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. 2022 Sep 12;61(38):15058–15069. doi: 10.1021/acs.inorgchem.2c02118

Metal–Metal Bond Umpolung in Heterometallic Extended Metal Atom Chains

Amelia M Wheaton 1, Jill A Chipman 1, Michael D Roy 1, John F Berry 1,*
PMCID: PMC9632685  PMID: 36094078

Abstract

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Understanding the fundamental properties governing metal–metal interactions is crucial to understanding the electronic structure and thereby applications of multimetallic systems in catalysis, material science, and magnetism. One such property that is relatively underexplored within multimetallic systems is metal–metal bond polarity, parameterized by the electronegativities (χ) of the metal atoms involved in the bond. In heterobimetallic systems, metal–metal bond polarity is a function of the donor–acceptor (Δχ) interactions of the two bonded metal atoms, with electropositive early transition metals acting as electron acceptors and electronegative late transition metals acting as electron donors. We show in this work, through the preparation and systematic study of a series of Mo2M(dpa)4(OTf)2 (M = Cr, Mn, Fe, Co, and Ni; dpa = 2,2′-dipyridylamide; OTf = trifluoromethanesulfonate) heterometallic extended metal atom chain (HEMAC) complexes that this expected trend in χ can be reversed. Physical characterization via single-crystal X-ray diffraction, magnetometry, and spectroscopic methods as well as electronic structure calculations supports the presence of a σ symmetry 3c/3e– bond that is delocalized across the entire metal-atom chain and forms the basis of the heterometallic Mo2–M interaction. The delocalized 3c/3e– interaction is discussed within the context of the analogous 3c/3e– π bonding in the vinoxy radical, CH2CHO. The vinoxy comparison establishes three predictions for the σ symmetry 3c/3e– bond in HEMACS: (1) an umpolung effect that causes the Mo–M interactions to become more covalent as Δχ increases, (2) distortion of the σ bonding and non-bonding orbitals to emphasize Mo–M bonding and de-emphasize Mo–Mo bonding, and (3) an increase in Mo spin population with increasing Mo–M covalency. In agreement with these predictions, we find that the Mo2···M covalency increases with increasing Δχ of the Mo and M atoms (ΔχMo–M increases as M = Cr < Mn < Fe < Co < Ni), an umpolung of the trend predicted in the absence of σ delocalization. We attribute the observed trend in covalency to the decreased energic differential (ΔE) between the heterometal Inline graphic orbital and the σ bonding molecular orbital of the Mo2 quadruple bond, which serves as an energetically stable, “ligand”-like electron-pair donor to the heterometal ion acceptor. As M is changed from Cr to Ni, the σ bonding and nonbonding orbitals do indeed distort as anticipated, and the spin population of the outer Mo group is increased by at least a factor of 2. These findings provide a predictive framework for multimetallic compounds and advance the current understanding of the electronic structures of molecular heteromultimetallic systems, which can be extrapolated to applications in the context of mixed-metal surface catalysis and multimetallic proteins.

Short abstract

This work describes how use of a metal−metal quadruply bonded metalloligand can reverse expected trends in metal−metal bond polarity through the preparation and systematic study of a novel series of Mo2M(dpa)4(OTf)2 (M = Cr, Mn, Fe, Co, and Ni) heterotrimetallic extended metal atom chain (HEMAC) complexes. These complexes feature a 3c/3e− metal−metal bond that is delocalized across the entire metal atom chain and is compared to the 3c/3e− π bonding in the vinoxyl radical.

Introduction

Bond polarity is a fundamental property of chemical systems, with donor–acceptor interactions at the heart of acid–base reactions,1−4 organic chemistry,5,6 inorganic solid state chemistry,7−11 and coordination chemistry.12−16 For example, the modern lexicon of coordination chemistry now contains X-, L-, or Z-type ligands, which indirectly describes the polarity of a metal–ligand bond by identifying the donor and acceptor involved in the interaction.17−19 Reversal of expected bond polarity, or umpolung, is also an important concept with impacts in organic synthesis,20−27 catalysis,6,28−32 frustrated Lewis pairs,33,34 and bonding in main group chemistry.35−37 Another area in which bond polarity plays an important but underappreciated role is in coordination compounds containing heterometallic metal–metal bonds. The recent explosion of interest in heterometallic compounds38−45 has produced a number of systematic series that illustrate how polarity impacts heterometallic bonding generally. Bond polarity is most commonly assessed using electronegativity, χ, and we use here the Allred–Rochow definition,46 in which χ is effectually proportional to effective nuclear charge, Zeff, according to the following equation

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Here, e is the electron charge, r is the atomic radius, and a is a constant that brings the Allred–Rochow scale in line with Pauling’s electronegativity scale.

Because χ scales with Zeff, it increases as the transition series is traversed from the early, electropositive metals to the later, more electronegative ones. Thus, in a heterobimetallic complex with a metal–metal bond, the expected bond polarity will have a late metal as a donor and an early metal as an acceptor. For example, Thomas and co-workers have extensively studied the chemistry of Co–Zr bonded compounds, in which they explicitly draw donor–acceptor arrows from the Co to the Zr to emphasize this bond polarity.47−49 Two further systematic series of compounds serve to illustrate this effect as well.

First, Lu and co-workers have reported many heterobimetallic metal–metal bonded compounds built off of a trisamidoamine-phosphine ligand scaffold.50−54 Of particular note is their series in which Cr, an early transition metal, occupies the trisamidoamine pocket and a later metal, Mn, Fe, Co, or Ni, occupies the phosphine site (Figure 1A).54 From a molecular orbital (MO) standpoint of the heterometallic interaction, the Cr atoms act as electron acceptors and the Mn/Fe/Co/Ni atoms serve as donors. With respect to the heterometallic interaction covalency, Ni has the highest χ of the four later metals, resulting in a large energetic mismatch (ΔE) between the frontier d orbitals of the Cr and Ni atoms, and therefore, the Ni atom engages least covalently with the early metal site. Metal–metal bonding thus strengthens with decreasing χ of the heterometal going from Ni to Co to Fe to Mn (Figure 1A) as reflected in the change in the calculated effective metal–metal bond orders (EBO of 0.87 for M′ = Ni; 3.94 for M′ = Mn). While the occupation of metal–metal π- and δ-symmetry antibonding orbitals contributes to the dramatic change in the calculated EBOs, a similar trend of strengthening the metal–metal σ bond with decreasing Δχ (and therefore decreasing ΔE) is also observed for a series of V–M′ (M′ = Fe, Co, Ni) bimetallics51 as well as for other bimetallic complexes containing early transition metals of Ti55,56 and Nb.57 These series help substantiate the assessment that the Δχ between the two metal atoms needs to be considered when assessing the corresponding metal–metal bond covalency.

Figure 1.

Figure 1

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(A) Schematic MO diagram depicting polar metal–metal σ bonding in bimetallics with a static early transition metal and variable later metals. (B) Similar MO diagram for bimetallics with a constant late transition metal and variable early metal. (C) Schematic MO diagram depicting polar metal–metal σ bonding in asymmetric heterotrimetallic extended metal atom chain (HEMAC) complexes with the M≣M group held constant and a variable heterometal. (D) Diagram of the Mo2M(dpa)4(OTf)2 HEMAC complexes 1–5 studied in this work. The structure of the dpa ligand, 2,2′-dipyridylamide, is shown.

Another illuminating set of compounds has been reported by Doerrer and co-workers, who have used thiocarboxylate ligands to support heterometallic interactions featuring the late transition metal Pt and a variable first-row heterometal M, where M = Mn, Fe, Co, and Ni (Figure 1B).58,59 Here, Pt is the donor and the Mn/Fe/Co/Ni atoms are the acceptors. In this set of compounds, the Pt–Mn pair is the least covalent and Pt–M bonding becomes stronger with decreasing Δχ going from Mn to Fe to Co and Ni and decreasing ΔE. As in the case of the Cr/V heterobimetallics, Δχ and ΔE are directly correlated, and decreasing the energetic mismatch in orbital energies leads to greater heterometallic covalency.

In contrast to the aforementioned bimetallic systems, our research group has utilized HEMAC complexes to probe heterometallic metal–metal interactions within multimetallic systems.40 Broadly defined, HEMAC complexes contain a linear chain of three metal atoms in idealized C4 symmetry and are supported by a helical arrangement of bridging equatorial 2,2′-dipyridylamido ligands (dpa, Figure 1D). Specifically, the HEMAC complexes that our work focuses on have a general formula of M2M′(dpa)4X2, where M2 represents two quadruply bonded early transition metal atoms (M = Cr, Mo, or W) and M′ is a third, paramagnetic, divalent transition metal ion typically from the first row of the transition series. X is a halide or pseudohalide axial ligand.

The key bonding feature of HEMACs is the three-center/three-electron (3c/3e–) bond formed via overlap of the three Inline graphic orbitals of the three metal atoms in the chain. If all three metal atoms have the same identity, then the bonding scheme becomes analogous to 3c/3e– bonded systems such as H3 or the π system of the allyl radical, as was pointed out by Bénard and co-workers in 2001.60 For HEMACs, the most straightforward comparison is to the vinoxy radical, CH2CHO, though to our knowledge this bonding analogy has not been explored before. A comparison of the relevant orbital pictures is given in Scheme 1. The three atomic orbitals (Inline graphic for HEMAC and p for vinoxy) combine to form three MOs of bonding, non-bonding, and antibonding character that are filled with the three available valence electrons of the system. For the vinoxy system, one expects a C–C bond to be more covalent than a C–O bond because O has higher Zeff than C. However, the three-center nature of the bonding results in polarity umpolung where the C–O π bond is more covalent than the C–C π bond and therefore resonance structure I is favored over II in the ground state.61 From an MO standpoint, this polarity umpolung is manifested as the π bonding orbital combination having a larger contribution from the O atom than the terminal C atom. The non-bonding combination has a small amount of central C character (rather than a node as in the allyl radical), which is slightly π bonding with respect to the C–C bond and π antibonding with respect to the C–O bond. The main radical character of vinoxy, therefore, is localized on the terminal C atom rather than the O atom.

Scheme 1. Schematic Diagram Comparing the Bonding (b.), Non-Bonding (n.b.), and Antibonding (a.b.) MOs of the Three-Center Interaction in HEMAC Complexes (Left) and the Vinoxyl Radical (Right).

Scheme 1

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M = early transition metal (Cr, Mo, or W), M′ = heterometal ion, X = anionic axial ligand.

Analogy of the π bonding in vinoxy to the 3c/3e– bond in HEMAC complexes is straightforward if we substitute the C atoms of vinoxy for the metal atoms of the quadruple bond and the O atom for the heterometal ion. Using the first reported HEMAC, Cr2Fe(dpa)4Cl2, as a representative example,62 the two individual Cr2+ ions are more electropositive than the Fe2+ ion. Based on the expected correlation of Δχ and ΔE described above for bimetallic systems, one would expect Cr ← Fe bond polarity. However, like the C p orbitals of vinoxy, the valence d orbitals of the two Cr2+ ions combine to form a set of bonding (σ, 2 × π, δ) and antibonding (σ*, 2 × π*, δ*) MOs that are energetically stabilized and destabilized, respectively, relative to those of the Fe2+ ion. The filled σ-symmetry Cr2 MO, formed via a collinear combination of two Cr dz2 atomic orbitals, could therefore serve as an electron pair donor (i.e., a dative “ligand”) to the more electronegative Fe, resulting in an umpolung of the Cr–Fe interaction and a Cr → Fe bond polarity.

Comparison to vinoxy allows us to make three important predictions about HEMACs: (1) the umpolung effect should cause the M–M′ interactions to become more covalent as the Zeff of M′ increases (Figure 1C), (2) increasing M–M′ covalency should lead to distortion of the nonbonding orbital such that it takes on M–M bonding and M–M′ antibonding character, and (3) increasing M–M′ covalency should increase the spin population of the terminal M atom. However, despite extensive development of HEMAC chemistry,40,63−67 the use of this framework for understanding the polarity of heterometallic interactions within HEMAC complexes (and more broadly within multimetallic systems) has not been explored fully. These efforts have in the past been hampered by the lack of a synthetic method to access a complete and systematic series of M2M′(dpa)4X2-type complexes with a varied scope of heterometal ions, as well as the multireference nature of the Cr–Cr multiple bond,68,69 which prevented us from fully interrogating the HEMAC complexes computationally. Thus, complete validation of this framework for understanding the trend in covalency of the metal–metal interactions in HEMAC complexes requires investigation of a broad, systematic series of M2M′(dpa)4X2-type complexes that are both synthetically accessible and computationally tractable, which is now accomplished in the work reported here.

In this work, we report the preparation of a novel series of Mo2M(dpa)4(OTf)2 HEMACs. We chose the Mo2M(dpa)4(OTf)2 HEMACs as targets due to the synthetic utility reported for Mo2Ni(dpa)4(OTf)2 (5).70 The Mo2M(dpa)4(OTf)2 HEMACs are challenging synthetic targets due to the presence of the labile OTf– ligands, which contributes to extreme air sensitivity of several members of the series. Nevertheless, we report the successful synthesis and characterization of Mo2M(dpa)4(OTf)2 HEMACs with heterometals M = Cr, Mn, Fe, and Co (1, 2, 3, and 4, Figure 1C,D) to complement the more stable 5 that we previously reported.70 Electronic structure calculations reveal that the Mo2 σ MO does indeed act as a ligand-like electron pair donor to the third heterometal ion, with Mo2–M′ covalency increasing with the Zeff of the heterometal ion. This increase in covalency stems from a decrease in the ΔE between the Mo2 σ bonding MO and the Inline graphic orbital of the heterometal ion. The decreasing ΔE, in turn, is the result of an increase in ΔχMo–M′, in direct opposition to the influence of Δχ in previously reported heterobimetallic systems with early transition metals. Comparison to the Mo2M(dpa)4Cl2 series reveals that this heterometallic umpolung is a broader trend among HEMAC complexes and whose characteristics are analogous to the π bonding in the vinoxy radical. These results provide straightforward guiding principles that may be used to understand and modulate the covalency of metal–metal bonds within other types of multimetallic systems, such as those involved in surface catalysis43,71,72 or metalloenzyme active sites.73−81 They are also relevant within the context of studies devoted to ennobling Earth-abundant base metals for various applications,82−88 as in this work we demonstrate that the early transition metal Mo can take on electronegativity characteristics of a later transition metal (e.g., Pt) when participating in a metal–metal multiple bond.

Results and Discussion

Synthesis

M(OTf)2 Salts

Compound 5 is best prepared via reaction of Mo2(dpa)4 with 1.5 equiv of Ni(OTf)2.70 Use of an analogous synthetic route for other Mo2M(dpa)4(OTf)2 HEMACs is complicated by the fact that Cr(OTf)2 is not commercially available and that other anhydrous M(OTf)2 salts, despite being commercially available, are inconsistently characterized89 and in our hands give inconsistent results. We therefore considered several methods for preparing M(OTf)2 starting materials,89−93 ultimately settling on the precipitation of an insoluble salt.89,91 We found that reaction of MCl2 (M = Cr, Mn, Fe, Co, and Ni) with 2.0 equiv of TlOTf in CH3CN affords the M(OTf)2 salts in good yields and high purity (Scheme 2A). This preparation involves the initial formation of a soluble, heteroleptic M(CH3CN)x(OTf)2 complex and precipitation of TlCl. After filtration and drying, stretches in the 2300 cm–1 range of the infrared (IR) spectra indicate residual CH3CN coordination to the M(OTf)2 salt (Figure S1).93 Further drying of these powders under vacuum for 12 h at 80 °C afforded the rigorously solvent-free M(OTf)2 salts as demonstrated by disappearance of the nitrile stretches in the IR (Figure S2) as well as by elemental analysis. The M(OTf)2 salts were also characterized for the first time by 19F NMR spectroscopy in CD3CN solutions (Figure S3), and the spectra indicate an equilibrium between metal-bound and free triflate. The Fe(OTf)2 precursor was also characterized by Mossbauer spectroscopy (Figures S4 and S5) and is consistent with the previously reported spectrum of the compound prepared by reaction of triflic acid and Fe metal.94,95 Ultimately, we observe that using the M(OTf)2 salts prepared by our synthetic method for preparation of the heterometallic complexes as described below improves overall product purity and reaction yields.

Scheme 2. Overview of the Synthetic Methods Used for This Work (A) Preparation of the M(OTf)2 Salts. (B) Preparation of 1, 3, 4, and 5. (C) Preparation of 2.

Scheme 2

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Heterometallic Complexes

1, 3, and 4 were straightforward to prepare in an analogous manner to the synthetic route we previously used to access 5 (Scheme 2B). However, all attempts to prepare 2 using a molten naphthalene route failed to provide an analytically pure crystalline product. Instead, this general method afforded a green-brown powder that was characterized by UV–vis (Figure S6) and IR spectroscopies (Figure S7). The electronic absorption spectra of the powder material in CH2Cl2 shows a broad feature at 800 nm corresponding to the reported energy and characteristic shape of the δ → δ* transition of the [Mo2(dpa)4]+ cation.96 The IR spectrum contains stretches corresponding to the Mo2(dpa)467 unit as well as stretches at 1030 and 623 cm–1 that are characteristic of a triflate SO3 moiety.90 This green-brown powder is therefore likely a mixture of 2 and [Mo2(dpa)4][OTf]. We were unable to isolate or characterize any monometallic MnII containing products; however, we hypothesize that the MnII(OTf)2 reagent can act as an oxidant to the Mo2(dpa)4, forming [Mo2(dpa)4][OTf]- and MnI(η6-C10H8)2-type complexes, which are well known.97−99

An alternative synthetic route for 2 was therefore developed and employed based upon the observation that 5 could be prepared via reaction of Mo2Ni(dpa)4Cl2 and 2 equiv of TlOTf.70 A slow but steady color change from brown to green is observed upon stirring Mo2Mn(dpa)4Cl2 in CH2Cl2 with excess TlOTf for 2 weeks (Scheme 2C). The sparing solubility of TlOTf in CH2Cl2 limits the reaction rate and yields of 2.

Crystallography

In this work, the entire Mo2M(dpa)4(OTf)2 (M = Cr, Mn, Fe, Co, Ni) series has now been structurally characterized via X-ray crystallography (Tables S1 and S2). A representative structure is shown in Figure 2, with the other structures shown in the Supporting Information. The complete structural characterization of the Mo2M(dpa)4(OTf)2 series also invites comparison to the structures of the Mo2M(dpa)4Cl2 [M = Cr (6),63 Mn (7),100 Fe (8),66 Co (9),67 and Ni (10)101] series, reported elsewhere and whose distances are reproduced in Table S3. Table S2 also contains the density-functional theory (DFT) optimized geometric parameters for the Mo2M(dpa)4(OTf)2 HEMACs with the M2+ ions, and in general, the calculated geometries are in good agreement with the experimental ones. This provides support for the computational methods employed in the spectroscopic and electronic structure analyses of the Mo2M(dpa)4(OTf)2 HEMAC complexes, described later.

Figure 2.

Figure 2

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Molecular drawing of 2 with selected atom labels. All atoms are shown at 50% probability levels; all H atoms and minor disordered components are omitted for clarity.

Focusing on the heterometallic interactions in 1–10, we see a steady decrease in Mo–M distance across both series from M = Mn to Ni. The low-spin compound 4, as well as the Cr compounds, have anomalously short Mo–M distances. One possible reason for the general contraction in Mo–M distance is the decreasing size of the M atoms. However, the contraction in Mo–M distance, ΔdMo–M = 0.26 to 0.27 Å, is steeper than the contraction in M–N distances, ΔdM–N = 0.08 to 0.10 Å. The M–X distances to the axial OTf in 2–5 or Cl in 7–10 actually increase slightly by 0.07 Å, a possible trans influence of the shorter Mo–M interaction. It is therefore reasonable to assign a degree of covalency to the Mo–M bonds, which increases as M is changed from Mn to Ni.

SQUID Magnetometry and EPR Spectroscopy

Comparison to the vinoxy radical suggested that the outer Mo atom of the metal–metal quadruple bond in 1–5 would display some paramagnetic character. However, as has been observed for the [Mo2Ni(dpa)4Cl2]+ cation,101 the σ-symmetry spin delocalization will reinforce the spin on M additively and would therefore not be detectable by SQUID magnetometry. Thus, for the purpose of modeling the magnetic susceptibility data, the paramagnetism of the heterometal ion is the dominant feature and 1–5 may be considered to a first-order approximation to contain a diamagnetic Mo2 quadruply bonded unit attached to a paramagnetic metal site. For example, 5 was previously described as having an S = 1 ground spin state stemming from the high-spin Ni2+ ion.70 To establish the spin states of the paramagnetic metals in 1, 3, and 4 (low yields of 2 prevented physical measurements), magnetic susceptibility, EPR, and Mössbauer spectroscopies are used. Magnetic susceptibility and reduced magnetization measurements for 1, 3, and 4 are shown in Figure 3 (Table 1 for fitting parameters), and EPR spectra of 4 at 10 K are shown in Figure 4. The Mössbauer spectrum of 3 is also shown in Figure 3.

Figure 3.

Figure 3

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Left: Magnetic susceptibility data and fits for 1, 3, and 4. Right (counterclockwise from top left): reduced magnetization data and fits for 1 (A), 3 (B), and 4 (C, data not fitted). The Mössbauer spectrum of 3 at 80 K is shown in the figure on the bottom right (D): δ = 1.05 mm/s, ΔEQ = 2.48 mm/s, Γ = 0.27 mm/s, χ2 = 2.37; δ(calc.) = 0.93 mm s–1; ΔEQ(calc.) = 3.33 mm s–1.

Table 1. SQUID Fit Parameters for 1, 3, and 4.

  1 3   4
g∥ 1.9332(6) 2.32(1) gLS 2.23b
g⊥ 2.0704(3) 2.152(3) gHS 2.53c
D (cm–1) –2.052(3) –6.42(8) ΔH 15.7(5)
Dcalc (cm–1) –3.132 –3.375 TC 339(2) K
Ecalc (cm–1) 0.008 0.016 PHS 6.8(1)%
TIP –0.00029(6) 0.0004(1) TIP 0.00201(4)
residuala 0.000034 0.1016    
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a

Inline graphic; where Mexp/Mcalc = measured/calculated magnetization and χexp/χcalc = measured/calculated susceptibility.

b

Fixed to the value determined by EPR.

c

Fixed to the g value determined for 9.

Figure 4.

Figure 4

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X-band EPR spectrum of 4; frequency = 9.38 MHz, temperature = 10 K. Simulation parameters: [gx, gy, gz] = [2.34, 2.25, 2.01]; [Ax, Ay, Az] = [160, 230, 322] MHz; H-strain = [700, 100, 200] MHz.

The measured values of χPT at 300 K (Figure 3) for 1 and 3 are 2.98 and 3.97 cm3 K mol–1. These values are consistent with S = 2 systems with g-factors slightly below and moderately above g = 2 and are expected for systems with less than and greater than five valence d electrons, respectively. The sharp downturn of the χPT values at low temperatures and nested variable temperature magnetization curves for 1 and 3 (Figure 3A,B) are consistent with strong magnetic anisotropy that can be modeled as zero-field splitting (ZFS). The data for both 1 and 3 were fitted with axial ZFS parameters with D < 0 for both HEMACs. The value of D for 1 (−2.052 cm–1) is slightly smaller than that of 6 (−2.187 cm–1).89 For 3, D is slightly smaller in magnitude and opposite in sign to that of Cr2Fe(dpa)4Cl2 (D = 8.3 cm–1).62 The sign and magnitude of D for 1 and 3 match those predicted from the DFT-modeled electronic structure (Table 1, vide infra). The negative values for Dcalc and the small values of Ecalc (ca. 0.01 cm–1) indicate easy-axis anisotropy of the Cr and Fe sites in 1 and 3, with an elongation along the metal–metal axis.102−104 The electron delocalization through the Mo–Mo–M chain likely contributes to this axial anisotropy. This idea is further supported by the observed magnetic anisotropy of M2Cr(dpa)4Cl2 systems where the magnitude of D increases in the order Cr < Mo < W. This increase is consistent with an increase in spin–orbit coupling (i.e., the heavy ion effect), in which the M≣M unit is considered as a ligand to the paramagnetic site.105

The Mössbauer spectrum of 3 displays an isomer shift, δ = 1.05 mm s–1, and quadrupole splitting, ΔEQ = 2.48 mm s–1, similar to that reported for the Mössbauer spectrum of 8 (δ = 1.02 mm s–1 and ΔEQ = 2.02 mm s–1)66 and supports the assignment of a high spin S = 2 ion in the trimetallic complex. The good agreement between measured and DFT-calculated Mössbauer parameters for 3 [δ(calcd) = 0.93 mm s–1; ΔEQ(calcd) = 3.33 mm s–1] provides further support for the calculated electronic structure discussed in more detail below. Notably, the ΔEQ for 3 is substantially larger than that of the Fe(OTf)2 precursor (ΔEQ = 2.04 mm s–1, Figure S4), indicating an increase in the electric field gradient about the Fe nucleus in 3 in the presence of the Mo2 metalloligand. This increased asymmetry is consistent with the axial anisotropy observed for 3 in the SQUID magnetometry measurements.

Crystallographic data at 100 K, particularly the short Co–N bond lengths, suggest that the Co2+ ion in 4 is low spin, S = 1/2, which is consistent with the measured χPT (ca. 0.6 cm3 K mol–1) at the low temperature limit (spin-only: χ·T = 0.375 cm3 K mol–1). This assignment is also corroborated by superimposable reduced magnetization curves (Figure 3C) and the lack of a downturn in χPT at low temperatures, which, taken together, indicate that there is no ZFS present for 4, as an S = 1/2 state is not subject to ZFS. The X-band EPR spectrum of 4 at 10 K shows one major slightly rhombic signal centered at gavg = 2.23. The observed g-value and extensive hyperfine (due to coupling with the I = 7/2 59Co nucleus) is consistent with an S = 1/2 ground state. Above 150 K, however, the χPT data for 4 increases steeply toward a value of 2.5 cm3 K mol–1 at 300 K. This behavior cannot be explained by the S = 1/2 ground state or by temperature-independent paramagnetism (an effect that would be present at all temperatures). The most plausible explanation for the high-temperature magnetic data is that 4 displays partial spin crossover, where thermal energy can populate the higher S = 3/2 spin state. The parameters that resulted from fitting the magnetic susceptibility data for 4 are listed in Table 1, and further discussion of the spin-equilibrium model used for the fit can be found in the Supporting Information.

Three previous HEMAC complexes with Co2+ heterometals have been characterized via SQUID magnetometry, and 4 is the third to display spin-crossover behavior. A summary of these results is outlined in Table 2. For all three HEMAC complexes with spin-crossover behavior, ΔS is positive, as a result of the high-spin state having more degrees of freedom than its low-spin counterpart. Compound 9 is the only HEMAC with a high spin, S = 3/2 Co2+ ion at all temperatures; as such, ΔH < 0 and cannot be measured. For 4, the ΔH needed to overcome spin pairing is 15.7 kJ/mol, which is higher than those of all of the other HEMACs, which bear π-donating (and thus weaker field) axial halides.

Table 2. Comparison of the Electronic Properties of the Co2+ Ions in M2Co(dpa)4X2 HEMACsa.

compound S (Co2+) ΔH (kJ mol–1) ΔS (J mol–1 K–1) refs
4 (M = Mo, X = OTf–) 1/2 → 3/2 15.7(5) 46 this work
Mo2Co(dpa)4Br2 1/2 → 3/2 13.95 67 (106)
9 (M = Mo, X = Cl–) 3/2 n/a n/a (67)
Cr2Co(dpa)4Cl2 1/2 → 3/2 5.7 27 (67)
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a

S = spin state of the Co2+ ion, where 1/2 → 3/2 indicates spin-crossover behavior, ΔH = enthalpy difference between low and high spin states, and ΔS = entropy difference between low and high spin states.

Electronic Absorption Spectroscopy

Electronic absorption spectra for 1, 3, 4, and 5 in CH2Cl2 show three major transitions at ca. 420 nm (A), 550 nm (B), and 630 nm (C) (Figure 5, Table S4). Time-dependent DFT (TDDFT) studies of 5 indicated that the major contributions to the A and B transitions arise from Mo≣Mo δ donation to the pyridyl rings of the dipyridylamine ligand (metal to ligand charge transfer, or MLCT) while a Mo≣Mo δ → δ* transition gives rise to the lower energy and weaker intensity C bands.70 The similarity of transition intensities and band shapes indicate a similar assignment of the A, B, and C transitions for all four compounds as MLCT (A and B) and δ → δ* (C). Assignment of the low-energy features as Mo≣Mo δ → δ* transitions (with the exception of 3; see below) is supported by TDDFT calculations as shown in Figure S18. The differences between calculated and experimental transition energies are of a similar magnitude to that observed for other HEMAC complexes.63,70

Figure 5.

Figure 5

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Electronic absorption spectra for 1, 3, 4, and 5.

Transition C for 3 deserves special comment, as it is red-shifted and broadened compared to the other HEMACs.66 Calculations on 3 indicate that the experimental C transition corresponds to a feature at 910 nm in the calculated spectrum (Figure S18 inset) that corresponds to a Fe dxy → Mo≣Mo δ* transition (Figure S19). This assignment is consistent with the calculation of a doubly occupied δ symmetry Fe dxy-based MO that is higher in energy than the Mo≣Mo δ orbital (Figure S24). Additionally, the calculated 910 nm feature also has two Fe dxy → dpa π* transitions as the other major contributors to its calculated intensity instead of Mo≣Mo δ → δ*-based transitions. This effect is unique to the Fe compound, as the Co dxy orbital in 4 is lowered below the energy of the Mo2 δ orbital due to increased Zeff (Figure S25).

Electronic Structure

Single-point DFT calculations of 1–5 were performed in order to determine the nature of the metal–metal bonding in 1–5 and to allow comparison to 6–10. The Mo2M(dpa)4X2 (X = anionic axial ligand) HEMACs have similar electronic structures, the basis of which can be understood by a ligand field theory (LFT) description of the Mo≣Mo quadruple bond, particularly its low-energy, filled σ-symmetry component, acting as an electron-pair donor to the heterometal ion within a C4 symmetric ligand field.

Figure 6 presents a representative MO diagram using this treatment on the Mo≣Mo···Cr complex. The four lowest occupied and four highest unoccupied energy MOs of the HEMAC are Mo≣Mo based and conform to the σ, two π, and δ bonding and antibonding orbitals of the Mo≣Mo quadruple bond with a, e, and b symmetry, respectively, in the C4 point group. The middle five MOs receive their predominant contribution from the five d orbitals of the Cr2+ ion (two δ orbitals, each of b symmetry, two e-symmetry π orbitals, and the σ orbital of a symmetry). The Mo≣Mo π and the δ orbitals of e and b symmetry are of the proper symmetry to overlap with e- and b-symmetry orbitals on Cr, but the spatial overlap is not effective. Thus, these sets of orbitals are primarily localized to the Mo≣Mo or to Cr, respectively. The a-symmetry Inline graphic orbitals, conversely, overlap sufficiently well to form bonding, non-bonding, and antibonding σ-symmetry combinations. The doubly occupied σ and singly occupied σnb orbitals form a 3c/3e– bond that is delocalized across the entire metal-atom chain. As outlined in the Introduction, with comparison to the vinoxy radical, in EMACs and symmetric HEMACs, the σnb orbital node is on the central metal atom (Scheme 1). For asymmetric HEMACs, the exact placement of this node varies with the identity of the heterometal.

Figure 6.

Figure 6

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Ligand field theory treatment of metal–metal bonding in asymmetric HEMAC complexes. The dark gray box is used to emphasize the HEMAC MOs that involve the heterometal d orbitals.

This MO picture is complicated by the fact that 1–5 possess C1, not C4, symmetry. However, our calculated results (see Figures S22–S27 for MO diagrams for 1–5) do conform to our expectations from this LFT model and thus validate the use of this framework for the discussion herein. For 1–5, the δ and π symmetry orbitals are localized to either the Mo2 quadruple bond or the paramagnetic heterometal. Notably, the singly occupied orbitals (with exception of the σnb, vide infra) are heterometal based. This is consistent with our observation that the magnetic behavior of 1–5 is well modeled under the assumption of a diamagnetic Mo2 moiety and an isolated, paramagnetic heterometal ion.

The isosurface plots and metal atom contributions to the β σ and σnb orbitals are shown in Figure 7 (see Figure S20 for α component contributions, which, due to spin polarization, suffer from substantial ligand contributions that obfuscate the analysis of metal contributions). For 1–5, the lowest-energy σ MO is primarily Mo≣Mo in character, while the σnb orbital is primarily of heterometal character. This situation arises from the relative stability and strength of the σ bonding contribution of the Mo≣Mo quadruple bond, leaving it significantly lower in energy than the heterometal ion’s σ-symmetry Inline graphic orbital. As the energetic difference (ΔE) between the Mo2 σ orbital and the heterometal Inline graphic orbital decreases, the Mo2 contribution to the three-center σnb orbital increases. These contributions are predicted by the LFT model and the analogy to the vinoxy radical, as the filled Mo≣Mo σ-component acts as a “metalloligand” to the heterometal ion; the reproduction of this effect by DFT lends credence to these models.

Figure 7.

Figure 7

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Isosurface plots and orbital occupations for the β components of the σ and σnb orbitals of (from left to right) 1, 2, 3, 4, and 5.

The extent of delocalization, or covalency, of the 3c/3e– σ bond therefore depends on the identity of the heterometal ion in the metal-atom chain. We calculate a gradual increase in the heterometal ion’s contribution to the β σ orbital (from ca. 13.2 to 37.7%) moving from the left to the right across the periodic table. The calculated trends indicate a polarization of the 3c/3e– metal–metal bond toward the heterometal ion with increasing Zeff of the heterometal ion (Cr < Mn < Fe < Co < Ni). This polarization also corresponds to an increase in the Mo2···M antibonding character in the σnb orbital, with the contribution of the central Mo atom to the β σnb orbital increasing from ca. 2.0 to 12%. This shift from a truly non-bonding to a relatively Mo2···M antibonding σnb MO can be rationalized as an increase in the energetic destabilization of the σnb MO with an increase in the covalency of the Mo2···M interaction. In the most covalent case, the Mo2–Ni σ and σnb orbitals are highly reminiscent of the 3c/3e– π and πnb orbitals of vinoxy radical (Scheme 1). The trends in polarization of both the σ and σnb MOs can be understood as an increase in the heterometallic bond covalency with increased Zeff of the heterometal. Specifically, increasing the Zeff of the heterometal ion (i.e., increasing the Mo–M Δχ) stabilizes the heterometal d orbitals, bringing them closer in energy to the bonding MOs of the Mo≣Mo metalloligand, decreasing the energetic difference ΔE between the two contributors as we originally outlined in Figure 1C. The improved energetic overlap strengthens the Mo2···M interaction, resulting in a polarization of the σ and σnb MOs (with Mo2···M antibonding interactions) toward the increasingly electronegative heterometal and a more covalent 3c/3e– bond. Comparison of 1–5 with 6–10 supports this analysis, as the σ and σnb orbital contributions follow the same general trends (Figure S21), with the 6–10 orbitals also having significant contributions from the Cl pz orbitals. The most pronounced discrepancies in the calculated contributions are between 4 and 9, which are expected given that the two Co2+ ions have different ground spin states. The observation of polarization of the σ MOs for both series indicates that the heterometal ion can be used to modulate the covalency of the 3c/3e– metal–metal bond independent of the choice of axial ligand.

The calculated Mayer bond orders (MBO) for the Mo≣Mo and Mo–M bonds provide an unambiguous display of the influence of the identity of the heterometal ion on the 3c/3e– metal–metal bond covalency. The leftmost and center plots in Figure 8 show the calculated Mo≣Mo and Mo–M MBOs, respectively, plotted versus the heterometal electronegativity χ, which is directly proportional to Zeff.46,107 The calculated Mo–Mo MBO values between 2 and 3 indicate a significant weakening from a true Mo≣Mo quadruple bond,108 and Mo–M MBO values < 1 but non-zero indicate partial σ bonding. Most importantly, we observe a concomitant decrease in the Mo≣Mo MBO and increase in the Mo2–M MBO with increasing χ of the heterometal ion. These trends demonstrate a shift of the electron density associated with the 3c/3e– σ bond toward the heterometal ion and therefore an increase in heterometallic metal–metal bond covalency. As outlined in the Introduction, this trend in heterometallic bonding is opposite of that based purely on polarity expectations—for Mo–Cr, having a smaller ΔχMo–Cr = 0.26, the bonding interaction is the weakest, while the Mo–Ni interaction is strongest despite the larger ΔχMo–Ni = 0.45. This umpolung of the heterometallic bond from initial expectation is due to the fact that the strong Mo≣Mo bond yields a low-energy, σ-symmetry electron pair that acts as a ligand to the third metal. Rather than behaving as a typical early, electropositive transition metal, the Mo2 unit has bond polarity characteristics that are more like those of late, relatively electronegative transition metals like Pt.

Figure 8.

Figure 8

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Plots of the Mo≣Mo (left) and Mo–M (center) MBO v. Allred–Rochow electronegativity of the heterometal ion (M). Right: Ratio of the outer Mo atom and heterometal ion Löwdin spin densities.

Just as the vinoxy radical is described by limiting resonance structures I and II, we can propose resonance forms III and IV for Mo2–M HEMACs, where the σ bonding can be localized to either the Mo–M(III) or the Mo–Mo(IV) interaction (Scheme 3). The π and δ bonds remain localized to the Mo2 unit, and only the Inline graphic spin of M is indicated. We anticipate that both resonance forms III and IV contribute to the Mo2–M HEMAC series, with greater contribution from form III as M is changed from Cr to Mn to Fe to Co to Ni.

Scheme 3. Resonance Forms of the Vinoxy vs the Mo2–M 3c/3e– Bonding System.

Scheme 3

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As highlighted in Scheme 3, the distribution of σ-symmetry spin is expected to change with the metal atom identity as well. Like the π non-bonding orbital of the vinoxy radical, we predicted that the σnb orbital of HEMAC complexes would show an increase in the spin density localized to the terminal Mo atom with an increase in the Zeff of the heterometal ion. The rightmost plot in Figure 8 shows the ratio of the calculated Löwdin spin densities of the terminal Mo atom and the heterometal ion in 1–3, 4 (S = 3/2), and 5 versus the heterometal χ (Table S5). The calculated spin density ratios show a consistent increase from M′ = Mn to Ni, indicating an increase in the localization of paramagnetic character on the terminal Mo atom with an increase of heterometal χ. The relatively large Mo/M′ spin density ratio for 1 can be rationalized in light of the fact that both the crystallographic and calculated structures of 1 have exceptionally short Mo–Cr distances, resulting in a greater degree of bond covalency that what would be independently predicted from the M′ identity alone. The increase in spin density with increasing M′ χ also suggests that the outer Mo atom in HEMACs with relatively covalent Mo–M′ bonds (i.e., 4 and 5) could engage in radical-like chemical reactivity, a possibility we intend to explore in future work.

Conclusions

This work aimed to answer two questions that have underpinned previous studies of HEMAC systems: can a metal–metal quadruple bond serve as a dative ligand to a transition metal ion and result in an umpolung of the heterometallic interaction, and can we use the characteristics of that transition metal ion to modulate the covalency of the heterometallic interactions? This work now serves as a verification of those hypotheses via a systematic preparation and characterization of a series of Mo2M(dpa)4(OTf)2 (M = Cr, Mn, Fe, Co, Ni) HEMACs. The synthetic route that previously afforded 5 was used to prepare 1, 3, and 4, and we also report an improved synthetic route to the anhydrous M(OTf)2 salts of Cr - Ni. Crystallographic and physical measurements of 1, 2, 3, and 4 allow assignment of the heterometal ions as high spin species with the exception of 4, which displays incomplete spin-crossover behavior. The negative measured and calculated values of D for 1 and 3 are attributed to the large easy-axis magnetic anisotropy that the Mo≣Mo quadruple bond enforces on the heterometal ion. Electronic structure calculations of the Mo2M(dpa)4(OTf)2 HEMACs support the presence of a σ symmetry 3c/3e– bond that is delocalized across the entire metal-atom chain. The extent of this delocalization increases with increasing Zeff of the heterometal ion (Cr < Mn < Fe < Co < Ni), as systematized in a linear trend in heterometallic bond order against Allred–Rochow electronegativity, χ. This trend is opposite to the expectation for bonding of these metals to a typical early transition metal ion, indicating that the strong Mo≣Mo quadruple bond causes an umpolung of the heterometallic 3c/3e– bond and is analogous to the π bonding in the vinoxy radical. Such effects can be used in future design of HEMAC compounds to tune the heterometallic covalency and stability and guide future synthetic efforts at accessing novel trimetallic chains. Ultimately, these findings provide new tools to understand the electronic structure of multimetallic systems. These tools can also be applied to fields of mixed-metal heterogenous (ad-atom or single-atom) catalysis109,110 and biology in the realm of metalloenzymes. For example, it is notable that the Mo–Fe distances in 3 and 8 are similar to the Mo–Fe distance in the FeMo cofactor of nitrogenase, which contains several iron atoms and a Mo site.80,111−113 We also anticipate the results reported here to be explored within the context of base metal catalysis, as we demonstrate earth-abundant base metals to be ennobled by the presence of a neighboring metal–metal bond.

Acknowledgments

We thank the US National Science Foundation for supporting this work via CHE-1953924. The Bruker ImpactTM II MALDI mass spectrometer and the Bruker Quazar APEXII diffractometer were supported by a generous gift from Paul J. and Margaret M. Bender. The Bruker EleXsys E500 EPR spectrometer was supported by NSF CHE-0741901. The SeeCo Mössbauer spectrometer with a Janis SHI-850 4K cryostat and the Quantum Design MPMS3 SQUID magnetometer with an EverCool System were additionally supported by the UW-Madison Department of Chemistry. MO graphics were generated with UCSF Chimera, developed by the Resource for Biocomputing, Visualization, and Informatics at the University of California, San Francisco, with support from NIH grant P41-GM103311.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.2c02118.

  • Cartesian coordinates (ZIP)

  • Experimental details, additional characterization data, and computational methods (PDF)

The authors declare no competing financial interest.

Dedication

We dedicate this work in memory of Prof. Carlos A. Murillo, who was an exceptional synthetic chemist, friend, and mentor.

Supplementary Material

ic2c02118_si_001.zip (19KB, zip)
ic2c02118_si_002.pdf (3.6MB, pdf)

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