The reactivity of the reduced Zr/Co and Ti/Co complexes (THF)Zr(MesNPiPr2)3CoN2 (1, Mes = 2,4,6-trimethylphenyl) and (THF)Ti(XylNPiPr2)3CoN2 (7, Xyl = 3,5-dimethylphenyl) toward diaryl ketones is explored in an effort to gain mechanistic insight into C═O bond cleavage processes. Complex 1 reacts with 4,4′-dimethoxybenzophenone to generate ((p-OMeC6H4)2CO)Zr(MesNPiPr2)3CoN2 (2), which exists as a mixture of valence tautomers in solution that interconvert via electron transfer from Co–I to the Zr-bound ketone in 2S to form a Zr-bound ketyl radical in 2T. The geometry of 2 in the solid state is most consistent with the singlet ketone adduct tautomer 2S. Upon removal of the Co-bound N2 under vacuum, complex 2 cleanly coverts to the μ-oxo carbene product (η2-MesNPiPr2)Zr(MesNPiPr2)2(μ-O)Co═C(C6H4p-OMe)2 (5) at room temperature in solution. A diamagnetic intermediate, tentatively assigned as ketone-bridged species (η2-MesNPiPr2)Zr(MesNPiPr2)2Co(μ2,η1η2-OC(p-OMeC6H4)2) (6), is observed spectroscopically during the transformation of 1 to 5. Similar reactions between the Ti/Co analogue 7 and diaryl ketones reveal no evidence for electron-transfer to form triplet ketyl radical species. Complex 7 reacts with 4,4′-dimethoxybenzophenone to afford diamagnetic ((p-OMeC6H4)2CO)Ti(XylNPiPr2)3CoN2 (8). In contrast, addition of benzophenone to 7 under N2 generates a mixture of (η2-XylNPiPr2)Ti(XylNPiPr2)2Co(η2-OCPh2) (9) and (Ph2CO)Ti(XylNPiPr2)3CoN2 (10) in solution, and C3-symmetric 10 is found to be favored in the solid state. Complex 9 can be generated exclusively and isolated in the absence of N2. Ti/Co complexes 8–10 are thermally stable and do not undergo C═O bond cleavage even at elevated temperature, in stark contrast to their Zr/Co congeners.

Heterometallic multiple bonds between niobium and other transition metals have not been reported to date, likely owing to the highly reactive nature of low-valent niobium centers. Herein, a C3-symmetric tris(phosphinoamide) ligand framework is used to construct a Nb/Fe heterobimetallic complex Cl–Nb(iPrNPPh2)3Fe−Br (2), which features a Fe→Nb dative bond with a metal–metal distance of 2.4269(4) Å. Reduction of 2 in the presence of PMe3 affords Nb(iPrNPPh2)3Fe–PMe3 (6), a compound with an unusual trigonal pyramidal geometry at a NbIII center, a Nb≡Fe triple bond, and the shortest bond distance (2.1446(8) Å) ever reported between Nb and any other transition metal. Complex 6 is thermally unstable and degrades via P–N bond cleavage to form a NbV═NR imide complex, iPrN═Nb(iPrNPPh2)3Fe−PMe3 (9). The heterobimetallic complexes iPrN═Nb(iPrNPPh2)3Fe−Br (8) and 9 are independently synthesized, revealing that the strongly π-bonding imido functionality prevents significant metal–metal interactions. The 57Fe Mössbauer spectra of 2, 6, 8, and 9 show a clear trend in isomer shift (δ), with a decrease in δ as metal–metal interactions become stronger and the Fe center is reduced. The electronic structure and metal–metal bonding of 2, 6, 8, and 9 are explored through computational studies, and cyclic voltammetry is used to better understand the effect of metal–metal interaction in early/late heterobimetallic complexes on the redox properties of the two metals involved.

Two cobalt complexes containing coordinated N-heterocyclic phosphenium (NHP+) ligands are synthesized using a bidentate NHP+/phosphine chelating ligand, [PP]+. Treatment of Na[Co(CO)4] with the chlorophosphine precursor [PP]Cl (1) affords [PP]Co(CO)2 (2), which features a planar geometry at the NHP+ phosphorus center and a short Co–P distance [1.9922(4) Å] indicative of a Co═P double bond. The more electron-rich complex [PP]Co(PMe3)2 (3), which is synthesized in a one-pot reduction procedure with 1, CoCl2, PMe3, and KC8, has an even shorter Co–P bond [1.9455(6) Å] owing to stronger metal-to-phosphorus back-donation. The redox properties of 2 and 3 were explored using cyclic voltammetry, and oxidation of 3 was achieved to afford [[PP]Co(PMe3)2]+ (4). The electron paramagnetic resonance spectrum of complex 4 features hyperfine coupling to both 59Co and 31P, suggesting strong delocalization of the unpaired electron density in this complex. Density functional theory calculations are used to further explore the bonding and redox behavior of complexes 2–4, shedding light on the potential for redox noninnocent behavior of NHP+ ligands.

•Three new nickel coordination compounds have been synthesized and characterized.•A Cp2Ni2 dimer bridged by a reduced N-heterocyclic phosphido ligand is presented.•PPh3 promotes migration of Cp− from Ni to the N-heterocyclic phosphenium center.Treatment of the N-heterocyclic chlorophosphine precursor (PPP)Cl (1) with two equivalents of nickelocene (NiCp2) affords the phosphorus-bridged dimer [(μ-PPP)Ni2Cp2]Cl (2). In contrast, an equimolar mixture of 1 and NiCp2 in the presence of PPh3 generates a different product, (PP(C5H5)P)NiCl2 (3), in which a cyclopentadienyl anion has migrated to the N-heterocyclic phosphenium center. The phosphorus-bound Cp ring in complex 3 has undergone a [1,5]-hydride shift to afford a vinylic C5H5− ring, and can be subsequently deprotonated to produce [(PP(C5H4)P)NiCl] (4).The coordination of an N-heterocyclic chlorophosphine ligand to nickel is explored using a nickelocene precursor, uncovering a reaction in which a cyclopentadienyl ring migrates to a cationic phosphenium center.Download high-res image (94KB)Download full-size image

To understand the metal–metal bonding and conformational flexibility of first-row transition metal heterobimetallic complexes, a series of heterobimetallic Ti/M and V/M complexes (M = Fe, Co, Ni, and Cu) have been investigated. The titanium tris(phosphinoamide) precursors ClTi(XylNPiPr2)3 (1) and Ti(XylNPiPr2)3 (2) have been used to synthesize Ti/Fe (3), Ti/Ni (4, 4THF), and Ti/Cu (5) heterobimetallic complexes. A series of V/M (M = Fe (7), Co (8), Ni (9), and Cu (10)) complexes have been generated starting from the vanadium tris(phosphinoamide) precursor V(XylNPiPr2)3 (6). The new heterobimetallic complexes were characterized and studied by NMR spectroscopy, X-ray crystallography, electron paramagnetic resonance, and Mössbauer spectroscopy, where applicable, and computational methods (DFT). Compounds 3, 4THF, 7, and 8 are C3-symmetric with three bridging phosphinoamide ligands, while compounds 9 and 10 adopt an asymmetric geometry with two bridging phosphinoamides and one phosphinoamide ligand bound η2 to vanadium. Compounds 4 and 5, on the other hand, are asymmetric in the solid state but show evidence for fluxional behavior in solution. A correlation is established between conformational flexibility and metal–metal bond order, which has important implications for the future reactivity of these and other heterobimetallic molecules.

A chelating diphosphine ligand with a central N-heterocyclic phosphenium cation (NHP+) has been used to explore the coordination chemistry of NHPs with nickel. Treatment of the chlorophosphine precursor [PPP]Cl (1) with stoichiometric Ni(COD)2 affords (PPP)NiCl (8), which is best described as a NiII/NHP− phosphido complex formed via oxidative addition of the P–Cl bond. In contrast, treating [PPP]Cl (1) with excess Ni(COD)2 results in a mixture of the trimetallic complex (PPP)2Ni3Cl2 (9) and the reduced NHP-bridged dimer [(PPP)Ni]2 (10). Compound 9 is found to be a NiIINiIINi0 complex in which the two NHP ligands act as bridging NHP− phosphidos, while complex 10 is a NiINiI complex that is highly delocalized throughout the symmetric Ni2P2 core. In contrast, the reaction of [PPP][PF6] (11) with Ni(COD)2 affords an asymmetrically-bridged dication [(PPP)Ni]2[PF6]2 (12), which is found to contain two bridging NHP+ cations bridging two Ni0 centers. Comproportionation of 10 and 12 affords monocationic [(PPP)Ni]2[PF6] (13), completing the redox series. Nickel complexes 8–10 and 12 are largely similar to their Pd and Pt analogues, but a paramagnetic monocation such as 13 was not observed in the Pd and Pt case. Computational studies lend further insight into the electronic structure and bonding in complexes 8–10 and 12–13, and further support the potential redox non-innocent properties of NHP ligands.

The reactivity of a reduced heterobimetallic Co−I/ZrIV complex, (tBuNC)Co(iPr2PNMes)3Zr(THF) (2), with a series of azido and diazo reagents is explored to demonstrate the feasibility of facilitating two-electron redox processes at a formally d0 Zr(IV) center using the appended Co fragment exclusively as an electron-reservoir. Addition of mesityl or adamantyl azide to 2 affords the terminal (tBuNC)Co(iPr2PNMes)3ZrNMes (3) and bridging (tBuNC)Co(iPr2PNMes)2(μ-NAd)Zr(iPr2PNMes) (4) CoI/ZrIV imido products, respectively. Similarly, diphenyldiazomethane reacts with 2 to afford the terminal Ph2CN22−-bound product (tBuNC)Co(iPr2PNMes)3ZrN–NCPh2 (5) via a two-electron oxidation of the Co center. Thermolysis of 5 results in a structural rearrangement to the diazomethane-bridged isomer (tBuNC)Co(iPr2PNMes)2(μ-N2CPh2)Zr(iPr2PNMes) (6). In contrast, treatment of 2 with 0.5 equivalents of the conjugated diazo reagent ethyl diazoacetate affords a tetranuclear ZrIV/Co0 complex, (tBuNC)Co(iPr2PNMes)3Zr(μ2–κ1-O-η2-N,N-OC(OEt)CHN2)Zr(MesNPiPr2)3Co(CNtBu) (7), bridged through enolate and η2-bound diazo functionalities.

Herein we report the synthesis and characterization of new heterobimetallic Zr/Co complexes incorporating chiral phosphinoamide ligands. The chiral phosphinoamine ligand (R-PhCH3CH)NHPiPr2 (1) is used to synthesize a tris(phosphinoamide) Zr metalloligand ClZr((R-PhCH3CH)NPiPr2)3 (2), which can be further treated with CoI2 to afford the chiral Zr/Co heterobimetallic complex ClZr((R-PhCH3CH)NPiPr2)3CoI (3). Complex 3 can be reduced by two electrons to produce Zr((R-PhCH3CH)NPiPr2)3CoN2 (4). In an alternative strategy, a mixed-ligand derivative featuring two achiral phosphinoamide ligands and one chiral ligand was targeted. Thermolysis of Zr(NMe2)4 with iPr2PNHXyl in toluene produces (Me2N)2Zr(iPr2PNXyl)2 (5, Xyl = 3,5-dimethylphenyl), which can be transformed into I2Zr(iPr2PNXyl)2 (6) via in situ treatment with Me3SiI. A third chiral phosphinoamide ligand can then be installed to generate IZr((R-PhCH3CH)NPiPr2)(XylNPiPr2)2 (7). Treatment of complex 7 with CoI2 affords IZr((R-PhCH3CH)NPiPr2)(XylNPiPr2)2CoI (8). Complexes 3, 7, and 8 were structurally characterized and a preliminary screening of the catalytic hydrosilylation of acetophenone with reduced complex 4 revealed a very poor yield and essentially no enantioselectivity.Chiral phosphinoamide ligands are used to construct chiral heterobimetallic Zr/Co complexes with either three linking chiral ligands or a 2:1 ratio of achiral to chiral ligands.

To explore metal–metal multiple bonds between first row transition metals, Ti/Co complexes supported by two phosphinoamide ligands have been synthesized and characterized. The Ti metalloligand Cl2Ti(XylNPiPr2)2 (1) was treated with CoI2 under reducing conditions, permitting isolation of the Ti/Co complex [(μ-Cl)Ti(XylNPiPr2)2CoI]2 (2). One electron reduction of complex 2 affords ClTi(XylNPiPr2)2CoPMe3 (3), which features a metal–metal triple bond and an unprecedentedly short Ti–Co distance of 2.0236(9) Å. This complex is shown to promote the McMurry coupling reaction of aryl ketones into alkenes, with concomitant formation of the tetranuclear complex [Ti(μ3-O)(NXylPiPr2)2CoI]2 (4). A cooperative mechanism involving bimetallic CO bond activation and a cobalt carbene intermediate is proposed.

Treatment of the tris(phosphinoamide) titanium precursor ClTi(XylNPiPr2)3 (1) with CoI2 leads to the heterobimetallic complex (η2-iPr2PNXyl)Ti(XylNPiPr2)2(μ-Cl)CoI (2). One-electron reduction of 2 affords (η2-iPr2PNXyl)Ti(XylNPiPr2)2CoI (3), which can be reduced by another electron under dinitrogen to generate the reduced diamagnetic complex (THF)Ti(XylNPiPr2)3CoN2 (4). The removal of the dinitrogen ligand from 4 under vacuum affords (THF)Ti(XylNPiPr2)3Co (5), which features a Ti–Co triple bond. Treatment of 4 with hydrazine or methyl hydrazine results in N–N bond cleavage and affords the new diamagnetic complexes (L)Ti(XylNPiPr2)3CoN2 (L = NH3 (6), MeNH2 (7)). Complexes 4, 5, and 6 have been shown to catalyze the disproportionation of hydrazine into ammonia and dinitrogen gas through a mechanism involving a diazene intermediate.

A novel bidentate ligand featuring an N-heterocyclic phosphenium cation (NHP+) linked to a phosphine side arm is used to explore the coordination chemistry of NHP+ ligands with nickel. Direct P–Cl bond cleavage from a chlorophosphine precursor [PP]-Cl (1) by Ni(COD)2 affords the asymmetric bimetallic complex [Cl2Ni(μ-PP)2Ni] (2) via a nonoxidative process. Abstraction of the halide with either NaBPh4 or K[B(C6F5)4] prior to metal coordination to form the free phosphenium ligand [PP]+ in situ, followed by coordination to Ni(COD)2, afforded the halide-free Ni0 complexes [(PP)Ni(COD)] [B(C6F5)4] (4) and [(PP)Ni(COD)][BPh4] (5). Chloride abstraction from 1 is problematic in the presence of a PF6– counterion, however, as evident by the formation of [(PP)Ni(PP-F)][PF6] (3). The COD ligand in 5 can be readily displaced with PMe3 or PPh3 to afford [(PP)NiL2][BPh4] (L = PMe3 (6), PPh3 (7)). Complexes 2–7 feature planar geometries about the NHP+ phosphorus atom and unusually short Ni–P distances, indicative of multiple bonding resulting from both P → Ni σ donation and Ni → P π backbonding. This bonding description is supported by theoretical studies using natural bond orbital analysis.

•Open shell (S = 3/2) heterobimetallic Cr/Rh and Cr/Ir complexes are reported.•These complexes feature the shortest Cr–Rh and Cr–Ir distances reported to date.•DFT calculations suggest a relatively covalent metal–metal interaction.•A metal–metal bond order of 0.5 between Cr and Rh or Ir is described.The chromium(III) tris(phosphinoamide) precursor Cr(iPrNPiPr2)3 (1) has been used to synthesize Cr/Rh and Cr/Ir heterobimetallic complexes. Treatment of [MCl(COD)]2 with 1 affords the open shell S = 3/2 complexes ClCr(μ-iPrNPiPr2)2M(η2-iPrNPiPr2) (M = Rh (2), Ir (3)). Instead of simple coordination of the late transition metal fragment to the C3-symmetric tris(phosphinoamide) binding pocket, the preference of RhI and IrI for a square planar environment leads to a chloride/amide ligand exchange process. Complexes 1–3 have been structurally characterized using X-ray crystallography, revealing short Cr–Rh and Cr–Ir distances (2.6095(3) Å (2); 2.6064(4) Å (3)) indicative of metal–metal bonds. A computational investigation of the electronic structures of 2 and 3 reveals substantial metal–metal orbital overlap, but the high spin nature of the CrIII center leads to population of metal–metal antibonding orbitals and relatively weak metal–metal bonding. The effects of the metal–metal interaction on the redox properties of the CrIII center are investigated using cyclic voltammetry.A new chromium tris(phosphinoamide) precursor is used to synthesize open shell (S = 3/2) heterobimetallic Cr/Rh and Cr/Ir complexes featuring metal–metal bonds.

The reduced heterobimetallic Co/Zr complex N2Co(iPr2PNMes)3Zr(THF) (1) has been previously reported to react with the C═O bonds of CO2 and benzophenone to generate Zr/Co μ-oxo complexes OC-Co(iPr2PNMes)2(μ-O)Zr(iPr2PNMes) (1-CO2) and Ph2C═Co(iPr2PNMes)2(μ-O)Zr(iPr2PNMes) (1-Ph2CO), respectively. Herein, we report a similar reaction of 1 with pyridine-N-oxide to form an analogous complex (pyridine)Co(iPr2PNMes)2(μ-O)Zr(iPr2PNMes) (2) with a more labile ligand bound to cobalt. Much like 1-CO2 and 1-Ph2CO, compound 2 reacts with Ph3SiH via formation of a Si–O linkage to form (N2)(H)Co(iPr2PNMes)3ZrOSiPh3 (5). The dinitrogen ligand in 5 is weakly bound and can be readily removed in vacuo or displaced by other L-type ligands. This allows complex 5 to undergo insertion reactions with unsaturated substrates, including diphenyldiazomethane, CO2, benzonitrile, and phenylacetylene to give hydrazonato (Ph2C═NNH)Co(iPr2PNMes)3ZrOSiPh3 (7), formate (OC(H)O)Co(iPr2PNMes)3ZrOSiPh3 (8), ketimide (PhHC═N)Co(iPr2PNMes)3ZrOSiPh3 (9), and ylide Co(PhHC═CHPiPr2NMes)(iPr2PNMes)2ZrOSiPh3 (10) products, respectively. Compound 5 was also found to catalyze the isomerization of 1-hexene to internal isomers.

A series of tris(phosphinoamide) heterobimetallic Cr–M (M = Fe, Co and Cu) complexes has been investigated in an effort to probe and contribute to the understanding of the electronic structure and metal–metal bonding in heterobimetallic complexes of the first row transition metals. The chromium tris(phosphinoamide), [Cr(iPrNPPh2)3] (1), is a useful isolable precursor and can be treated with MI2 under reducing conditions to form [Cr(iPrNPPh2)3M–I] (M = Fe (2), Co (3)). Both of these complexes can be reduced by one electron to generate [Cr(iPrNPPh2)3M–PMe3] (M = Fe (4), Co (5)). The Cr–Cu complex [Cr(iPrNPPh2)3Cu–I] (6) has also been synthesized for comparison. The solid state structures of 2–6 have been determined crystallographically, revealing relatively short metal–metal interatomic distances. Mössbauer spectroscopy, cyclic voltammetry, and computational methods have been used to evaluate the electronic structure and metal–metal interactions in these unique bimetallic complexes in an effort to uncover the underlying factors that affect metal–metal bonding between elements of the first row transition series.

Metal–metal multiple bonds have been an intense area of focus in inorganic chemistry for many decades as a result of their fundamentally interesting bonding properties, as well as their potential applications in multielectron transfer and small molecule activation processes. Much of what is known in this field revolves around 2nd and 3rd row transition metals, with fundamental knowledge lacking in the area of bonds between elements of the first transition series. The smaller size and tendency of first row ions to adopt high-spin electron configurations weaken metal–metal interactions and serve to complicate the interpretation of the electronic structure and bonding in bimetallic species containing first row transition metals. Furthermore, traditional tetragonal “paddlewheel” complexes dominate the metal–metal multiple bond literature, and only recently have researchers begun to take advantage of the weaker ligand field in three-fold symmetric bimetallic complexes to encourage more favourable metal–metal bonding interactions. In the past 5 years, several research groups have exploited three-fold symmetric frameworks to investigate new trends in metal–metal bonding involving the first row transition metals. This feature article serves to highlight recent achievements in this area and to use C3-symmetric systems as a model to better understand the fundamental aspects of multiple bonds featuring first row transition metals.

The reactivity of the reduced heterobimetallic complex Zr(iPrNPiPr2)3CoN2 (1) toward aryl azides was examined, revealing a four-electron redox transformation to afford unusual heterobimetallic zirconium/cobalt diimido complexes. In the case of p-tolyl azide, the diamagnetic C3-symmetric bis(terminal imido) complex 3 is formed, but mesityl azide instead leads to asymmetric complex 4 featuring a bridging imido fragment.

The chemical oxidation and subsequent group transfer activity of the unusual diiron imido complexes Fe(iPrNPPh2)3Fe≡NR (R = tert-butyl (tBu), 1; adamantyl, 2) was examined. Bulk chemical oxidation of 1 and 2 with Fc[PF6] (Fc = ferrocene) is accompanied by fluoride ion abstraction from PF6– by the iron center trans to the Fe≡NR functionality, forming F–Fe(iPrNPPh2)3Fe≡NR (iPr = isopropyl) (R = tBu, 3; adamantyl, 4). Axial halide ligation in 3 and 4 significantly disrupts the Fe–Fe interaction in these complexes, as is evident by the >0.3 Å increase in the intermetallic distance in 3 and 4 compared to 1 and 2. Mössbauer spectroscopy suggests that each of the two pseudotetrahedral iron centers in 3 and 4 is best described as FeIII and that one-electron oxidation has occurred at the tris(amido)-ligated iron center. The absence of electron delocalization across the Fe–Fe≡NR chain in 3 and 4 allows these complexes to readily react with CO and tBuNC to generate the FeIIIFeI complexes F–Fe(iPrNPPh2)3Fe(CO)2 (5) and F–Fe(iPrNPPh2)3Fe(tBuNC)2 (6), respectively. Computational methods are utilized to better understand the electronic structure and reactivity of oxidized complexes 3 and 4.

The effect of modifying the N-aryl substituent (aryl = mesityl vs. m-xylyl) of the phosphinoamide ligands linking Zr and Co in tris(phosphinoamide)-linked heterobimetallic complexes has been investigated. Treatment of the metalloligand (iPr2PNXyl)3ZrCl (2) (Xyl = m-xylyl) with CoI2 affords the iodide-bridged product ICo(iPr2PNXyl)2(μ-I)Zr(η2-iPr2PNXyl) (3) rather than the C3-symmetric isomer observed using the N-mesityl derivative, ICo(iPr2PNMes)3ZrCl. Upon two-electron reduction of complex 3, ligand rearrangement occurs to generate the three-fold symmetric reduced complex N2Co(iPr2PNXyl)3Zr(THF) (4). Comparison of 4 with the previously reported mesityl-substituted complex N2Co(iPr2PNMes)3Zr(THF) (1) reveals similar structural features but with a less sterically hindered Zr apical site in complex 4. An obvious electronic difference between these two complexes is also present based on the drastically different infrared N2 stretching frequencies of 1 and 4. These notable differences lend themselves to different reactivity in both stoichiometric and catalytic reactions. Alkyl halide addition to complex 4 results in homo-coupling products resulting from alkyl radicals rather than the alkyl-bridged or intramolecular C–H activation products formed upon addition of RX to 1. This difference in reactivity with alkyl halides renders complex 3 a less effective catalyst for the Kumada cross-coupling of alkyl halides with n-octylMgBr than ICo(iPr2PNMes)3ZrCl, as a greater proportion of homocoupling products are formed under catalytic conditions.

The reduced heterobimetallic complex (THF)Zr(MesNPiPr2)3CoN2 (1) has been examined along with a series of structurally similar reference compounds using X-ray absorption near edge structure (XANES) spectroscopy. Complex 1 has been shown to be highly reactive, often via one-electron pathways that might be expected for a d1 ZrIII complex. However, the presence of two strongly interacting metals in complex 1 renders the assignment of oxidation states ambiguous. Both Zr and Co K-edge XANES spectra reveal that the most accurate description of complex 1 is that of a ZrIV/Co−I zwitterion. Electronic structure calculations support this assignment.

•We explore the coordination of N-heterocyclic phosphenium cations to copper.•Cu-bound phosphenium ligands abstract Ph− from BPh4−.•Cu-bound N-heterocyclic iodophosphine ligands ring-open THF.•We provide evidence for electrophilic phosphenium/copper(I) intermediates.The coordination chemistry of a diphosphine pincer ligand incorporating an N-heterocyclic phosphenium cation (PPP+) and its halophosphine precursors [PP(X)P] has been explored using copper(I) reagents. In contrast to the P–X bond cleavage observed when other low valent transition metals are used, simple coordination compounds [PP(X)P]CuY (X = Y = Cl (1); X = Cl, Y = I (2); X = Y = I (3)) are formed when halophosphine precursors are treated with CuY starting materials. Exposure of the N-heterocyclic iodophosphine complex 3 to THF results in ring-opening insertion to form the alkoxyphosphine complex [PP(O-CH2CH2CH2CH2-I)P]CuI (4). Since this reaction does not occur with the chlorophosphine compounds 1 and 2, it is proposed that iodide dissociation exposes the electrophilic metal-bound phosphenium cation to allow THF coordination and subsequent ring-opening. The electrophilicity of the N-heterocyclic phosphenium is also apparent when the halide-free ligand precursor [PPP][BPh4] is employed: CuCl reacts with [PPP][BPh4] to form [PP(Ph)P]CuCl (5), the product of phenyl group abstraction from [BPh4]−. A similar arylphosphine product [PP(Mes)P]CuCl (6, Mes = 2,4,6-trimethylphenyl) is obtained via transmetallation between [PP(Cl)P] and mesitylcopper(I). The isolation of complexes 4–6 provide indirect evidence for the formation of an electrophilic copper–phosphenium intermediate {[PPP]CuCl}+.The copper(I) coordination chemistry of an N-heterocyclic phosphenium-containing pincer ligand and its halophosphine precursors is explored, revealing highly electrophilic reactivity including ring-opening of tetrahydrofuran and phenyl group abstraction from the tetraphenylborate anion.

Single-electron transfer from the ZrIVCo–I heterobimetallic complex (THF)Zr(MesNPiPr2)3Co-N2 (1) to benzophenone was previously shown to result in the isobenzopinacol product [(Ph2CO)Zr(MesNPiPr2)3Co-N2]2 (4) via coupling of two ketyl radicals. Thermolysis of 4 led to cleavage of the C═O bond to generate a Zr/Co μ-oxo species featuring an unusual terminal Co═CPh2 carbene linkage (3). In this work monomeric ketyl radical complexes have been synthesized, and the reactivity of these compounds has been explored. The electronic preference for the formation of a ketyl radical complex or a coordination complex has been investigated computationally. Furthermore, thione substrates were allowed to react with 1, generating new complexes that bind the thione to the Co rather than undergoing single-electron transfer (12, 14). The preference of thiones to coordinate to Co can be overridden if the Co is ligated by CO, in which case a thioketyl radical complex forms (13) analogous to 4. The reaction between 1 and imines resulted in N–H bond activation, affording a μ-methyleneamido Co–H complex (16) that can undergo cyclometalation and loss of H2 (15).

Single electron transfer from the ZrIIICo0 heterobimetallic complex (THF)Zr(MesNPiPr2)3Co–N2 (1) to benzophenone was previously shown to result in the isobenzopinacol product [(Ph2CO)Zr(MesNPiPr2)3Co–N2]2 (2) via coupling of two ketyl radicals. In this work, thermolysis of 2 in an attempt to favor a monomeric ketyl radical species unexpectedly led to cleavage of the C–O bond to generate a Zr/Co μ-oxo species featuring an unusual terminal Co═CPh2 carbene linkage, (η2-MesNPiPr2)Zr(μ-O)(MesNPiPr2)2Co═CPh2 (3). This complex was characterized structurally and spectroscopically, and its electronic structure is discussed in the context of density functional theory calculations. Complex 3 was also shown to be active toward carbene group transfer (cyclopropanation), and silane addition to 3 leads to PhSiH2O–Zr(MesNPiPr2)3Co–N2 (5) via a proposed Co–alkyl bond homolysis route.

A series of V/Fe heterobimetallic complexes supported by phosphinoamide ligands, [Ph2PNiPr]−, is described. The V(III) metalloligand precursor [V(iPrNPPh2)3] can be treated with Fe(II) halide salts under reducing conditions to afford [V(iPrNPPh2)3FeX] (X = Br (2), I (3)). These complexes feature multiple bonds between Fe and V, leading to an intermetallic distance of ∼2.07 Å. Exploration of the one-electron reduction of complex 3 allows isolation of [V(iPrNPPh2)3Fe(PMe3)] (5), which also features metal–metal multiple bonding and a nearly identical Fe–V distance. Mössbauer spectroscopy of complexes 2 and 5 suggest that the most reasonable oxidation state assignments for these complexes are VIIIFeI and VIIIFe0, respectively, and that reduction occurs solely at the Fe center in these bimetallic complexes. A theoretical investigation confirms this description of the electronic structure, providing a description of the metal–metal bonding manifolds as (σ)2(π)4(Fenb)3 and (σ)2(π)4(Fenb)4 for complexes 3 and 5, consistent with a metal–metal bond order of three. One electron-oxidation of complex 3 results in halide abstraction from PF6−, forming FV(iPrNPPh2)3FeI (6). Complex 6 has a much weaker V–Fe interaction as a result of axial fluoride ligation at the V center.

A tris(phosphino)amide-ligated Zr–Co heterobimetallic complex has been shown to activate N–H bonds of hydrazine derivatives via a proton-coupled electron transfer process. Such reactivity is highly unusual for an early metal such as Zr, but is promoted by the adjacent redox active Co atom.

The synthesis and preliminary coordination chemistry of two new redox-active bidentate ligands containing amido and phosphido donors are described. Treatment of the [RNP]2– (R = Ph, 2,4,6-trimethylphenyl) ligands with CuCl2 and PMe3 results in a dimeric copper(I) P–P coupled product via ligand oxidation. The intermediate of this reaction is proposed to involve a ligand radical generated via oxidation of the [RNP]2– ligand by copper(II), and the existence of such an intermediate is probed using computational methods. Significant radical character on the phosphorus atoms of the alleged [RNP]•–/copper(I) intermediate leads to P–P radical coupling.

A series of tris- and tetrakis(phosphinoamide) U/Co complexes has been synthesized. The uranium precursors, (η2-Ph2PNiPr)4U (1), (η2-iPr2PNMes)4U (2), (η2-Ph2PNiPr)3UCl (3), and (η2-iPr2PNMes)3UI (4), were easily accessed via addition of the appropriate stoichiometric equivalents of [Ph2PNiPr]K or [iPr2PNMes]K to UCl4 or UI4(dioxane)2. Although the phosphinoamide ligands in 1 and 4 have been shown to coordinate to U in an η2-fashion in the solid state, the phosphines are sufficiently labile in solution to coordinate cobalt upon addition of CoI2, generating the heterobimetallic Co/U complexes ICo(Ph2PNiPr)3U[η2-Ph2PNiPr] (5), ICo(iPr2PNMes)3U[η2-(iPr2PNMes)] (6), ICo(Ph2PNiPr)3UI (7), and ICo(iPr2PNMes)3UI (8). Structural characterization of complexes 5 and 7 reveals reasonably short Co–U interatomic distances, with 7 exhibiting the shortest transition metal–uranium distance ever reported (2.874(3) Å). Complexes 7 and 8 were studied by cyclic voltammetry to examine the influence of the metal–metal interaction on the redox properties compared with both monometallic Co and heterobimetallic Co/Zr complexes. Theoretical studies are used to further elucidate the nature of the transition metal–actinide interaction.

By incorporating an N-heterocyclic phosphenium/phosphide (NHP) ligand into a chelating pincer ligand framework (PPP+/PPP–), we have elucidated several different and unprecedented binding modes of NHP ligands in homobimetallic, heterobimetallic, and trimetallic metal complexes. One-electron reduction of the previously reported (PPP)−/MII complexes (PPP)M-Cl (M = Pd (1), Pt (2)) results in clean formation of the symmetric homobimetallic MI/MI complexes [(μ-PPP)Pd]2 (5) and [(μ-PPP)Pt]2 (6). The tridentate NHP ligand has also been utilized as a bridging linker in the M/Co heterobimetallic compounds (OC)3Co(u-PPP)M(CO) (M = Pd (7), Pt (8)), synthesized via salt elimination from mixtures of 1 and 2 and Na[Co(CO)4]. Furthermore, an NHP-bridged trimetallic complex (PPP)2Pd3Cl2 (9) can be synthesized in a manner similar to precursor 1 (Pd(PPh3)4 + (PPP)Cl) via careful adjustment of reaction stoichiometry. Examination of the interatomic distances and angles in complexes 5–9, in tandem with density functional theory calculations have been used to evaluate and characterize the bonding interactions in these complexes.

The tris(phosphinoamide)-bridged FeIIFeII diiron complex Fe(μ-iPrNPPh2)3Fe(η2-iPrNPPh2) (1) can be reduced in the absence or presence of PMe3 to generate the mixed-valence FeIIFeI complexes Fe(μ-iPrNPPh2)3Fe(PPh2NHiPr) (2) or Fe(μ-iPrNPPh2)3Fe(PMe3) (3), respectively. Following a typical oxidative group transfer procedure, treatment of 2 or 3 with organic azides generates the mixed-valent FeIIFeIII imido complexes Fe(iPrNPPh2)3Fe≡NR (R = tBu (4), Ad (5), 2,4,6-trimethylphenyl (6)). These complexes represent the first examples of first-row bimetallic complexes featuring both metal–ligand multiple bonds and metal–metal bonds. The reduced complexes 2 and 3 and imido complexes 4–6 have been characterized via X-ray crystallography, Mössbauer spectroscopy, cyclic voltammetry, and SQUID magnetometry, and a theoretical description of the bonding within these diiron complexes has been obtained using computational methods. The effect of the metal–metal interaction on the electronic structure and bonding in diiron imido complexes 4–6 is discussed in the context of similar monometallic iron imido complexes.

Oxidative addition of CO2 to the reduced Zr/Co complex (THF)Zr(MesNPiPr2)3Co (1) followed by one-electron reduction leads to formation of an unusual terminal Zr–oxo anion [2][Na(THF)3] in low yield. To facilitate further study of this compound, an alternative high-yielding synthetic route has been devised. First, 1 is treated with CO to form (THF)Zr(MesNPiPr2)3Co(CO) (3); then, addition of H2O to 3 leads to the Zr–hydroxide complex (HO)Zr(MesNPiPr2)3Co(CO) (4). Deprotonation of 4 with Li(N(SiMe3)2) leads to the anionic Zr–oxo species [2][Li(THF)3] or [2][Li(12-c-4)] in the absence or presence of 12-crown-4, respectively. The coordination sphere of the Li+ countercation is shown to lead to interesting structural differences between these two species. The anionic oxo fragment in complex [2][Li(12-c-4)] reacts with electrophiles such as MeOTf and Me3SiOTf to generate (MeO)Zr(MesNPiPr2)3Co(CO) (5) and (Me3SiO)Zr(MesNPiPr2)3Co(CO) (6), respectively, and addition of acetic anhydride generates (AcO)Zr(MesNPiPr2)3Co(CO) (7). Complex [2][Li(12-c-4)] is also shown to bind CO2 to form a monoanionic Zr–carbonate, [(12-crown-4)Li][(κ2-CO3)Zr(MesNPiPr2)3Co(CO)] ([8][Li(12-c-4)]). A more stable version of this compound [8][K(18-c-6)] is formed when a K+ counteranion and 18-crown-6 are used. Binding of CO2 to [2][Li(12-c-4)] is shown to be reversible using isotopic labeling studies. In an effort to address methods by which these CO2-derived products could be turned over in a catalytic cycle, it is shown that the Zr–OMe bond in 5 can be cleaved using H+ and the CO ligand can be released from Co under photolytic conditions in the presence of I2.

Homobimetallic dicobalt complexes featuring metal centers in different coordination environments have been synthesized, and their multielectron redox chemistry has been investigated. Treatment of CoX2 with MesNKPiPr2 leads to self-assembly of [(THF)Co(MesNPiPr2)2(μ-X)CoX] [X = Cl (1), I (2)], with one Co center bound to two amide donors and the other bound to two phosphine donors. Upon two-electron reduction, a ligand rearrangement occurs to generate the symmetric species (PMe3)Co(MesNPiPr2)2Co(PMe3) (3), where each Co has an identical mixed P/N donor set. One-electron oxidation of 3 to generate a mixed valence species promotes a ligand reararrangement back to an asymmetric configuration in [(THF)Co(MesNPiPr2)2Co(PMe3)][PF6] (4). Complexes 1–4 have been structurally characterized, and their metal–metal interactions are discussed in the context of computational results.

An imidazolinium cation has been incorporated into an arene-linked diphosphine pincer ligand, [2]+, and the metalation of this ligand has been investigated via direct imidazolinium C–H activation to Pd0 and Pt0. The expected NHC-ligated metal-hydride species [5]PF6 (M = Pt) and 6 (M = Pd) are obtained if the halide-free imidazolinium salt [2]PF6 is used. In contrast, treatment of the imidazolinium chloride salt [2]Cl with M(PPh3)4 leads to isolation of N-heterocyclic alkyl MII species 3 (M = Pd) and 4 (M = Pt), in which the imidazolinium C–H bond remains intact. Interestingly, there are no apparent agostic interactions between the imidazolinium protons and the metal centers in 3 and 4, indicating that these species represent an unusual type of arrested C–H activation intermediate. While Pd complex 3 is thermally stable, Pt complex 4 undergoes C–H activation to afford the corresponding NHC-PtII-hydride species [5]Cl upon heating. Additionally, both complexes 3 and 4 undergo rapid C–H activation upon abstraction of the metal-bound halide to form 6 and [5]PF6, respectively. The nature of the bonding in the unusual N-heterocyclic alkyl species is investigated computationally.

A series of homobimetallic phosphinoamide-bridged diiron and dimanganese complexes in which the two metals maintain different coordination environments have been synthesized. Systematic variation of the steric and electronic properties of the phosphinoamide phosphorus and nitrogen substituents leads to structurally different complexes. Reaction of [iPrNKPPh2] (1) with MCl2 (M = Mn, Fe) affords the phosphinoamide-bridged bimetallic complexes [Mn(iPrNPPh2)3Mn(iPrNPPh2)] (3) and [Fe(iPrNPPh2)3Fe(iPrNPPh2)] (4). Complexes 3 and 4 are iso-structural, with one metal center preferentially binding to the three amide ligands in a trigonal planar arrangement while the second metal center is ligated by three phosphine donors. A fourth phosphinoamide ligand caps the tetrahedral coordination sphere of the phosphine-ligated metal center. Mössbauer spectroscopy of complex 4 suggests that the metals in these complexes are best described as FeII centers. In contrast, treatment of MnCl2 or FeI2 with [MesNKPiPr2] (2) leads to the formation of the halide-bridged species [(THF)Mn(μ-Cl)(MesNPiPr2)2Mn(MesNPiPr2)] (5) and [(THF)Fe(μ-I)(MesNPiPr2)2FeI (7), respectively. Utilization of FeCl2 in place of FeI2, however, leads exclusively to the C3-symmetric complex [Fe(MesNPiPr2)3FeCl] (6), structurally similar to 4 but with a halide bound to the phosphine-ligated Fe center. The Mössbauer spectrum of 6 is also consistent with high spin FeII centers. Thus, in the case of the [iPrNPPh2]− and [MesNPiPr2]− ligands, zwitterionic complexes with the two metals in disparate coordination environments are preferentially formed. In the case of the more electron-rich ligand [iPrNPiPr2]−, complexes with a 2:1 mixed donor ligand arrangement, in which one of the ligand arms has reversed orientation relative to the previous examples, are formed exclusively when [iPrNLiPiPr2] (generated in situ) is treated with MCl2 (M = Mn, Fe): (THF)3LiCl[Mn(NiPrPiPr2)2(PiPr2NiPr)MnCl] (8) and [Fe(NiPrPiPr2)2(PiPr2NiPr)FeCl] (9). Bimetallic complexes 3–9 have been structurally characterized using X-ray crystallography, revealing Fe–Fe interatomic distances indicative of metal–metal bonding in complexes 6 and 9 (and perhaps 4, to a lesser extent). All of the complexes appear to adopt high spin electron configurations, and magnetic measurements indicate significant antiferromagnetic interactions in Mn2 complexes 5 and 8 and no discernible magnetic superexchange in Fe2 complex 4. The redox behavior of complexes 3–9 has also been investigated using cyclic voltammetry, and theoretical investigations (DFT) were performed to gain insight into the metal–metal interactions in these unique asymmetric complexes.

The coordination chemistry of an N-heterocyclic phosphenium (NHP)-containing bis(phosphine) pincer ligand has been explored with Pt0 and Pd0 precursors. Unlike previous compounds featuring monodentate NHP ligands, the resulting NHP Pt and Pd complexes feature pyramidal geometries about the central phosphorus atom, indicative of a stereochemically active lone pair. Structural, spectroscopic, and computational data suggest that the unusual pyramidal NHP geometry results from two-electron reduction of the phosphenium ligand to generate transition metal complexes in which the Pt or Pd centers have been formally oxidized by two electrons. Interconversion between planar and pyramidal NHP geometries can be affected by either coordination/dissociation of a two-electron donor ligand or two-electron redox processes, strongly supporting an isolobal analogy with the linear (NO+) and bent (NO–) variations of nitrosyl ligands. In contrast to nitrosyls, however, these new main group noninnocent ligands are sterically and electronically tunable and are amenable to incorporation into chelating ligands, perhaps representing a new strategy for promoting redox transformations at transition metal complexes.

The heterobimetallic complexes [Mn(iPrNPPh2)3Cu(iPrNHPPh2)] (1) and [Fe(iPrNPPh2)3Cu(iPrNHPPh2)] (2) have been synthesized by the one pot reaction of LiNiPrPPh2, MCl2 (M = Mn, Fe), and CuI in high yield. Addition of excess CuI into 2 or directly to the reaction mixture led to the formation of a heterotrimetallic [Fe(iPrNPPh2)3Cu2(iPrNPPh2)] (3) in good yield. Complexes 1–3 have been characterized by means of elemental analysis, paramagnetic 1H NMR, UV–vis spectroscopy, cyclic voltammetry, and single crystal X-ray analysis. In all three complexes, Mn or Fe are in the +2 oxidation state and have a high spin electron configuration, as evidenced by solution Evans’ method. In addition, the oxidation state of Fe in complex 3 is confirmed by zero-field 57Fe Mössbauer spectroscopy. X-ray crystallography reveals that the three coordinate Mn/Fe centers in the zwitterionic complexes 1–3 adopt an unusual trigonal planar geometry.

The reactivity of E–H bonds (E = S, O, Cl) with Pt(II) complexes ligated by an N-heterocyclic phosphido-containing diphosphine ligand have been investigated. Addition of PhSH to [(PPP)Pt(PPh3)][PF6] (1) results in clean formation of [(PP(H)P)Pt(SPh)][PF6] (3), in which the substrate has added across the Pt–PNHP bond. Similar reactivity occurs when 1 is treated with ROH (R = Ph, Me), but in this case the O–H bond adds across the Pt–P bond in the opposite direction producing [(PP(OR)P)Pt(H)(PPh3)][PF6] (R = Ph (4), Me (5)). HCl addition to 1 cleanly generates [(PP(H)P)PtCl][PF6] (6PF6PF6PF6). The neutral Pt–NHP complex (PPP)PtCl (2) exhibits similar reactivity; however, in the presence of the nucleophilic Cl− anion, the (PP(OR)P)Pt(H)Cl species presumably generated via addition of ROH (R = Me, Et) undergoes an Arbuzov-like dealkylation reaction to exclusively form the N-heterocylic phosphinito species (PP(O)P)Pt(H) (7).

Phosphinoamide ligands have been utilized to link a CoI center to a Nb or Ta imido fragment. The resulting heterobimetallic complexes, ICo(Ph2PNiPr)3MNtBu, have weakened dative metal–metal interactions as a result of the strongly donating imido ligand. These complexes can be reduced by 2 electrons to generate dinitrogen-bound complexes.

The Pd(I)−Pd(I) dimer [(FPNP)Pd−]2 reacts with O2 upon exposure to light to produce either the superoxide (FPNP)PdO2 or the peroxide [(FPNP)PdO−]2, which exist in equilibrium with free O2. Both complexes contain square-planar Pd(II) centers. The unpaired electron density in (FPNP)PdO2 is localized on the superoxide ligand.

At room temperature, the early/late heterobimetallic complex Co(iPr2PNMes)3Zr(THF) has been shown to oxidatively add CO2, generating (OC)Co(iPr2PNMes)2(μ-O)Zr(iPr2PNMes). This compound can be further reduced under varying conditions to generate either the Zr oxoanion (THF)3Na–O–Zr(MesNPiPr2)3Co(CO) or the Zr carbonate complex (THF)4Na2(CO3)-Zr(MesNPiPr2)3Co(CO). Additionally, reactivity of the CO2-derived product has been observed with PhSiH3 to generate the Co-hydride/Zr-siloxide product (OC)(H)Co(iPr2PNMes)3ZrOSiH2Ph.

The synthesis of a new pincer ligand containing a central cationic N-heterocyclic phosphenium donor is described. The electrophilic nature of this cationic ligand renders it non-innocent, and coordination of this ligand to a PtCl2 fragment leads to chloride migration from Pt to the cationic phosphorus center.

The phosphinoamide-linked Co/Hf complexes ICo(Ph2PNiPr)3HfCl (4), ICo(iPr2PNMes)3HfCl (5), and ICo(iPr2PNiPr)3HfCl (6) have been synthesized from the corresponding tris(phosphinoamide)HfCl complexes (1–3) for comparison with the recently reported tris(phosphinoamide) Co/Zr complexes. Very minor structural and electronic differences between the Zr and Hf complexes were found when the N-iPr-substituted phosphinoamide ligands [Ph2PNiPr]− and [iPr2PNiPr]− were utilized. The reduction products [(THF)4Na-{N2-Co(Ph2PNiPr)3HfCl}2]Na(THF)6 (7) and N2-Co(iPr2PNiPr)3Hf (9) are also remarkably similar to the corresponding Zr/Co analogues. In the case of Hf/Co and Zr/Co complexes linked by the N-Mes ligand [iPr2PNMes]− (Mes = 2,4,6-trimethylphenyl), however, more pronounced differences in structure, bonding, and reactivity are observed. While differences associated with 5 are still modest, larger variations are observed when comparing the two-electron reduction product [N2-Co(iPr2PNMes)3Hf-X][Na(THF)5] (8) with its Zr congener. In addition to structural and spectroscopic differences, vastly different reactivity is observed, with 8 undergoing one-electron oxidation to form ClHf(MesNPiPr2)3CoN2 (11) in the presence of MeI, while a two-electron oxidative addition process occurs in a similar reaction with the Zr derivative. The activity of 5 toward Kumada coupling was investigated, finding significantly diminished activity in comparison to Co/Zr complexes.

A tridentate pincer ligand featuring a central N-heterocyclic phosphenium (NHP+) donor has been coordinated to a Co(CO)2 fragment to generate the Co NHP complex [PPP]Co(CO)2 (2). The NHP unit adopts an unusual pyramidal geometry with a relatively long Co–P distance, suggesting a stereochemically active nonbonding phosphorus lone pair. Interestingly, treatment of 2 with trimethylamine N-oxide affords [P(P═O)P]Co(CO)2 (3), in which the Co-bound central phosphorus donor has been oxidized to an unprecedented N-heterocyclic phosphinito species. The bonding and electronic properties of these complexes are discussed in the context of DFT and NBO computational data.

The reactivity of heterobimetallic Zr/Co complexes linked by phosphinoamide ligands towards the oxidative addition of I2 and alkyl halides is reported. These reactions are accompanied by dissociation of one phosphine ligand from Co and η2-coordination to Zr. Addition of H2 leads to both oxidative addition across the M–M bond and P–N bond cleavage.

The bidentate metalloligand (NMe2)2Zr(iPrNPPh2)2 (1) has been synthesized and treated with (COD)PtMe2 to generate the heterobimetallic Zr/Pt species (NMe2)2Zr(iPrNPPh2)2PtMe2 (2). Complex 2 can be treated with TMSCl to generate the dichloride species Cl2Zr(iPrNPPh2)2PtMe2 (3), a useful precursor for generating a range of Zr/Pt complexes featuring different Zr substituents. Upon treatment with lithio or Grignard reagents, Cl(Me3SiCH2)Zr(iPrNPPh2)2PtMe2 (4), (Me3SiCH2)2Zr(iPrNPPh2)2PtMe2 (5), and Me2Zr(iPrNPPh2)2PtMe2 (6) have been synthesized. Complexes 1−6 have been characterized using X-ray crystallography, revealing an unmistakable trend in Pt−Zr distance as a function of the electron-releasing ability of the Zr-bound X-type ligands. In addition, the solid-state structure of 6 reveals isomerization of the chelating metalloligand from a cis to a trans configuration about Pt and an unusual anagostic interaction between one of the Pt-bound Me groups and the electron-deficient Zr center. This isomerization is accompanied by a thermoneutral methyl group exchange process between Zr and Pt.

Reduction of Zr/Co heterobimetallic complexes ICo(MesNPiPr2)3ZrCl (1) and ICo(iPrNPiPr2)3ZrCl (2) with excess Na/Hg under N2, followed by subsequent benzene extraction to remove coordinated Na halide salts, leads to neutral two-electron reduced, dinitrogen-bound complexes (THF)Zr(MesNPiPr2)3Co−N2 (4) and Zr(iPrNPiPr2)3Co−N2 (5). Upon halide loss, a THF solvent molecule coordinates to the axial site of the Zr center in 4, while this axial site remains unoccupied in 5. X-ray crystallography reveals short Co−Zr distances in 4 and 5, indicative of metal−metal multiple bonding, and an unprecedented trigonal monopyramidal geometry about the Zr center in 5. Reduction of 4 under an Ar atmosphere (in the absence of N2) results in another unusual structure type: an unoccupied axial Co coordination site and a trigonal monopyramidal Co center in (THF)Zr(MesNPiPr2)3Co (6). X-ray crystallography reveals that, in the absence of coordinated N2, the Co−Zr bond can attain full triple bond character with a Co−Zr distance of 2.14 Å, the shortest M−M distance in an early/late heterobimetallic complex reported to date. To further assess the electronic structure and bonding in 4, 5, and 6, calculations were performed on these molecules using DFT and the results of these theoretical investigations will be discussed.

To assess the effect of dative M→M interactions on redox properties in early/late heterobimetallic complexes, a series of Co/Zr complexes supported by phosphinoamide ligands have been synthesized and characterized. Treatment of the Zr metalloligands (Ph2PNiPr)3ZrCl (1), (iPr2PNMes)3ZrCl (2), and (iPr2PNiPr)3ZrCl (3) with CoI2 leads to reduction from CoII to CoI and isolation of the heterobimetallic complexes ICo(Ph2PNiPr)3ZrCl (4), ICo(iPr2PNMes)3ZrCl (5), and ICo(iPr2PNiPr)3ZrCl (6), respectively. Interestingly, treatment of CoI2 with the phosphinoamine Ph2PNHiPr in the absence of a bound Zr center leads to the disubstituted CoII complex (Ph2PNHiPr)2CoI2 (7). The tris(phosphinoamine) CoI complex (Ph2PNHiPr)3CoI (8) can only be generated in the presence of an added reductant such as Zn0, indicating that the reduction of CoII to CoI only occurs in the presence of Zr in the formation of complexes 4−6. Structural characterization of 4−6 reveals a Zr−Co interaction, with interatomic distances of 2.7315(5) Å, 2.6280(5) Å, and 2.6309(5) Å, respectively. This distance appears to decrease as the phosphine donors on Co become more electron-releasing, strengthening the dative Co→Zr interaction. Cyclic voltammetry of 4−6 shows that all three compounds can be further reduced by two electrons at relatively mild reduction potentials (−1.65 V to −2.07 V vs Fc/Fc+). The potentials at which these reductions occur in each of these complexes are largely governed by the extent to which electron-density is donated to Zr, as well as the electron-donating ability of the phosphine substituents. Moreover, cyclic voltammetry of complex 8 reveals that in the absence of Zr, the Co center is significantly more electron rich, and thus more difficult to reduce. Chemical reduction of 5 leads to the isolation of the two-electron reduced dinitrogen complex [N2Co(iPr2PNMes)3ZrX][Na(THF)5] (9). X-ray crystallography of 9 reveals that two-electron reduction is accompanied by a significant contraction of the Co−Zr interatomic distance from 2.6280(5) Å to 2.4112(3) Å. These heterobimetallic complexes have been studied computationally using density functional theory to examine the nature of the metal−metal interactions in these species.

A chelating diphosphine ligand with a central N-heterocyclic phosphenium cation (NHP+) has been used to explore the coordination chemistry of NHPs with nickel. Treatment of the chlorophosphine precursor [PPP]Cl (1) with stoichiometric Ni(COD)2 affords (PPP)NiCl (8), which is best described as a NiII/NHP− phosphido complex formed via oxidative addition of the P–Cl bond. In contrast, treating [PPP]Cl (1) with excess Ni(COD)2 results in a mixture of the trimetallic complex (PPP)2Ni3Cl2 (9) and the reduced NHP-bridged dimer [(PPP)Ni]2 (10). Compound 9 is found to be a NiIINiIINi0 complex in which the two NHP ligands act as bridging NHP− phosphidos, while complex 10 is a NiINiI complex that is highly delocalized throughout the symmetric Ni2P2 core. In contrast, the reaction of [PPP][PF6] (11) with Ni(COD)2 affords an asymmetrically-bridged dication [(PPP)Ni]2[PF6]2 (12), which is found to contain two bridging NHP+ cations bridging two Ni0 centers. Comproportionation of 10 and 12 affords monocationic [(PPP)Ni]2[PF6] (13), completing the redox series. Nickel complexes 8–10 and 12 are largely similar to their Pd and Pt analogues, but a paramagnetic monocation such as 13 was not observed in the Pd and Pt case. Computational studies lend further insight into the electronic structure and bonding in complexes 8–10 and 12–13, and further support the potential redox non-innocent properties of NHP ligands.

Metal–metal multiple bonds have been an intense area of focus in inorganic chemistry for many decades as a result of their fundamentally interesting bonding properties, as well as their potential applications in multielectron transfer and small molecule activation processes. Much of what is known in this field revolves around 2nd and 3rd row transition metals, with fundamental knowledge lacking in the area of bonds between elements of the first transition series. The smaller size and tendency of first row ions to adopt high-spin electron configurations weaken metal–metal interactions and serve to complicate the interpretation of the electronic structure and bonding in bimetallic species containing first row transition metals. Furthermore, traditional tetragonal “paddlewheel” complexes dominate the metal–metal multiple bond literature, and only recently have researchers begun to take advantage of the weaker ligand field in three-fold symmetric bimetallic complexes to encourage more favourable metal–metal bonding interactions. In the past 5 years, several research groups have exploited three-fold symmetric frameworks to investigate new trends in metal–metal bonding involving the first row transition metals. This feature article serves to highlight recent achievements in this area and to use C3-symmetric systems as a model to better understand the fundamental aspects of multiple bonds featuring first row transition metals.

A tris(phosphino)amide-ligated Zr–Co heterobimetallic complex has been shown to activate N–H bonds of hydrazine derivatives via a proton-coupled electron transfer process. Such reactivity is highly unusual for an early metal such as Zr, but is promoted by the adjacent redox active Co atom.

The synthesis of a new pincer ligand containing a central cationic N-heterocyclic phosphenium donor is described. The electrophilic nature of this cationic ligand renders it non-innocent, and coordination of this ligand to a PtCl2 fragment leads to chloride migration from Pt to the cationic phosphorus center.

The reactivity of heterobimetallic Zr/Co complexes linked by phosphinoamide ligands towards the oxidative addition of I2 and alkyl halides is reported. These reactions are accompanied by dissociation of one phosphine ligand from Co and η2-coordination to Zr. Addition of H2 leads to both oxidative addition across the M–M bond and P–N bond cleavage.

To explore metal–metal multiple bonds between first row transition metals, Ti/Co complexes supported by two phosphinoamide ligands have been synthesized and characterized. The Ti metalloligand Cl2Ti(XylNPiPr2)2 (1) was treated with CoI2 under reducing conditions, permitting isolation of the Ti/Co complex [(μ-Cl)Ti(XylNPiPr2)2CoI]2 (2). One electron reduction of complex 2 affords ClTi(XylNPiPr2)2CoPMe3 (3), which features a metal–metal triple bond and an unprecedentedly short Ti–Co distance of 2.0236(9) Å. This complex is shown to promote the McMurry coupling reaction of aryl ketones into alkenes, with concomitant formation of the tetranuclear complex [Ti(μ3-O)(NXylPiPr2)2CoI]2 (4). A cooperative mechanism involving bimetallic CO bond activation and a cobalt carbene intermediate is proposed.

A series of V/Fe heterobimetallic complexes supported by phosphinoamide ligands, [Ph2PNiPr]−, is described. The V(III) metalloligand precursor [V(iPrNPPh2)3] can be treated with Fe(II) halide salts under reducing conditions to afford [V(iPrNPPh2)3FeX] (X = Br (2), I (3)). These complexes feature multiple bonds between Fe and V, leading to an intermetallic distance of ∼2.07 Å. Exploration of the one-electron reduction of complex 3 allows isolation of [V(iPrNPPh2)3Fe(PMe3)] (5), which also features metal–metal multiple bonding and a nearly identical Fe–V distance. Mössbauer spectroscopy of complexes 2 and 5 suggest that the most reasonable oxidation state assignments for these complexes are VIIIFeI and VIIIFe0, respectively, and that reduction occurs solely at the Fe center in these bimetallic complexes. A theoretical investigation confirms this description of the electronic structure, providing a description of the metal–metal bonding manifolds as (σ)2(π)4(Fenb)3 and (σ)2(π)4(Fenb)4 for complexes 3 and 5, consistent with a metal–metal bond order of three. One electron-oxidation of complex 3 results in halide abstraction from PF6−, forming FV(iPrNPPh2)3FeI (6). Complex 6 has a much weaker V–Fe interaction as a result of axial fluoride ligation at the V center.

A series of tris(phosphinoamide) heterobimetallic Cr–M (M = Fe, Co and Cu) complexes has been investigated in an effort to probe and contribute to the understanding of the electronic structure and metal–metal bonding in heterobimetallic complexes of the first row transition metals. The chromium tris(phosphinoamide), [Cr(iPrNPPh2)3] (1), is a useful isolable precursor and can be treated with MI2 under reducing conditions to form [Cr(iPrNPPh2)3M–I] (M = Fe (2), Co (3)). Both of these complexes can be reduced by one electron to generate [Cr(iPrNPPh2)3M–PMe3] (M = Fe (4), Co (5)). The Cr–Cu complex [Cr(iPrNPPh2)3Cu–I] (6) has also been synthesized for comparison. The solid state structures of 2–6 have been determined crystallographically, revealing relatively short metal–metal interatomic distances. Mössbauer spectroscopy, cyclic voltammetry, and computational methods have been used to evaluate the electronic structure and metal–metal interactions in these unique bimetallic complexes in an effort to uncover the underlying factors that affect metal–metal bonding between elements of the first row transition series.

The reactivity of a reduced heterobimetallic Co−I/ZrIV complex, (tBuNC)Co(iPr2PNMes)3Zr(THF) (2), with a series of azido and diazo reagents is explored to demonstrate the feasibility of facilitating two-electron redox processes at a formally d0 Zr(IV) center using the appended Co fragment exclusively as an electron-reservoir. Addition of mesityl or adamantyl azide to 2 affords the terminal (tBuNC)Co(iPr2PNMes)3ZrNMes (3) and bridging (tBuNC)Co(iPr2PNMes)2(μ-NAd)Zr(iPr2PNMes) (4) CoI/ZrIV imido products, respectively. Similarly, diphenyldiazomethane reacts with 2 to afford the terminal Ph2CN22−-bound product (tBuNC)Co(iPr2PNMes)3ZrN–NCPh2 (5) via a two-electron oxidation of the Co center. Thermolysis of 5 results in a structural rearrangement to the diazomethane-bridged isomer (tBuNC)Co(iPr2PNMes)2(μ-N2CPh2)Zr(iPr2PNMes) (6). In contrast, treatment of 2 with 0.5 equivalents of the conjugated diazo reagent ethyl diazoacetate affords a tetranuclear ZrIV/Co0 complex, (tBuNC)Co(iPr2PNMes)3Zr(μ2–κ1-O-η2-N,N-OC(OEt)CHN2)Zr(MesNPiPr2)3Co(CNtBu) (7), bridged through enolate and η2-bound diazo functionalities.

The effect of modifying the N-aryl substituent (aryl = mesityl vs. m-xylyl) of the phosphinoamide ligands linking Zr and Co in tris(phosphinoamide)-linked heterobimetallic complexes has been investigated. Treatment of the metalloligand (iPr2PNXyl)3ZrCl (2) (Xyl = m-xylyl) with CoI2 affords the iodide-bridged product ICo(iPr2PNXyl)2(μ-I)Zr(η2-iPr2PNXyl) (3) rather than the C3-symmetric isomer observed using the N-mesityl derivative, ICo(iPr2PNMes)3ZrCl. Upon two-electron reduction of complex 3, ligand rearrangement occurs to generate the three-fold symmetric reduced complex N2Co(iPr2PNXyl)3Zr(THF) (4). Comparison of 4 with the previously reported mesityl-substituted complex N2Co(iPr2PNMes)3Zr(THF) (1) reveals similar structural features but with a less sterically hindered Zr apical site in complex 4. An obvious electronic difference between these two complexes is also present based on the drastically different infrared N2 stretching frequencies of 1 and 4. These notable differences lend themselves to different reactivity in both stoichiometric and catalytic reactions. Alkyl halide addition to complex 4 results in homo-coupling products resulting from alkyl radicals rather than the alkyl-bridged or intramolecular C–H activation products formed upon addition of RX to 1. This difference in reactivity with alkyl halides renders complex 3 a less effective catalyst for the Kumada cross-coupling of alkyl halides with n-octylMgBr than ICo(iPr2PNMes)3ZrCl, as a greater proportion of homocoupling products are formed under catalytic conditions.

The reduced heterobimetallic complex (THF)Zr(MesNPiPr2)3CoN2 (1) has been examined along with a series of structurally similar reference compounds using X-ray absorption near edge structure (XANES) spectroscopy. Complex 1 has been shown to be highly reactive, often via one-electron pathways that might be expected for a d1 ZrIII complex. However, the presence of two strongly interacting metals in complex 1 renders the assignment of oxidation states ambiguous. Both Zr and Co K-edge XANES spectra reveal that the most accurate description of complex 1 is that of a ZrIV/Co−I zwitterion. Electronic structure calculations support this assignment.

The reactivity of E–H bonds (E = S, O, Cl) with Pt(II) complexes ligated by an N-heterocyclic phosphido-containing diphosphine ligand have been investigated. Addition of PhSH to [(PPP)Pt(PPh3)][PF6] (1) results in clean formation of [(PP(H)P)Pt(SPh)][PF6] (3), in which the substrate has added across the Pt–PNHP bond. Similar reactivity occurs when 1 is treated with ROH (R = Ph, Me), but in this case the O–H bond adds across the Pt–P bond in the opposite direction producing [(PP(OR)P)Pt(H)(PPh3)][PF6] (R = Ph (4), Me (5)). HCl addition to 1 cleanly generates [(PP(H)P)PtCl][PF6] (6PF6PF6PF6). The neutral Pt–NHP complex (PPP)PtCl (2) exhibits similar reactivity; however, in the presence of the nucleophilic Cl− anion, the (PP(OR)P)Pt(H)Cl species presumably generated via addition of ROH (R = Me, Et) undergoes an Arbuzov-like dealkylation reaction to exclusively form the N-heterocylic phosphinito species (PP(O)P)Pt(H) (7).

Phosphinoamide ligands have been utilized to link a CoI center to a Nb or Ta imido fragment. The resulting heterobimetallic complexes, ICo(Ph2PNiPr)3MNtBu, have weakened dative metal–metal interactions as a result of the strongly donating imido ligand. These complexes can be reduced by 2 electrons to generate dinitrogen-bound complexes.