Introduction

Multiply charged metal cations especially tripositive and tetrapositive ions are prevalent species in solution and solid state chemistry [1, 2], but the chemistry of these highly charged cations in the gas phase is limited due to the fact that the 3rd and 4th ionization energies (IEs) of most metals lie above the IEs of common neutral ligands. As a result, charge reduction through coulomb explosion usually occurs when complexes of M3+ and M4+ are transferred into the gas phase [3, 4]. In spite of the challenges, several techniques [4], such as charge-stripping, electron ionization, and electrospray ionization (ESI), have been employed to produce a series of complexes of M3+ ligated by DMSO [5, 6], DAA [7], acetonitrile [8,9,10], DMF [9], acetone [9], peptides [11], and diamides [12,13,14] in recent decades, and the structures and reactivities of these triply charged ions were investigated by both mass spectrometric studies and theoretical calculations. For the much more challenging M4+-containing complexes, the first successful example is the observation of Th(TMOGA)34+ (TMOGA = N,N,N′,N′-tetramethyl-diglycolamide) in which Th4+ is coordinated by nine oxygen atoms from three TMOGA ligands [15]. Subsequent experimental studies on the gas-phase tetrapositive TMOGA-supported uranium/neptunium/plutonium complexes revealed redox chemistry which agrees well with the behaviors of these actinide ions in solution [16]. Our recent work showed that even Zr4+ can be stabilized in the form of Zr(TMOGA)34+ and Zr(TMPDA)34+ (TMPDA = N,N,N′,N′-tetramethyl-pyridine-2,6-dicarboxamide) in the gas phase although the 4th IE of Zr (34.34 eV) is larger than the values of thorium (28.8 eV), uranium, neptunium, and plutonium (estimated to be above 32 eV) [17].

As the heavier analog of zirconium, the aqueous chemistry of hafnium(IV) has been extensively investigated as well, which contrasts the fact that little is known regarding the gas-phase behaviors of tetrapositive hafnium complexes presumably due to its very high 4th IE of 33.33 eV as well as the great tendency for Hf4+ toward hydrolysis even at very low pH [1, 18]. Herein, we report a combined experimental and theoretical study on the formation and fragmentation chemistry of tetrapositive Hf(TMPDA)43+ and Hf(TMOGA)43+ complexes in the gas phase. These cations were produced via ESI of Hf(ClO4)4/TMPDA and Hf(ClO4)4/TMOGA in acetonitrile, respectively, and their gas-phase fragmentation behaviors were investigated by collision-induced dissociation (CID). The structures of these tetrapositive cations and their CID products were obtained by density functional theory (DFT) calculations. Results regarding the effects of diamide ligand (Scheme 1), solvent and counter-ion on the formation of tetrapositive hafnium complexes in the gas phase were also provided.

Scheme 1
scheme 1

Structures of TMGA, TMOGA, TMTDA and TMPDA.

Full size image

Experimental and Theoretical Methods

All the experiments were performed on a ThermoScientific (San Jose, CA) LTQ-XL linear ion trap mass spectrometer (LIT-MS) equipped with a heated ion max electrospray ionization source. The mixtures of 0.5 mM Hf(ClO4)4 and 1.5 mM diamide (TMGA: N,N,N′,N′-tetramethyl glutaramide, TMOGA, TMTDA: N,N,N′,N′-tetramethyl 3-thio-diglycolamide, TMPDA) in acetonitrile or methanol were prepared for ESI mass spectrometric studies, and comparative experiments were also conducted using acetonitrile solutions of 0.5 mM HfCl4 and 1.5 mM TMOGA or TMPDA under similar ESI conditions. The hafnium solutions for ESI were diluted from the freshly prepared Hf(ClO4)4 (50 mM) or HfCl4 (50 mM) solutions. Four diamide ligands were synthesized in our laboratory according to the procedures reported previously [13, 14, 17]. In enhanced mode, the instrument has a detection range of m/z 50~2000 with a mass width (FWHM) of m/z ~0.25. ESI mass spectra were acquired in the positive polarity mode and the detailed instrumental parameters are described in Supporting Information. The MSn collision-induced dissociation (CID) capabilities of the LIT-MS allow isolation and excitation of Hf(TMPDA)34+ and Hf(TMOGA)34+ cations and ion dissociation is achieved by multiple energetic collisions with the He gas. The normalized collision energy is 7% for Hf(TMOGA)34+ and 11% for Hf(TMPDA)34+. The accurate m/z of the observed cations were obtained from a Bruker Daltonics (Bremen, Germany) SolariX XR 7.0T Fourier transform ion cyclotron resonance mass spectrometer (FTICR-MS) with extreme resolution (> 106) and ultrahigh mass accuracy (error < 0.6 ppm).

DFT calculations on the observed Hf(TMOGA)34+ and Hf(TMPDA)34+ complexes and their fragmentation products Hf(TMOGA)(TMOGA-H)3+ and Hf(TMPDA)(TMPDA-H)3+ as well as the unobserved Hf(TMTDA)34+ and Hf(TMGA)34+ were carried out with the Gaussian 09 package using the hybrid B3LYP density functional [19,20,21]. The 6-31G(d) basis sets for H, S, C, N, and O were used [22,23,24], and SDD pseudopotential basis set with 60 valence electrons was used for hafnium [25]. Vibrational frequency analysis was employed to ensure that the optimized structures correspond to minima on the potential energy surface, and zero point energy (ZPE) corrections were included in the calculations of relative energies and sequential binding energies. The natural bond orbital (NBO) analysis was performed using NBO version 6.0 on the basis of the geometries of tetrapositive hafnium ions obtained at the B3LYP level with the same basis sets [26]. Additional calculations on the structures and binding energies of Hf(TMGA)4+, Hf(TMOGA)4+, Hf(TMTDA)4+, and Hf(TMPDA)4+ at the M06/6-311++G(d,p)/SDD and B3LYP/6-311++G(d,p)/SDD levels are also given for comparison [20, 21, 25, 27,28,29].

Results and Discussion

The mixture of 1:3 Hf(ClO4)4 and TMOGA in acetonitrile was first prepared for the ESI experiments. Besides the metal independent protonated ligand HTMOGA+ (m/z 189.12), the mass spectrum (Figure 1, top) from LIT-MS is dominated by two sets of peaks starting at m/z 185.08 and 279.64. The overall profile of the peaks starting at m/z 185.08 as well as the 0.25 m/z interval between neighboring peaks indicate they should be due to a tetrapositive hafnium complex, Hf(TMOGA)34+. As shown in Figure S1, this assignment is further supported by the ultrahigh resolution mass spectrum obtained using FTICR-MS, and the observed accurate m/z and peak distribution arising from different isotopomers of Hf(TMOGA)34+ are in good agreement with the calculated ones. In addition to this tetrapositive ion, tripositively charged complex Hf(TMOGA)3(ClO4)3+ (starting at m/z 279.64) is also present in the spectrum with moderate abundance. The ESI mass spectrum of 1:3 Hf(ClO4)4 and TMPDA mixture in acetonitrile (Figure 1, bottom) is rather simple, and Hf(TMPDA)34+ (most intense isotopic peak located at m/z 210.76) was observed as a major species. This quadruply charged complex can be readily assigned following the Hf(TMOGA)34+ case (Figure S2). For the tripositive Hf(TMPDA)3(ClO4)3+ complex, the most intense isotopic peak was observed at m/z 313.96 in the ESI mass spectrum. Hf(TMPDA)34+ or Hf(TMOGA)34+ is always the only tetrapositive hafnium containing complex generated via ESI of Hf(ClO4)4/TMPDA or Hf(ClO4)4/TMOGA mixture in acetonitrile. No matter how the metal concentration (0.25, 0.50, 0.75 mM) and metal-to-ligand molar ratio (5:1 to 1:5) were varied, no other binary Hf4+ complex was observed.

Figure 1
figure 1

ESI mass spectra of 0.5 mM Hf(ClO4)4/L and Hf(ClO4)4/L′ acetronitrile solutions (L = TMOGA, top; L′ = TMPDA, bottom) with metal-to-ligand molar ratios of 1:3. The asterisks denote abnormal peaks probably arising from fragmentation of fragile ions during resonant ejection from the ion trap [30]

Full size image

In addition to TMOGA and TMPDA, the ESI experiments were also repeated by using acetonitrile solutions of Hf(ClO4)4/TMGA and Hf(ClO4)4/TMTDA to probe the effect of ligand on the stabilization of gas-phase tetrapositive ions toward charge reduction. The ESI mass spectra do not show the presence of any binary tetrapositive hafnium complexes, even though the metal-to-ligand molar ratio was increased to 1:10 as well as the ESI and ion trap parameters were adjusted. In contrast, the Hf(TMOGA)34+ and Hf(TMPDA)34+ peaks appeared in the ESI spectra after TMOGA and TMPDA (three times as much as Hf4+) were added into the original Hf(ClO4)4/TMGA and Hf(ClO4)4/TMTDA solutions (Figures S3 and S4).

Figure S5 shows the ESI spectra of methanol solutions of Hf(ClO4)4/TMOGA and Hf(ClO4)4/TMPDA. The use of protic methanol as solvent resulted in the absence of tetrapositively charged diamide-supported hafnium complexes, and only protonated ligands HTMOGA+ and HTMPDA+ as well as weak peaks due to singly charged cations such as Hf(TMPDA)(ClO4)2(OH)+, Hf(TMPDA)(ClO4)2(CH3O)+, and Hf(TMPDA)(ClO4)3+ were observed, which stands in stark contrast to what was observed when the aprotic acetonitrile solutions of Hf(ClO4)4/TMOGA and Hf(ClO4)4/TMPDA were subjected to ESI. For the effect of counter-ions, experimental studies were carried out using the acetonitrile solutions of HfCl4/TMOGA and HfCl4/TMPDA respectively with the metal-to-ligand molar ratio fixed at 1:3. ESI of these solutions produced some charge reduction species such as ternary Hf(L)2Cl22+ and Hf(L)Cl3+ as well as quaternary Hf(L)2Cl(OH)2+ and Hf(L)2Cl2(OH)+ rather than binary tetrapositive complexes Hf(L)x4+ (L = TMOGA or TMPDA) as shown in Figure S6.

The observation of Hf(TMOGA)34+ and Hf(TMPDA)34+ but not the 1:1, 1:2, and 1:4 complexes in the ESI spectra regardless of the metal-to-ligand molar ratio (5:1 to 1:5) suggests that the stabilization of gas-phase tetrapositive Hf4+ complexes against charge reduction requires three TMOGA or TMPDA ligands. To explore the coordination structures of Hf(TMOGA)34+ and Hf(TMPDA)34+ as well as the unobserved Hf(TMGA)34+ and Hf(TMTDA)34+ complexes, DFT calculations at the B3LYP level were performed on these tetrapositive ions. Geometry optimizations on Hf(TMOGA)34+ and Hf(TMGA)34+ resulted in two structures with D3 symmetries (Figure 2a and b), and a similar geometry was obtained for the Hf(TMPDA)34+ complex but the pyridine ring of TMPDA slightly deviates from the ligand plane resulting in a C3 symmetry (Figure 2c). As shown in Figures 2d and S7, six isomers were obtained for Hf(TMTDA)34+ and the most stable one possesses a C3h symmetry in which all three ligands prefer boat conformation (S-CH2-C=O) with the raised S atoms forming trans arrangements. There are two other isomers with lower symmetries in which the Hf-S distances are around 2.9 Å, and both are less stable than the C3h isomer by 24.4(C3) and 20.8(C1) kcal/mol. For the rest three structures, much longer Hf-S distances around 4.7 Å were given by the calculations, and they were predicted to be 14.7 (C3), 11.3 (C1) and 8.5 (D3) kcal/mol higher in energy respectively. The Hf-Ocarbonyl distances for the optimized geometries of these four hafnium-diamide complexes are between 2.199 and 2.229 Å, which are slightly longer than the single Hf-O bond length of 2.15 Å [31], and close to the known Hf-Ocarbonyl distances in some crystal structures (2.18–2.22 Å) [32,33,34]. The computed Hf-Oether and Hf-Npyridine distances are 2.430 and 2.460 Å, respectively, which are consistent with the values obtained from the crystals containing similar moieties (Hf-Oether: 2.38 Å, Hf-Npyridine: 2.37~2.46 Å) [32, 35,36,37,38]. In contrast, the calculated Hf-Ccenter (2.892 Å) distance and Hf-S (2.995 Å) distance of the most stable isomer are much longer than the values of single Hf-C (2.27 Å) and Hf-S (2.55 Å) bonds [31]. These results suggest that the hafnium center should have bonding interactions with Ocarbonyl, Oether, and Npyridine albeit the latter two are weaker, but not with S or Ccenter. Hence, the Hf4+ center in Hf(TMOGA)34+ and Hf(TMPDA)34+ is nine coordinate with three ligand molecules bound in tricapped trigonal prismatic geometry while unsaturated six-coordinate geometries were obtained for Hf(TMGA)34+ and Hf(TMTDA)34+. The TMOGA and TMPDA ligands are bound to Hf4+ in a tridentate fashion which is also found in the cases of UO2(TMOGA)22+, Th(TMOGA)34+, Zr(TMOGA)34+, and Zr(TMPDA)34+ [15, 17, 39].

Figure 2
figure 2

Optimized structures of Hf(TMOGA)34+, Hf(TMGA)34+, Hf(TMPDA)34+, and Hf(TMTDA)34+ at the B3LYP/6-31G(d)/SDD level of theory (Hf cyan, C gray, O red, N blue, S yellow). Hydrogen atoms are omitted for clarity

Full size image

As shown in Table 1, the natural population analysis (NPA) charge on the hafnium center is significantly reduced upon coordination by diamide ligand with 1.26, 1.41, 1.19, and 1.21 e transferred from TMGA, TMTDA, TMOGA, and TMPDA to Hf4+ respectively in the 1:1 complexes. The NPA charge on hafnium decreases as the number of ligand increases. For a complex with a given stoichiometry, the charge on Ocarbonyl is almost the same regardless of the ligand type. The charge on Oether of TMOGA or Npyridine of TMPDA becomes more negative upon addition of the first ligand to Hf4+ as a result of the electron transfer from other parts of the ligand when Oether or Npyridine donates electrons to the metal center, which is similar to the cases of UO22+, Zr4+ and Th4+ [15, 17, 39]. However, the charge on S becomes more positive when the first TMTDA ligand is coordinated to Hf4+. This effect is valid even if the conformation of TMTDA (NPA charge on S of this isomer: 0.31 e) is similar to that in the Hf(TMTDA)4+ complex. Since S is uncoordinated, it is reasonable that the electrons flow from S to Ocarbonyl when TMTDA forms complex with Hf4+.

Table 1 NPA Charges and Sequential Binding Energies (kcal/mol) Computed at the B3LYP/6-31G(d)/SDD Level of Theorya
Full size table

The sequential binding energies of Hf4+ with four diamides are also summarized in Table 1. The binding energies for the first TMPDA (844.2 kcal/mol) and TMOGA (817.5 kcal/mol) are much larger than that for TMGA (774.8 kcal/mol), which are in support of the presence of Hf-Npyridine and Hf-Oether bonding in the corresponding tetrapositive complexes. However, the energy release upon coordination of the first bidentate TMTDA ligand to Hf4+ is 827.0 kcal/mol, which is even larger than that of the first tridentate TMOGA ligand. As listed in Table S1, the Hf-S distance in the Hf(TMTDA)4+ complex was computed to be 2.637 Å, which is 0.358 Å shorter than that in Hf(TMTDA)34+ and about 0.1 Å longer than the value of Hf-S single bond (2.55 Å) [31]. Such difference is very close to those obtained for Hf-Oether of Hf(TMOGA)34+ and Hf-Npyridine of Hf(TMPDA)34+, respectively, suggesting there are some bonding interactions between Hf4+ and S of TMTDA in the 1:1 complex. This is in accord with the fact that the binding energy of the first TMTDA ligand is comparable with those of the first TMOGA and TMPDA ligands. In fact, both of the Hf-X (X = Ccenter, S, Oether, and Npyridine) and Hf-Ocarbonyl distances increase by about 0.3 Å from Hf(L)4+ to Hf(L)34+, which is consistent with the decrease in sequential binding energies owing to the reduction of charge on hafnium as well as the increase of steric hindrance for ligand addition. Although the structures of diamide ligands are flexible, it has negligible effect on the gas-phase binding energy as demonstrated previously [39]. The binding energies of the first ligand calculated using the M06 functional and larger basis set are very close to those obtained at the B3LYP/6-31G(d)/SDD level of theory (Table S2).

Neither Hf(TMGA)34+ nor Hf(TMTDA)34+ was observed in the experiments despite the fact that both were predicted to be stable in the gas phase. The observation of tetrapositive Hf(TMOGA)34+ and Hf(TMPDA)34+ cations after TMOGA and TMPDA were added into either Hf(ClO4)4/TMGA or Hf(ClO4)4/TMTDA implies that the ESI process should be responsible for the absence of tetrapositive Hf(TMGA)34+ and Hf(TMTDA)34+ complexes in the gas phase. Since both Hf(TMGA)34+ and Hf(TMTDA)34+ are six-coordinate complexes, it should be more facile for small molecules such as H2O, which could result from either the moisture during the spray and ion transfer processes or the residual in the solvent, to enter the inner coordination sphere of the unsaturated Hf4+ center during ESI. This eventually leads to the formation of hafnium hydroxide due to the much higher tendency of Hf4+ toward hydrolysis.

It is worth noting that although both Hf(TMOGA)34+ and Hf(TMPDA)34+ were observed upon ESI of the corresponding acetonitrile solutions, neither of them was detected when the solvent was changed to methanol. It is most likely that these tetrapositive ions undergo alcoholysis when the hafnium complex was transferred from solution to the gas phase, during which the charge of the whole cationic complex was reduced [40]. This is in agreement with the observation of Hf(TMPDA)(ClO4)2(CH3O)+ in the spectrum shown in Figure S5. Similar methoxide cations such as Ln(TMGA)2(CH3O)2+ also appeared in the ESI mass spectra of the methanol solutions of TMGA and lanthanide chloride although the Ln(TMGA)33+ ions were the dominate species [13].

As shown in Figure S6, the ESI spectra of HfCl4/TMOGA and HfCl4/TMPDA mixtures are completely different from those of Hf(ClO4)4/TMOGA and Hf(ClO4)4/TMPDA. The much stronger affinity of Cl− toward Hf4+ than ClO4− leads to the formation of a series of ternary and quaternary cations in which the total charges of the cations are reduced upon chloride addition [41]. As a result, it is nearly impossible to observe tetrapositive Hf(L)34+ complexes when the solution for ESI contains chloride ions. Although the interaction between Hf4+ and Cl− is strong enough such that Cl− can compete with neutral diamide in binding the Hf4+ center, binary tripositive cations such as Ln(TMGA)33+, Ln(TMTDA)33+, and Ln(TMOGA)33+ were still the major species when the solutions of lanthanide chloride and diamides were subjected to ESI [12,13,14].

The observation of Hf(TMOGA)34+ and Hf(TMPDA)34+ in the ESI mass spectra makes it possible to investigate the gas-phase reactivities of these tetrapositively charged species. Only single isotopomers of Hf(TMOGA)34+ and Hf(TMPDA)34+ with highest intensities were mass selected and subjected to CID. As shown in top of Figure 3, the major CID product of Hf(TMOGA)34+ is Hf(TMOGA)(TMOGA-H)3+ (m/z 185.04, reaction 1) whose m/z is very close to that of Hf(TMOGA)34+. Accompanied by the formation of Hf(TMOGA)(TMOGA-H)3+, the protonated ligand HTMOGA+ was observed at m/z 189.08. The (TMOGA-44)+ (m/z 143.96) fragment arises from the cleavage of Ccarbonyl-N bond in TMOGA, and the weak peaks located at m/z 203.60 and 240.48 are secondary fragmentation products which were observed upon CID of Hf(TMOGA)(TMOGA-H)3+ (Figure S8). The CID spectrum of Hf(TMPDA)34+ (Figure 3, bottom) seems a little more complicated, but it is also dominated by Hf(TMPDA)(TMPDA-H)3+ (m/z 206.96) and HTMPDA+ (m/z 222.08), both of which were produced via reaction 1. Other peaks in the spectrum come from the fragmentations of Hf(TMPDA)(TMPDA-H)3+ (Figure S9). In all of the CID products, the IV oxidation state of hafnium is retained.

Figure 3
figure 3

CID mass spectra of Hf(L)34+ and Hf(L′)34+ (L = TMOGA, top; L′ = TMPDA, bottom). A1, A2 are secondary fragmentation products of Hf(L)34+ and A1′, A2′, A3′, A4′ are secondary fragmentation products of Hf(L′)34+. Single Hf(L)34+ and Hf(L′)34+ isotopomers were mass selected for CID

Full size image
$$ {\displaystyle \begin{array}{l}\mathrm{Hf}{\left(\mathrm{L}\right)}_3^{4+}\to \mathrm{Hf}\left(\mathrm{L}\right){\left(\mathrm{L}-\mathrm{H}\right)}^{3+}+{\mathrm{HL}}^{+}\\ {}\Delta \mathrm{E}=-37.6\ \mathrm{kcal}/\mathrm{mol}\left(\mathrm{TMPDA}\right),-74.3\ \mathrm{kcal}/\mathrm{mol}\left(\mathrm{TMOGA}\right)\end{array}} $$
(1)

To further understand the structure of the tripositive CID product Hf(L)(L-H)3+, geometry optimizations were carried out at the B3LYP level. For TMOGA, there are three isomers of TMOGA-H depending on the deprotonation site. The most stable structure results from the deprotonation of CH3 cis to Ocarbonyl. As shown in Figure 4a, one of the C-Oether bonds in TMOGA-H is completely broken resulting in the formation of one neutral ligand and one anionic ligand which are bound to the Hf4+ center via two Ocarbonyl and one Oalkoxyl atoms. As a result, the total charge of the Hf(TMOGA)(TMOGA-H)3+ complex is reduced to 3+ due to the formation of the Hf-Oalkoxyl bond while the oxidation state of Hf remains IV. The other two isomers where deprotonation occurs in CH3 trans to Ocarbonyl and CH2 were predicted to be 55.8 and 28.8 kcal/mol higher in energy respectively (Figure S10). For TMPDA, three isomers of Hf(TMPDA)(TMPDA-H)3+ were predicted to be stable, and two of them originate from the deprotonation of CH3 either cis or trans to Ocarbonyl with the cis-to-Ocarbonyl isomer being more stable by 12.1 kcal/mol. The third isomer in which the proton is lost from the pyridine ring is 30.0 kcal/mol higher in energy than the most stable cis-to-Ocarbonyl isomer (Figure S11). From the geometric parameters of the most stable Hf(TMPDA)(TMPDA-H)3+ isomer listed in Figure 4b, the Hf-N1 (pyridine N in TMPDA) distance is 0.184 Å longer than the Hf-N2 (pyridine N in TMPDA-H) distance. While the first distance (2.383 Å) approaches that of a typical Hf-Npyridine bond as found in Hf(TMPDA)34+, the Hf-N2 distance (2.199 Å) is very close to those of Hf-Namino bond in a series of hafnium complexes containing diamido ligands [42, 43]. This is consistent with the conversion of a pyridine N to an alkyl amino N when the CH3 group cis to Ocarbonyl is deprotonated as indicated by the elongation of the C-Npyridine bond lengths by 0.03 and 0.04 Å. The formation of the Hf-Namino bond results in a total charge of 3+ for the Hf(TMPDA)(TMPDA-H)3+ complex with the oxidation state of Hf retained as IV. On the basis of the computed Hf-O and Hf-N distances, the hafnium center in both tripositive Hf(TMOGA)(TMOGA-H)3+ and Hf(TMPDA)(TMPDA-H)3+ complexes is six coordinate, which is similar to the Ln(TMGA)33+ and Ln(TMTDA)33+ cases where six-fold coordination was found for the trivalent lanthanide center [13, 14].

Figure 4
figure 4

Optimized structures of the most stable isomers of (a) Hf(TMOGA)(TMOGA-H)3+ (Hf-O1: 2.037 Å, Hf-O2: 1.968 Å, Hf-O3: 2.182 Å, Hf-O4: 2.123 Å, Hf-O5: 2.335 Å, Hf-O6: 2.126 Å), and (b) Hf(TMPDA)(TMPDA-H)3+ (Hf-N1: 2.383 Å, Hf-N2: 2.199 Å, Hf-O1: 2.129 Å, Hf-O2: 2.129 Å, Hf-O3: 2.005 Å, Hf-O4: 2.089 Å) at the B3LYP level of theory. The deprotonation sites in both structures are labeled by the dash circles. Hf, C, O, N, and H are labeled in cyan, gray, red, blue, and white respectively

Full size image

The fragmentation behaviors of Hf(TMPDA)34+ and Hf(TMOGA)34+ upon CID are in analogous to those of Zr(TMPDA)34+ and Zr(TMOGA)34+ reported in our previous work which are dominated by the loss of a protonated ligand [17]. This is consistent with the similar chemical behaviors of both ions in solution. In addition to proton transfer (reaction 1), competitive reactions including neutral ligand loss (reaction 2) and electron transfer (reaction 3) are known during CID of multiply charged cations [3, 4]. However, no peak can be assigned to the fragments of Hf(TMPDA)34+ and Hf(TMOGA)34+ resulting from either neutral ligand loss or electron transfer upon CID. To understand the difference in the gas-phase fragmentation chemistry, computations were carried out on the energetics of these reactions. It can be found that proton transfer is exothermic for both TMOGA and TMPDA by 74.3 and 37.6 kcal/mol, respectively, while neutral ligand loss and electron transfer are endothermic with the former being more than 140 kcal/mol higher in energy than the proton transfer reaction. This is in line with the CID results that proton transfer is the major fragmentation pattern observed for both TMOGA and TMPDA complexes. Although both proton and electron transfer reactions involve charge reducing fragmentation, it is energetically unfavorable for the latter reaction to occur due to the formation of cations with Hf(III) which is an uncommon oxidation state in the coordination chemistry of Hf [1]. The absence of Hf(TMPDA)1,24+ and Hf(TMOGA)1,24+ in the CID experiments further demonstrates that nine donor atoms from three TMPDA or TMOGA ligands are needed to stabilize Hf4+ against charge reduction in the gas phase.

$$ {\displaystyle \begin{array}{l}\mathrm{Hf}{\left(\mathrm{L}\right)}_3^{4+}\to \mathrm{Hf}{\left(\mathrm{L}\right)}_2^{4+}+\mathrm{L}\\ {}\Delta \mathrm{E}=109.0\mathrm{kcal}/\mathrm{mol}\left(\mathrm{TMPDA}\right),119.2\mathrm{kcal}/\mathrm{mol}\left(\mathrm{TMOGA}\right)\end{array}} $$
(2)
$$ {\displaystyle \begin{array}{l}\mathrm{Hf}{\left(\mathrm{L}\right)}_3^{4+}\to \mathrm{Hf}{\left(\mathrm{L}\right)}_2^{3+}+{\mathrm{L}}^{+}\\ {}\Delta \mathrm{E}=13.0\ \mathrm{kcal}/\mathrm{mol}\left(\mathrm{TMPDA}\right),5.9\ \mathrm{kcal}/\mathrm{mol}\left(\mathrm{TMOGA}\right)\end{array}} $$
(3)

Conclusion

ESI of Hf(ClO4)4/TMPDA and Hf(ClO4)4/TMOGA in acetonitrile produced binary tetrapositively charged Hf(TMPDA)34+ and Hf(TMOGA)34+ complexes, respectively, which were computed to possess D3 and C3 symmetries with the hafnium center coordinated by six Ocarbonyl and three Npyridine/Oether atoms at the B3LYP level of theory. In contrast, neither Hf(TMGA)34+ nor Hf(TMTDA)34+ was observed when the acetonitrile solutions of Hf(ClO4)4/TMGA and Hf(ClO4)4/TMTDA were subjected to ESI under similar conditions. The hafnium center in both Hf(TMGA)34+ and Hf(TMTDA)34+ is coordinated by six Ocarbonyl atoms, and there is no bonding interaction between hafnium and S/Ccenter. Such geometries make it more facile for small molecules such as H2O to enter the inner coordination sphere of Hf4+, which should be responsible for the formation of hafnium hydroxide and the absence of Hf(TMGA)34+ and Hf(TMTDA)34+ in the gas phase.

In addition to the influence of ligand, solvent and counter-ion also have significant effects on whether tetrapositive hafnium ions can be formed in the gas phase. Neither Hf(TMOGA)34+ nor Hf(TMPDA)34+ was observed when the solvent of solution for ESI was changed from aprotic acetonitrile to protic methanol. This is most likely due to the alcoholysis of Hf4+ when it was transferred from solution to the gas phase. No tetrapositive hafnium ions were observed when HfCl4 instead of Hf(ClO4)4 was used as hafnium source for ESI due to the much stronger affinity of Cl− toward Hf4+ than ClO4−.

CID of Hf(TMPDA)34+ and Hf(TMOGA)34+ mainly led to the loss of protonated ligand to produce Hf(TMPDA)(TMPDA-H)3+ and Hf(TMOGA)(TMOGA-H)3+ respectively. Both tripositive hafnium products were formed via the deprotonation of the CH3 group cis to Ocarbonyl, and the Hf(IV) center in each case is coordinated by six oxygen atoms. On the basis of DFT calculations, it is much more favorable for the loss of protonated ligand to occur as observed in the experiments while loss of neutral or cationic ligand was much higher in energy. No other tetrapositive hafnium ions were observed during ESI and CID, suggesting nine donor atoms from three TMPDA or TMOGA ligands are required to stabilize Hf4+ against charge reduction in the gas phase.