Gas-Phase Metal Trications in Protic Solvent Complexes Alexandre A. Shvartsburg DiVision of Chemistry, National Center for Toxicological Research, HFT-233, 3900 NCTR Road, Jefferson, Arkansas 72079 Received January 31, 2002 Ligated multiply charged metal (M) ions in the gas phase have become topical lately. These species were not accessible until the 1990s, because, unlike M + , metal polycations generally do not ligate when passing through the ligand (L) vapor. 1 Instead, they are normally reduced by L on contact (The second ionization potentials (IPs) of all metals except Ca, Sr, and Ba are above 12 eV, while the IPs of most solvent molecules are 8-12 eV. The third IPs of metals lie above 19 eV). However, solvated M 2+ and M 3+ can be generated by electrospray ionization (ESI) that transfers the solvated ions into the gas phase, inside a solvent shell. 2-10 This circumvents the small size range where the charge transfer occurs. An alternative is to ligate M or M + and ionize the ML n (M + L n ) complex, as implemented in the pick-up 11,12 and charge-stripping 13 techniques. These methods have produced clusters of M 2+ with both protic and aprotic solvents, including water, 2-4,6-8,10-13 alcohols, 4,5,11,12 ethers, 11 ketones, 4,8,11,12 dimethyl sulfoxide, 2,4 benzene, 12 acetonitrile, 4,5,9,11,12 and pyridine. 5,12 However, known complexes of M 3+ had been limited to four aprotic ligands: DMF, 14 DMSO, 4,14 acetonitrile, 4,15 and acetone. 4,15 Despite sustained efforts for water and simple alcohols, no complexes of metal trications with any protic ligand (HOR) could be produced. 4,14,15 Instead, charge-reduced species such as M 2+ X(HOR) n (X is the counterion of dissolved metal salt) or M 2+ OR(HOR) n were typically observed in ESI. The reason for that is not obvious, considering that the third IPs of two metals (La 19.2 eV and Ce 20.2 eV) are below the second IP of Cu (20.3 eV), but Cu 2+ readily forms gas-phase complexes with water and alcohols. 3,4,7,12,13 Here we report the observation of metal trication complexes with a protic ligand (L), diacetone alcohol (below termed DAA), (CH 3 ) 2 C(OH)CH 2 COCH 3 . Dissociation of these species is studied to elucidate the size ranges of their existence. This work was performed using the Finnigan TSQ 7000 MS/ MS system. Ligated M 3+ ions were generated from M(NO 3 ) 3 dissolved in aqueous DAA. The ESI needle was at 4 kV, sheath gas flow was low, and auxiliary gas was off. The heated capillary was at 110-150 °C. M 3+ L n ions found in the Q1 scan usually had n ) 7-11. They were fragmented in Q2 at the lab energies (E) of 0-240 eV by collisions with Ar at 0.3 mTorr (close to single- collision conditions) or 1.3 mTorr (multicollisional CID). To verify the assignments, most experiments were reproduced using d 12 -DAA/ D 2 O and, where possible, different metal isotopes. Lanthanum has the lowest third IP of all trivalent metals. The Q1 scan (Figure 1A) reveals (along with usual charge-reduced species) a substantial yield of La 3+ (DAA) n for n ) 8-12, with a maximum at n ) 9. Could those features be La 3+ (acetone) 2n arising from an adventitious acetone impurity in DAA? (Acetone com- plexes of La 3+ and Ho 3+ were reported. 4,15 ) The absence of La 3+ (acetone) 2n+1 peaks convinces that the features observed truly are DAA complexes. This would be confirmed in the MS/MS experiments. M 3+ (DAA) n complexes were also identified for Y and all other rare-earth elements studied: Ce, Pr, Sm, Eu, Gd, Tb, Ho, Yb, Lu (Nd, Dy, Er, and Tm were not tried). The third IPs of these metals range from 20 eV (Ce) to 25 eV (Eu and Yb). Increasing third IP values favor charge reduction, and the yields of trications go down while the distribution maxima shift to slightly higher sizes at n ) 10-11 (Figure 1B). The hallmark property of ligated metal polycations is the dissociative electron or proton transfer. As reviewed above, the difference in IPs makes nearly all M k+ -ligand heterodimers thermodynamically unstable (although metastability due to the energy barrier preventing charge transfer is common 16 ). In the other extreme, metal di- and trications exist in the bulk solvents, and macroscopic droplets of such solutions evaporate by the “neutral ligand loss” only. Hence, there must be a critical size (n crit ) at which a shrinking droplet first charge-reduces. There also may be a minimum size (n min e n crit ) for which the M k+ L n complex could still be observed. For precursors with n min < n e n crit , the ligand loss and dissociative charge transfer compete. To elucidate the size range of their existence (i.e., determine n crit and n min ), M 3+ (DAA) n species for all metals listed above were probed using CID. Experiments at the low- and high-collision gas pressures produced essentially identical findings. Fragmentation * To whom correspondence should be addressed. E-mail: ashvartsburg@ nctr.fda.gov. Figure 1. Q1 mass spectra of M(NO3)3 dissolved in diacetone alcohol/ H2O for M ) La (A) and Yb (B). Underlined numerals n are for M 3+ (DAA)n trications, numerals with apostrophes m′ are for M 2+ (DAA-H)(DAA)m-1. The symbol “L” stands for DAA ligand. Published on Web 06/14/2002 7910 9 J. AM. CHEM. SOC. 2002, 124, 7910-7911 10.1021/ja025763w Not subject to U.S. copyright. Publ. 2002 Am. Chem. Soc.