One-electron bonds in copper–aluminum and copper–gallium complexes

Odd-electron bonds have unique electronic structures and are often encountered as transiently stable, homonuclear species. In this study, a pair of copper complexes supported by Group 13 metalloligands, M[N((o-C6H4)NCH2PiPr2)3] (M = Al or Ga), featuring two-center/one-electron (2c/1e) σ-bonds were synthesized by one-electron reduction of the corresponding Cu(i) ⇢ M(III) counterparts. The copper bimetallic complexes were investigated by X-ray diffraction, cyclic voltammetry, electron paramagnetic spectroscopy, and density functional theory calculations. The combined experimental and theoretical data corroborate that the unpaired spin is delocalized across Cu, M, and ancillary atoms, and the singly occupied molecular orbital (SOMO) corresponds to a σ-(Cu–M) bond involving the Cu 4pz and M ns/npz atomic orbitals. Collectively, the data suggest the covalent nature of these interactions, which represent the first examples of odd-electron σ-bonds for the heavier Group 13 elements Al and Ga.

Odd-electron s-bonds, where the electrons are delocalized between two atoms, can occur as two-center/one-electron (2c/1e) or two-center/three-electron (2c/3e) interactions. Proposed by Pauling in 1931, 1 odd-electron s-bonds have garnered attention because of their fundamental importance to chemical bonding and their relationship to radical species generated during oxidative stress in biological systems. [2][3][4][5][6][7][8][9][10][11][12][13][14] Examples of compounds exhibiting odd-electron bonding are typically homonuclear (like H 2 + , He 2 + , and alkali metal dimers) and transiently stable, limiting them to spectroscopic characterization. 1,11,[15][16][17][18] The rst solid-state structure of a formally one-electron sbond was a tetraphosphabenzene species (Fig. 1a) which was formed by the coupling of two diphosphirenyl radicals. 19 Following this discovery, the formation of discrete 2c/1e sbonds, where the odd-electron is delocalized between two homonuclear main group centers, was reported for B$B and then extended to P$P. 8,17,20 Of note, the rst solid-state structure of a B$B compound was reported in only 2014 (Fig. 1b). 21 Examples of 2c/1e s-bonds between the heavier Group 13 congeners are even more lacking because of the greater propensity for their unpaired spins to couple, forming larger more stable clusters. 8 To our knowledge, there are only three structurally characterized examples of odd-electron bonds for the heavy Group 13 atoms, 22 and these examples are all homonuclear p-radicals (Fig. 1c). [23][24][25][26] Heteronuclear odd-electron s-bonds are also rare. The Cu(TPB) complex, where TPB is a trisphosphinoborane, is the single structural example of a 2c/1e bond between heteroatoms (Fig. 1d). 27 The authors described the bonding as Cu$B, where the unpaired electron is heavily polarized toward B. A theoretical study predicted that such a bond would also exist between Cu and Al, but no heavier analogues of Cu(TPB) have been synthesized to date. 28 Furthermore, the heavier Group 13 elements by virtue of their lower electronegativity compared to B should facilitate greater covalent interactions with the Cu center.
Hence, we sought to target formally zerovalent Cu complexes supported by Al(III) or Ga(III) as an extension of the previously reported isoelectronic nickelate species and Cu(TPB). 29 Herein, we describe the synthesis, structure, spectroscopic characterization, and DFT calculations of cationic [CuML] + complexes (L ¼ [N((o-C 6 H 4 )NCH 2 P i Pr 2 ) 3 ] 3À ; M ¼ Al and Ga) as well as their one-electron reduced metalloradical counterparts that feature discrete 2c/1e bonds.

Synthesis and cyclic voltammetry
Initial synthetic attempts to produce cationic [CuML] + complexes via metalation of ML with a copper halide followed by halide abstraction produced intractable mixtures. Hence, we sought an alternative Cu(I) precursor that eschews the halide and has labile ligands, such as [Cu(COD) 2 ]A (COD ¼ 1,5-cyclooctadiene and A ¼ counteranion). This cation has received little attention as a precursor to low-coordinate copper complexes, which may stem from the known salts (A ¼ ClO 4 À , NO 3 À , SbF 6 À , etc.) lacking sufficient solubility in weakly coordinating solvents. 30 ) and CuGaL (2 red ), respectively. However, high-yielding syntheses of 1 red and 2 red were achieved by rst metalating ML with CuBr to generate an in situ Cu(I) halide complex followed by addition of KC 8 in THF at À78 C (Scheme 1). The resultant reductions produced deeply colored maroon (1 red ) and dark red (2 red ) solutions, from which the products were puried by removing all volatiles and then extracting into benzene.  Cl 12 ] were achieved by the oxidation of 1 red and 2 red , respectively, with Ag 2 [B 12 Cl 12 ]. [37][38][39] Crystals of the neutral copper species were grown from saturated Et 2 O at À30 C (1 red ) or toluene/pentane vapor diffusion at room temperature (2 red ). The solid-state structures of the four copper-Group 13 complexes are shown in Fig. 2, and the structural metrics are summarized in Table 1 43,44 and anionic Co(-I) 45 bimetallics, which have r values <1, because the localized cationic change on the Cu(I) center should render it a weaker donor. The Cu center is nearly trigonal planar with minimal perturbation due to the Group 13 ion as both P (;P-Cu-P) and P (;N eq ÀMÀN eq ) approach 360 . For comparison, mononuclear [Cu(LH 3 )]BAr F exhibits nearly perfect trigonal symmetry in the solid state ( P (;P-Cu-P) ¼ 359.96 (2) and Cu-to-P3-plane ¼ 0.027Å, Table  S2 and Fig. S2 †) Cl 12 ], which indicates that all three complexes possess a Cu(I) d 10 center.

X-ray crystallography
Aer reduction to 1 red and 2 red , the Cu-M bond contracts signicantly by 0.094Å (r ¼ 1.00) and 0.120Å (r ¼ 0.97), respectively, indicating an increase in the metal-metal bonding. Of relevance, bonding between copper and a Group 13 centers have been structurally characterized within clusters 46 and unsupported complexes, [47][48][49] although these prior examples do not feature odd-electron bonds. In comparison to 1 red and 2 red , the known Cu-Al 49 and Cu-Ga 47-49 complexes have lower r values of 0.90-0.91. The greater r values for 1 red and 2 red are expected considering that their bonding involves only one electron. Another indication of a metal-metal interaction is the pyramidalization of the Group 13 center as indicated by the P (:N eq -M-N eq ) values of 355.01(1) and 350.36(4) for 1 red and 2 red , respectively, as both the M-to-(N 3 -plane) and M-N ap distances increase as the M center moves closer to Cu. 50 The local coordination environment around the Cu center also changes, with the slight lowering of the Cu center into the P 3plane and the contraction of the Cu-P bonds upon reduction to the {Cu-M} 11 core. The Cu-P bond contraction is consistent with an increased Cu electron density that more strongly pbackbonds into the phosphine ligands. Moreover, the signicant perturbation of the Group 13 centers suggests their direct participation in stabilizing the reduced {Cu-M} 11 core, presumably via bonding interactions to the Cu center.

EPR spectroscopy
To gain insight into the radical nature of the one-electron bonds, room-temperature EPR spectra were collected for solutions of 1 red and 2 red in a 1 : 1 solvent mixture of 2-MeTHF and toluene (Fig. 3). Complex 1 red gave rise to a complex signal centered at g $ 2.0, which corresponds to a highly overlapped manifold of hyperne lines that can be decomposed into a leading 1 : 1:1 : 1:1 : 1 sextet due to hyperne coupling (hfc) of the electronic spin S ¼ 1/2 with 27 Al nuclei (I ¼ 5/2, 100% natural abundance) and additional splitting of each of these hyperne transitions into two 1 : 1:1 : 1 quartets due to additional coupling to the isotopes 63 29 The distribution of the unpaired spin density across the Cu and Group 13 valence orbitals in the {Cu-M} 11 core can be evaluated by comparing the corresponding experimental A iso values  with tabulated reference values (a iso 0 ) for each specic elemental orbital. 51 Using the reference values for a localized valence s-based spin for Al (3911 MHz) and Ga (12,210 MHz), the 27 Al 3s and the 69 Ga 4s character in 1 red and 2 red , respectively, were found to be small at 6.9 and 9.8%, respectively. 51 A similar calculation was performed for the 63 Cu 4s orbital (a iso 0 ¼ 5995 MHz) resulting in even smaller contributions of 2.9 and 2.2% in 1 red and 2 red , respectively. The frozen EPR spectra of 1 red and 2 red were also collected ( Fig. S43 and S46 †). However, only in the former case were we able to model the axial signal and thus extract the 27 , 64] MHz, was then compared to that of a fully localized spin in an Al 3p z orbital, where b 0 ¼ [À83, À83, 166] MHz. We found that the 27 Al 3p z character is much greater at 38%. Similarly, the 63 Cu 4p z contribution was found to be 19.9%. 52 The total estimated spin density between Cu and Al is ca. 67%, wherein the majority of the unpaired spin likely resides in the bonding axis between the two metals. Only a handful of formal Cu(0) complexes are known in the literature, and they exhibit a range of isotropic A( 63 Cu) values. The complexes Cu(PMe 3 ) 3 and Cu(TPB) have A( 63 Cu) values of 120 and 191 MHz, respectively, which compare well to those measured for 1 red and 2 red . 27,53,54 For other literature complexes, the A( 63 Cu) values are signicantly smaller (<30 MHz) because the unpaired spin is strongly delocalized over the ligand. For example, Cu(Me 2 -cAAC) 2 (cAAC ¼ cyclic (alkyl)(amino)carbene) displayed a small A( 63 Cu) coupling of 22.2 MHz, and the DFTcalculated spin density revealed that the majority of the spin density resides in the C carbene -N p-bond. 55 Similarly, the unpaired spin in Cu(B 2 P 2 ) (B 2 P 2 ¼ diphosphine-diboraanthracene) was proposed to reside on the redox-active ligand core as no Cu hyperne was discerned. 56 Further comparison can be made with hfc to the Group 13 supporting ions with 27

Theoretical computations
To further elucidate the electronic structure of these species, theoretical calculations were performed using Kohn-Sham density functional theory (KS-DFT). Geometry optimizations were performed on the structures without truncation using the M06-L functional 62 with def2-series basis sets 63 and strong agreement was found between the optimized geometries and the X-ray structures (Table S3 and S4 †). Based on the in depth study by Hedegård et al., 64 the PBE0 (ref. 65 and 66) functional in conjunction with def2-TZVP and def2/J basis sets 67 were used to determine the EPR parameters with spin-orbit coupling included. The DFT-calculated EPR parameters also matched well to the experimental g and isotropic A values, as well as the anisotropic A tensors, when available (Fig. 3, Tables S12 and S13 †). The good agreement further supports the validity of the calculated electronic structures as models for the experimental species.
Examination of the molecular orbitals (MOs) of cationic species [Cu(LH 3 )] + , [1] + , and [2] + revealed ve doubly lled dorbitals as expected for trigonal Cu(I) species. In both bimetallics, the lowest unoccupied molecular orbital (LUMO, Fig. 4) was found to be a hybrid MO containing large contributions from the empty Cu 4p z and Group 13 ns and np z -orbitals. 68 The neutral radical species 1 red and 2 red each contain a singly occupied molecular orbital (SOMO) that is similar to the LUMO of the cations, suggesting that one-electron reduction of the cations results in occupancy of the LUMO. The SOMO of 1 red contains major contributions from Cu (8.4%), Al (33.8%), and P (35.2%). Similarly, the SOMO for 2 red possesses contributions from Cu (12.7%), Ga (32.0%), and P (35.2%).
The contributions of the Group 13 atoms to the SOMOs have signicant s-character (ca. 19% for each species), while the Cu center only possesses contributions from the 4p z . The s/p-mixing predicted to be present at both Al and Ga likely contributes to the pyramidalization of the Group 13 center. This result is consistent with the greater hfc values observed in the EPR spectra (vide supra). Notably, the P contribution nearly doubles upon reduction (from ca. 17% to 35% for both 1 red and 2 red ), further indicating the importance of the ancillary ligand in supporting the more electron-rich Cu center.
The Mulliken spin density of 1 red and 2 red revealed delocalization across Cu, Al/Ga, P, and N atoms as visualized through spin-density plots (Fig. 4, Table S11 †). Comparison of spin densities between the two metal pockets CuP 3 : MN 3 revealed a ratio of 1.0 : 1.2 suggesting the unpaired electron is almost equally delocalized across the two binding pockets. For 1 red , spin densities were calculated to be 0.18 on Cu and 0.52 on Al, with a moderate amount distributed throughout the ancillary atoms (0.09 per P and 0.01 per N eq ), and these values are in good agreement to those estimated by EPR spectroscopy (vide supra). The calculated spin densities for 2 red were found to be similar to that of 1 red with 0.24 on Cu, 0.44 on Ga, and the remainder on the supporting donor atoms (0.08 per P and 0.03 per N eq ).
By considering the SOMOs and noting the extent of spin delocalization in the spin density plots, the radical species 1 red and 2 red can therefore best be described as possessing sbonding interactions between the Cu (4p z ) and Al/Ga (np z /ns) orbitals, as well as a p-backbonding interaction between Cu and the three P atoms. 53,54, 69 The covalent nature of the s-(Cu-M) bonds is supported by the signicant contraction of the metalmetal bond lengths in the solid-state structures and the nearly equal spin delocalization between the two metal pockets, which matches well with the experimental EPR hfc values.

Conclusion
In summary, two cationic copper-Group 13 complexes were synthesized and their subsequent reduction provided access to unusual 17 e À {Cu-M} 11 complexes. These radical species are characterized by short Cu-M bond lengths and exhibit signicant hfc to Cu, M, and P atoms, collectively consistent with the highly covalent nature of SOMOs that involve both metal centers and the ancillary ligand donor atoms. These neutral radical {Cu-M} 11 species possess rarely observed 2c/1e s-bonds and are the rst report of any such bond for the heavier elements Al and Ga. Along with the previously reported Cu(TPB), 1 red and 2 red complete a triad of copper-Group 13 complexes that demonstrate an increase in Cu bonding character and Cu-M covalency when traversing down the group in the order B < Al < Ga. 27 Investigation of the metalloradical reactivity of these bimetallics is ongoing.

Data availability
Additional data for synthesis, characterization, and DFT calculations are available in the ESI. † The combined XYZ coordinates of DFT-optimized geometries have been uploaded as ESI. †

Author contributions
BJG and MVV performed the experimental work supervised by CCL. TRS performed the theoretical calculations supervised by LG. MJD assisted with the EPR data collection, and EB assisted with the EPR analysis and interpretation. VGY provided X-ray support. The original dra was written by BJG with guidance from CCL, and all authors edited the manuscript.

Conflicts of interest
There are no conicts to declare.