbitals in transition metals can increase bond multiplicity, enabling high-order bonding in metal main-group complexes, , whereas the comparable radial extents of the 2s and 2p orbitals in second-period elements favor sp hybridization and multiple-bond formation. The CUO molecule possesses a distinct quadruple bond between carbon and uranium, composed of two σ and two π bonds, a structure that has become the classical paradigm of quadruple bonds. Quadruple bonding between boron and transition metals has been identified in the complexes BFe­(CO)₃ ^– and RhB, , as demonstrated by the work of Zhou and Wang using infrared photodissociation spectroscopy and photoelectron spectroscopy, respectively. Recently, the characterization of the noble gas-containing complex ArBO⁺, via combined experimental and theoretical approaches, shows a very strong Ar–B covalent bond with distinct multiple-bond character. Similarly, Liu and Li first proposed the existence of a ThN quadruple bond. In addition, theoretical predictions have further indicated the possibility of quadruple bonding involving alkaline earth metals and the main-group elements F and O. − Consequently, the multiple bond involving main-group elements has attracted considerable attention. − Beyond advancing our fundamental understanding of chemical bonding, research on multiple bonds is increasingly directed toward harnessing their unique properties for technological applications. This inquiry warrants serious consideration, as the formation and cleavage of chemical bonds, processes inherently involving the accumulation and release of electrons, are potentially pivotal for enabling efficient redox reactions. This aspect is not only of fundamental interest but also bears significant industrial relevance.

The widespread adoption of metal carbides (e.g., V₃C₄ ^–, FeTaC₂ ^–, InNbC₂ ^–, etc.), − nitrides (e.g., Ta₂N^+/–, Ta₃N₃H_0,1 ^–, V₅N₅ ^–, etc.), − and borides (e.g., NbB₃O₂ ^–, AuNbBO^–, Nb₂BO^–, etc.) − in dinitrogen activation, despite its associated challenges, offers a viable pathway to introduce and exploit multiple-bond architectures of metal main-group elements within multiphase catalytic reactions. Herein, we employed a combined theoretical and experimental approach, involving the design of a triple-bonded RhC^– anion, followed by structural validation through mass spectrometry and photoelectron spectroscopy. Investigations into dinitrogen cleavage mediated by RhC^– revealed a mechanism wherein the triple bond serves as an electron-storage unit, facilitating fixation upon bond rupture. This finding provides the first demonstration that a metal–carbon multiple bond functions as an electron reservoir in dinitrogen activation and cleavage.

Experimental and Theoretical Methods

The experiments were conducted using a custom-built apparatus comprising a laser vaporization source, a dual-channel time-of-flight mass spectrometer, and a velocity map imaging spectrometer. A detailed description of the instrument has been provided elsewhere; only an essential overview is given here. Both RhC^– and RhCN₂ ^– cluster anions were synthesized via laser vaporization of a rhodium–carbon composite target (Rh/C molar ratio = 1:2). The RhC^– anions were generated using pure helium or helium carrier gas doped with a small percentage of N₂, whereas for RhCN₂ ^–, the helium carrier gas was seeded with 10% N₂ and pressurized to a stagnation pressure of approximately 2–5 atm. Following cooling via supersonic expansion into the source chamber, the anions were mass-selected using a McLaren Wiley time-of-flight mass spectrometer. For the detachment of these anionic clusters, photons with wavelengths of 532 nm (2.331 eV) and 355 nm (3.496 eV) were used. The photoelectron signal was accumulated over 15000 laser shots at a 10 Hz repetition rate and recorded using a charge-coupled device camera. The resulting raw image constitutes a two-dimensional projection of the 3D photoelectron velocity distribution in the laboratory frame. The original 3D distribution was reconstructed from this projection using the Basis Set Expansion inverse Abel transform method. The photoelectron spectrum was then obtained by angular integration of a central slice of the reconstructed 3D distribution. The kinetic energy scale was calibrated against the reference spectrum of Au^–. The experimental energy resolution was better than 5%, which translates to an absolute resolution of 50 meV at 1 eV electron kinetic energy.

To elucidate the electronic and geometrical structures of RhC^– and RhCN₂ ^–, theoretical calculations were performed using the Gaussian 09 program. The structures were optimized with density functional theory using the Perdew–Burke–Ernzerhof (PBE) functional with the 6-311+G­(d) basis set for C and N and aug-cc-pVTZ-PP for the Rh atom. , All plausible spin states and structures were considered. Harmonic frequency analysis was performed to ensure that the obtained structures were real minima on the potential energy surfaces. The vertical detachment energy (VDE) was computed as the energy difference between the neutral and anionic species at the optimized geometry of the anion. The adiabatic detachment energy (ADE) was obtained from the energy difference between the fully optimized neutral and anionic structures. Notably, the relative energies of these clusters have been reported at the CCSD­(T) level. Single-point energies were calculated at the CCSD­(T)//PBE/6-311+G­(d)/aug-cc-pVTZ-PP level to obtain the ADEs and VDEs. A detailed analysis of the RhC^– and RhCN₂ ^– bonding mechanism was conducted using the high-performance hybrid functional PBE0. The chemical bonding of RhCN₂ ^– was investigated using the adaptive natural density partitioning (AdNDP) method. To understand the structure of these clusters, the bond lengths and electron localization function (ELF) were determined, and natural population analysis (NPA) and canonical molecular orbital analysis were performed using the Gaussian 09 package.

Results and Analysis

Theoretical Verification of the Rh–C Triple Bond

Figure a depicts the Frontier Kohn–Sham molecular orbitals (KS-MOs) of the RhC^– anion. The HOMO (3σ) of RhC^– is a weakly antibonding orbital between the Rh 5s and the C 2p orbitals, a finding that has also been confirmed in RhB. The HOMO-1 (1δ) of RhC^– consists of two degenerate nonbonding Rh 4d orbitals. HOMO-2 (2σ) is a σ-bonding orbital formed between the Rh 4d and C 2p atomic orbitals. HOMO-3 (1π) comprises the doubly degenerate π-bonding orbitals with contributions from Rh 4d and C 2p orbitals. The HOMO-4 (1σ) orbital is commonly featured in the KS-MOs of systems with quadruple bonds between transition metals and main-group elements. ,,, However, the strong delocalization of these canonical MOs, which do not directly correspond to localized chemical bonds, precludes a direct assessment of bond formation involving the 1σ-type orbitals in RhC^–. To overcome this limitation of canonical MOs, we utilized Pipek–Mezey localized molecular orbitals (LMOs) to unambiguously characterize the multiple bonding in RhC^– (Figure b). LMOs analysis reveals that the Rh–C bond comprises one σ bond and two degenerate π bonds. In addition, LMOs also exhibit two 4d orbitals and one 5S orbital of Rh. Importantly, the significantly low energy (−39.779 eV) of the C 2s orbital allows for unambiguous assignment of its occupancy. Consequently, the LMOs offers direct evidence for the Rh–C triple-bond character.

1.

Bonding analysis of the RhC^– anion using KS-MOs and LMOs (isosurface value = 0.03 a.u.), calculated at the PBE0/6-311+G­(d)/aug-cc-pVTZ-PP level of theory. In KS-MOs, the symmetry for each MO is indicated, while in LMOs, the energies for the orbitals are also provided.

Verification of Structure by Photoelectron Spectroscopy

Informed by theoretical insights, a subsequent experimental investigation was undertaken. Figure S1 displays a representative mass spectrum under 10% N₂ in a helium atmosphere, demonstrating that the prepared RhC^– anion readily reacts with N₂. Notably, the Rh^– anion also reacts with N₂ to form RhN₂ ^–, providing a relevant control for future studies on nitrogen reduction catalyzed by RhC^–. The photoelectron spectra of RhC^–, RhN₂ ^–, and RhCN₂ ^– recorded at 532 nm are shown in Figure a–c, respectively. The spectra of RhC^– and RhN₂ ^– exhibit distinct dominant bands at 1.46 ± 0.04 eV and 1.58 ± 0.04 eV (Table ), which correspond to the VDE (labeled X) of their ground-state structures. The spectral bands without vibrationally resolved structures prevent us from directly measuring the ground-state ADE, which can be alternatively estimated by the intersection of a line drawn along the rising edge of the main band with the binding energy axis. The ADE value of RhC^– and RhN₂ ^– is evaluated to be 1.40 ± 0.05 eV and 1.51 ± 0.04 eV (Table ). The additional feature marked X″ in Figure a is attributed to an excited state of RhC^–, consistent with prior reports. For RhCN₂ ^– (Figure c), two peaks are observed at 1.70 ± 0.03 eV and 2.22 ± 0.01 eV. However, the higher-energy peak is very close to the detachment threshold at 532 nm, resulting in an incomplete peak shape. To obtain the complete spectrum, measurements were repeated at 355 nm (Figure d). The full spectral profile confirms that the peak at 2.22 eV is the dominant feature.

2.

Photoelectron spectra of (a) RhC^– and (b) RhN₂ ^– at 532 nm and spectra of RhCN₂ ^– at (c) 532 and (d) 355 nm. X peak represents the VDE of the ground-state structure, X′ is the contribution of coexisting isomers, and the X″ peak comes from the excited state of RhC^–.

1. Comparisons between the Theoretical and Experimental VDEs and ADEs of RhC^–, RhN₂ ^–, and RhCN₂ ^–, along with States and Symmetries.

			VDE/eV		ADE/eV
cluster	state	symmetry	expt	calc	expt	calc
RhC–	1Σ+	C ∞v	1.46(4)	1.41	1.40(5)	1.39
RhN2 –	1Σ+	C ∞v	1.58(4)	1.60	1.51(4)	1.54
RhCN2 – (I)	1 A′	C s	2.22(1)	2.33	2.14(1)	2.19
RhCN2 – (II)	1 A′	C s	1.70(3)	1.61	1.66(3)	1.58

^aNumbers in parentheses represent the uncertainty of the last digit.

Determination of the low-lying isomer structures underlying the observed spectra involved geometry optimizations at the PBE/6-311+G­(d)/aug-cc-pVTZ-PP level for RhC^–, RhN₂ ^–, and RhCN₂ ^–, with subsequent high-level CCSD­(T) single-point calculations performed to refine the relative energies. Ground-state structures and selected low-lying isomers for RhC^–, RhN₂ ^–, and RhCN₂ ^– anions are shown in Figure S2. The lowest-lying isomer of RhC^– possesses C_∞v symmetry with a ¹Σ⁺ electronic state. For RhN₂ ^–, in the most stable isomer (labeled A), N₂ is bound to the Rh atom in an end-on C _∞v configuration. The calculated VDE/ADE values for the ground-state structures of RhC^– and RhN₂ ^– are 1.41/1.39 eV and 1.60/1.54 eV, respectively, which agree well with the experimental values (1.46 ± 0.04/1.40 ± 0.05 eV and 1.58 ± 0.04/1.51 ± 0.04 eV) (Table ). In the most stable isomer (I) of RhCN₂ ^–, N₂ is completely cleaved, forming an N–Rh–C–N bent chain configuration. As listed in Table , the calculated VDE and ADE values of I are 2.33 and 2.19 eV, respectively, in excellent agreement with the experimental results (2.22 ± 0.01 and 2.14 ± 0.01 eV). Isomer II (ΔE = +0.11 eV relative to isomer I) features complete cleavage of both the NN triple bond and the Rh–C bond, forming a bent chain-like Rh–N–C–N structure. The calculated VDE and ADE values for isomer II are 1.61 and 1.58 eV, respectively. This favorable agreement with the experimental values of the X′ band (VDE: 1.70 ± 0.03 eV, ADE: 1.66 ± 0.03 eV) strongly supports its assignment. Both types of low-energy isomers (I and II) exhibit C_s symmetry with a ¹ A′ electronic state (Table ). In contrast, the corresponding structures of other high-spin multiplicities and other isomers possess substantially higher relative energies (Table S1 and Figure S2), which are thus undetectable under the experimental conditions.

To further validate the consistency between experiments and theory, we compared the experimental photoelectron spectra of RhC^–, RhN₂ ^–, and RhCN₂ ^– with the corresponding simulated spectra, as shown in Figure . The experimental photoelectron spectra of RhC^– and RhN₂ ^– are characterized by single, resolved peaks, with simulations showing excellent agreement with the experimental data (Figures a,b). For RhCN₂ ^–, the observation of four distinct peaks (1.70, 2.22, 2.65, and 2.87 eV) indicates the presence of two coexisting isomers. Specifically, the peaks at 2.22 and 2.65 eV align with the simulated spectrum of ground-state isomer I, while those at 1.70 and 2.87 eV correspond to low-energy isomer II (Figure S3), supporting the coexistence of both structures. Quantitative analysis of the peak intensities reveals that isomer I is approximately ten times more abundant than isomer II (Figure c). The potential energy curve reveals that the interconversion between the two isomers requires overcoming an energy barrier of approximately 1.24 eV (Figure S4).

3.

Experimental photoelectron spectrum (red mark) and simulated spectra (blue mark) of (a) RhC^–, (b) RhN₂ ^–, and (c) RhCN₂ ^–. The spectrum of RhCN₂ ^– is a combination of two low-energy isomers I and II, where the ratio of I and II is approximately 10:1, and the relative energy (eV) is listed in parentheses.

Analysis of Bonding Characteristics and Electronic Structure

Following the experimental identification of the structure, we proceeded to investigate the bonding characteristics of the RhCN₂ ^– anion. The corresponding AdNDP and Frontier MO analyses for the two isomers are presented in Figures and S5–S7. The bonding analysis of structure I reveals an NRh–CN connectivity. This bonding pattern originates from the weakening of an initial Rh–C triple bond, which facilitates the simultaneous formation of a RhN and a CN triple bond. Consequently, the Rh–C interaction is reduced to a single σ bond (2c-2e), while the Rh–N and C–N linkages are formal triple bonds, each comprising one σ and two π components (all 2c–2e). Furthermore, the electronic structure involves three 4d lone pairs at the Rh center and one 2s lone pair on each N atom. Similarly, cleavage of the Rh–C bond in structure II results in a reorganization of the remaining bonds: the terminal CN triple bond, RhN a double bond, and central C–N single bond, yielding the chain configuration RhN–CN (Figure S5). In summary, the dinitrogen cleavage drives a significant transformation of the Rh–C bond, weakening it from a triple bond to a single bond or cleaving it entirely. This change both accompanies and facilitates the establishment of a stable C–N coupling. This perspective is supported by the orbital and component analyses presented in Tables S4 and S5.

4.

Bonding analysis of the ground-state structure (I) for the RhCN₂ ^– anion using ADNDP. The ON values represent the occupancy numbers of the electron (|e|), and the isosurface values are set to 0.03 a.u.

Following this, a detailed discussion and comparison were conducted to elucidate the nitrogen activation effect of triple-bonded RhC^– and its relative efficacy against the single Rh^– anion. The bond length analysis indicates a critical structural change: the Rh–C bond elongated from 1.636 Å to 1.958 Å upon N₂ cleavage and even broke (Figure a). The stepwise change in bond orders is consistent with the Rh–C triple bond (bond order: 2.813) to a single bond (bond order: 0.814) or even non-bonding characteristics. ELF Analysis (Figure b) identifies pronounced electron aggregation around the Rh center in RhC^–, marking them as prospective active sites. Subsequently, a significant weakening of the Rh–C interaction in the corresponding region (outlined by a yellow dashed box) is observed after nitrogen cracking, which is further validated by Localized Orbital Locator (LOL) analysis (Figure S8). Critically, this attenuation directly facilitates the electron density redistribution from the RhC moiety to the two N atoms, thereby promoting the C–N coupling. However, the Rh^– anion facilitates nitrogen activation solely via chemisorption. Frontier MO analysis (Figures c and S9) and their components (Table S6) demonstrate that the fixation mechanism involves a dual electronic mechanism: synergistic activation of N₂ is achieved by π-backdonation (Orbital 1) from metal Rh atoms and σ-donation (Orbital 2) from the N₂ molecule. In contrast, the presence of carbon-based triple bonds confers a fundamental improvement in the efficacy of nitrogen activation.

5.

Bond analysis of RhC^–, RhN₂ ^–, and RhCN₂ ^–. (a) The bond lengths, bond orders (in red parentheses), and (b) ELF of the RhC^– and RhCN₂ ^–, as well as (c) the bond length and two typical orbitals of the RhN₂ ^– anion. All the bond lengths are given in Å, and the isovalue for orbitals is 0.03 a.u.

Given the central role of charge transfer in nitrogen reduction, we focus on the NPA of key species: RhC^–, RhN₂ ^–, and RhCN₂ ^– isomers I and II (Table ). The analysis reveals that while both Rh^– and RhC^– activate N₂ through charge transfer, the extent is markedly greater for RhC^– (0.929 for RhCN₂ ^– isomers I vs. 0.568 for Rh^–), with structure II showing the highest transfer (1.076). These quantified values align with the electron density patterns seen in the ELF plots (Figures b and S10), providing consistent evidence for the proposed mechanism. Comparison of isomers I and II reveals that charge transfer scales with the degree of Rh–C bond dissociation. We constructed an adsorption complex (¹IV) analogous to RhN₂ ^– and examined its electronic properties (Figures S11 and S12) to test the conclusion. Although the Rh–C bond in ¹IV still retains triple-bond characteristics, the bond length is significantly stretched (from 1.636 Å to 1.690 Å), the bond order decreases (from 2.813 to 2.572), and the charge transfer amount is 0.289 (Table ), which is consistent with our prediction and confirms the previous conclusion. The relationship between bonding and charge transfer is demonstrated in Figure , which plots the Rh–C bond length, bond order, and fragment charges for RhC^–, ¹IV, I, and II. A consistent trend is observed: a weakened Rh–C bond is associated with a depleted charge on the RhC unit and an enhanced charge transfer to the N₂ moiety. Consequently, the Rh–C triple bond can be conceptualized as an electron reservoir. Weakening this “gate” directly modulates the release of electrons from the reservoir.

2. NPA Charges of RhC^–, RhN₂ ^–, and RhCN₂ ^–, where N1 is Bonded to Rh and N2 Is another Nitrogen Atom, ¹IV Represents the Structure of RhC^– Chemisorbed N₂, and the Spin Multiplicity Is Singlet.

species	NPA charge
	Rh	C	N1	N2
RhC–	–0.143	–0.857	--	--
RhN2 –	–0.432	--	–0.485	–0.083
RhCN2 – (I)	–0.046	–0.025	–0.396	–0.533
RhCN2 – (II)	–0.254	0.330	–0.562	–0.514
RhCN2 – (1IV)	–0.550	–0.162	–0.154	–0.135

6.

Bond lengths, bond orders, and NPA charge of RhC^–, ¹IV, I, and II. The black and red balls represent the bond length and bond order of Rh–C, while the blue and green triangles represent the charge distribution of RhC and N₂ units, respectively. All the bond lengths are given in Å.

Conclusion

In summary, this study establishes the functional role of a metal–carbon RhC triple bond as an electron reservoir in redox. We theoretically identified a triple-bond motif in the RhC^– anion and experimentally characterized it via laser vaporization supersonic cluster source gas-phase synthesis, combined with mass spectrometry and photoelectron spectroscopy. Furthermore, the investigation revealed the dinitrogen activation capability of the RhC^– anion. Quantum chemical calculations confirmed that the RhC^–-mediated dinitrogen cleavage yields two products characterized by complete cleavage of the NN triple bond alongside the formation of a stable CN triple bond. Electronic structure analysis further established the operative mechanism: the RhC triple bond in RhC^– functions as an electron reservoir, thereby enabling the release of stored electrons through cleavage of the Rh–C bond. Critically, this electron-reservoir mechanism, where multiple-bond cleavage directly fuels redox chemistry, provides a paradigm for designing catalytic systems that harness bond activation for small-molecule transformations.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.6c01036.

• Detailed analysis of mass spectra; optimized structures of the low-energy isomers for RhC^–, RhN₂ ^–, and RhCN₂ ^–; simulated spectra of RhCN₂ ^–; AdNDP analysis of II and the configuration of RhC^– chemisorbed N₂; ELF and LOL of RhC^–, RhN₂ ^–, and RhCN₂ ^–; and molecular orbital and orbital composition analysis of RhN₂ ^– and RhCN₂ ^– (PDF)

S.D.: Data curation, investigation, validation, formal analysis, and writingoriginal draft. J.L. and S.W.: Validation and formal analysis. H.W., W.X., and L.J.: Formal analysis. H.X.: Funding acquisition, writingreview and editing, supervision, conceptualization, and project administration. H.Z.: Supervision, writingreview and editing, and conceptualization. S.C.: Conceptualization, supervision, project administration, funding acquisition, and writingreview and editing.

The authors declare no competing financial interest.