We report dinuclear Mn(II) and Co(II) complexes supported by triaryl tetradentate ligands derived from o-phenylenediamide that are flanked by different metal-donor substituents (X = NMe2vs. SMe). Single-crystal XRD data revealed that the Mn complexes (Mn-1 and Mn-2) both possess Mn2N2 diamond cores with relatively similar bond distances, electronic structures, and magnetic properties regardless of the ligand identity. The Co complexes, by contrast, revealed dramatic substituent-dependent differences. Like Mn, the Co complexes were dinuclear, but their core structures varied from open (X = NMe2; no μ-N bridging; Co-1a) to closed (X = SMe; intact Co2N2 diamond core; Co-2), with a second structure isolated with X = NMe2 in between (semi-open core; Co-1b). Of the three structures, Co-2 had the shortest metal-metal distance of 2.4540(8) Å, just at the onset of that expected for Co-Co bonding. Evidence of an appreciable metal-metal interaction in Co-2 was revealed with a unique UV-vis absorption at 516 nm that was assigned to metal-metal charge transfer (MMCT). Moreover, magnetic measurements conducted on Co-2 revealed a magnetic moment of 1.2μB at room temperature, which was much lower than that of other Co and Mn complexes. Active space calculations corroborated the experimental observations and suggested that Co-2 possesses a weak metal-metal bond with a low effective bond order of 0.24. These findings, which are compared to those previously reported for Fe(II) and Cr(II) complexes with the same ligands, reveal the marked influence that metal identity has on the structures, magnetic properties, and metal-metal bonding within this family of triaryl tetradentate ligands.

Herein, we report a series of Cr(II) complexes containing triaryl, tetradentate ligands derived from o-phenylenediamide that illuminate how rigid, chelating amido ligands facilitate Cr-Cr bonding. The aminolysis reaction of Cr[N(SiMe3)2]2(thf)2 with H2(L1)─a protonated N4 proligand containing flanking NMe2 groups─yielded square planar and mononuclear Cr(L1) (1), whereas the same reaction with N2S2 and N2O2 proligands with flanking SMe and OMe groups yielded dinuclear [Cr(L2)]2 (2) and [Cr(L3)]2 (3). The structures of 2 and 3 revealed Cr-Cr distances of 2.3356(6) and 2.3481(5) Å, consistent with metal-metal bonding, which was confirmed by the complete active space methods. The theoretical results suggest that Cr-Cr bonding is assisted by the chelating nature of the bridging amido ligands, which fold the dinuclear structure and orient the metals so that side-on overlap of Cr 3d orbitals can occur. Variable-temperature SQUID magnetometry and spectroscopic data (e.g., UV-vis-NIR, Raman, and IR) reported for 1-3 show differences indicative of the change in nuclearity and electronic structure. Collectively, these results reveal bridging amido ligand characteristics that support metal-metal bonding with Cr(II), and they help account for the wide range of metal-metal distances observed in dinuclear (or binuclear) Cr(II) complexes containing Cr2N2 cores.

Ligand K-edge X-ray absorption spectroscopy (XAS), a technique that can measure variations in covalent metal-ligand bonding, has rarely been used to assess covalency in complexes containing metal-boron bonds. Here we describe ligand K-edge XAS and TDDFT studies of the Ni dicarbollide complex Ni-(C2B9H11)2 (1) and the Ni-free salt (HNMe3)-(C2B9H12) (L1). The XAS spectrum for 1 reveals a pre-edge feature indicative of covalent Ni-B bonding, which is corroborated by time-dependent density functional theory (TDDFT) calculations and comparative analysis to L1 and inner-shell electron energy loss spectroscopy (ISEELS) collected on the same Ni complex.

Metal borohydride complexes have long been the subject of intense fundamental interest because of their unconventional metal-ligand bonding that occurs via three-center, two-electron M-H-B bonds. This type of bonding implies significant delocalization of electron density over all three atoms, but the degree of orbital mixing between the metal and boron has been difficult to assess by direct experimental means. Herein, we demonstrate how ligand K-edge X-ray absorption spectroscopy (XAS) conducted at the B K-edge yields evidence of significant covalent M-H-B bonding with Zr and Hf. To accommodate the B K-edge XAS studies, which were conducted under ultra-high vacuum (<10-8 torr), we prepared a series of new [Zr(RBH3)4] and [Hf(RBH3)4] complexes with substituents that attenuate volatility (R = benzyl, phenyl, mesityl, 2,4,6-triisopropylphenyl, and anthryl). 1H and 11B NMR spectroscopy, IR spectroscopy, and single-crystal X-ray diffraction (XRD) studies revealed metal and ligand dependent differences in the BH3 chemical shifts that correlate to changes in M-B distances and select B-H vibrational stretching modes. The B K-edge XAS spectra of the Zr and Hf complexes yielded a pre-edge feature that was assigned as B 1s → M-H-B π* based on comparison to time-dependent density functional theory (TDDFT) calculations. The pre-edge transitions appear due to covalent mixing between boron and the metal, thereby demonstrating how B K-edge XAS can provide direct evidence of covalent three-center, two electron M-H-B bonding in borohydride complexes using boron as a spectroscopic reporter.

Actinide complexes containing the simplest borohydrides (BH4)1- and (MeBH3)1- can exhibit remarkably highly volatility, which creates unique hazards and handling challenges, especially when making measurements on solid samples under vacuum. Here we describe efforts to prepare new actinide borohydride complexes with attenuated volatility by adding bulkier benzyl (Bn) and cyclohexyl (Cy) substituents to boron. Reactions of ThCl4, UI3(thf)4, and NdI3 with the mixed alkali metal salt Li/K(BnBH3)(thf)nyielded Th(BnBH3)4(thf)2, U (BnBH3)4(thf)2, and K[Nd(BnBH3)4], respectively. Notable amongst these, the reaction with UI3(thf)4 proceeds via oxidation of U(III) to U(IV) despite the presence of reducing borohydride ligands. Similarly, reactions of the same metal halides with four equivalents of Li(CyBH3)(Et2O)nyielded Th(CyBH3)4, U(CyBH3)4(thf)2, and [Li (Et2O)3][Nd(CyBH3)4]. Single crystal X-ray diffraction studies of the M(BnBH3)4(thf)2 complexes with M =Th and U confirmed their formulations. The complexes have approximate D 2d point group symmetry and adopt bicapped hexagonal antiprismatic coordination geometries with axial thf ligands and kappa 3-BnBH3 ligands bound in the equatorial plane. K[Nd(BnBH3)4] and [Li(Et2O)3][Nd(CyBH3)4], which were prepared for comparison to U (III) complexes that were unsuccessfully targeted, were also structurally characterized to reveal complex anions with tetrahedral arrangements of trihydroborate ligands bound to Nd(III). Crystals obtained for Th(CyBH3)4 and U(CyBH3)4(thf)2 were not suitable for XRD studies, but 1 H and 11 B NMR spectra were consistent with their formulations. Collectively, these complexes represent rare examples of structurally characterized f-element trihydroborate complexes with carbon substituents other than methyl.

Here we describe the first coordination complexes containing a bulky m-terphenyltrihydroborate ligand. Treating [UI3(thf)4] and NdCl3 with three equiv. of Li(H3BArtBu4)(Et2O) (where ArtBu4 = 2,6-(3,5-tBu2C6H3)2C6H3) yielded [M(H3BArtBu4)3(thf)2] (M = U and Nd). [U(H3BArtBu4)3(dme)2] is also described, and structural comparisons reveal the influence of the Lewis base on H3BArtBu4 positioning.

Herein we report an electronic structure investigation of neutral and oxidized Ru complexes containing a redox noninnocent N2S2 ligand derived from o-phenylenediamide (L1). UV-vis spectroelectrochemistry (SEC) studies were conducted on the square pyramidal complex [RuII(L1)(PPh3)] (1) and the six-coordinate complexes [RuII(μ-BH3)(L1)(PPh3)] (2) - which has BH3 bound in a metal-ligand cooperative (MLC) fashion across Ru and L1 - and [RuII(L1)(PPh3)(MeCN)] (3). The SEC results yielded spectra assigned to singly and doubly oxidized 1 and 3, revealing electronic structure changes as a function of oxidation state and in response to the presence and absence of bound MeCN. By contrast, the SEC results of 2 showed that it rapidly loses MLC-bound BH3 upon oxidation. The SEC results for 1 and 3 were compared to single-crystal XRD data and UV-vis, EPR, and P K-edge, S K-edge, and Ru L3-edge X-ray absorption spectroscopy (XAS) data collected on isolated samples of chemically oxidized 3. The data revealed that the first two oxidations are primarily localized on the ligand, which was supported by DFT and TDDFT calculations. DFT calculations for the doubly oxidized species revealed a singlet ground state with a singlet-triplet gap of 8.9 kcal/mol. CASPT2 calculations corroborated the DFT calculations and further revealed that the singlet ground state is multiconfigurational with 21% radical character. Collectively, the results establish redox formalisms and the underlying electronic structure of Ru complexes containing a noninnocent tetradentate ligand in different oxidation states.

Here we report the synthesis and characterization of diiron complexes containing triaryl N4 and N2S2 ligands derived from o-phenylenediamine.

Despite the discovery of actinide borohydride complexes over 80 years ago, no plutonium borohydride complexes have been structurally validated using single-crystal X-ray diffraction (XRD). Here we describe Pu-2((H3BPBu2BH3)-Bu-t)(6), the first example of a Pu(III) borohydride complex authenticated by XRD and NMR spectroscopy. Theoretical calculations (DFT, EDA, and QTAIM) and experimental comparisons of metal-boron distances suggest that metal-borohydride covalency in M-2((H3BPBu2BH3)-Bu-t)(6) complexes generally decreases in the order M = U(III) > Pu(III) > Ln(III).

Here we describe the synthesis, structures, and volatility of lanthanide complexes containing N4O3 ligands decorated with different fluoroalkyl substituents. The results show how ligand fluorination does not necessarily enhance complex volatility.

Here, we report the mechanochemical synthesis and characterization of homoleptic uranium and lanthanide phosphinodiboranates with isopropyl and ethyl substituents attached to phosphorus. M(H3BPiPr2BH3)3 complexes with M = U, Nd, Sm, Tb, and Er were prepared by ball milling UI3(THF)4, SmBr3, or MI3 with three equivalents of K(H3BPiPr2BH3). M(H3BPEt2BH3)3 with M = U and Nd were prepared similarly using K(H3BPEt2BH3), and the complexes were purified by extraction and crystallization from Et2O or CH2Cl2. Single-crystal XRD studies revealed that all five M(H3BPiPr2BH3)3 crystallize as dimers, despite the significant differences in metal radii across the series. In contrast, Nd(H3BPEt2BH3)3 with smaller ethyl substituents crystallized as a coordination polymer. Crystals of U(H3BPEt2BH3)3 were not suitable for structural analysis, but crystals of U(H3BPMe2BH3)3 isolated in low yield by solution methods were isostructural with Nd(H3BPEt2BH3)3. 1H and 11B NMR studies in C6D6 revealed that all of the complexes form mixtures of monomer and oligomers when dissolved, and the extent of oligomerization was highly dependent on metal radius and phosphorus substituent size. A comprehensive analysis of all structurally characterized uranium and lanthanide phosphinodiboranate complexes reported to date, including those with larger Ph and tBu substituents, revealed that the degree of oligomerization in solution can be correlated to differences in B–P–B angles obtained from single-crystal XRD studies. Density functional theory calculations, which included structural optimizations in combination with conformational searches using tight binding methods, replicated the general experimental trends and revealed free energy differences that account for the different solution and solid-state structures. Collectively, these results reveal how steric changes to phosphorus substituents significantly removed from metal coordination sites can have a significant influence on solution speciation, deoligomerization energies, and the solid-state structure of homoleptic phosphinodiboranate complexes containing trivalent f-metals.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

This chapter describes the synthesis and structures of organometallic complexes containing pincer ligands with chromium, molybdenum, and tungsten.

The high fluorophilicity of borane-containing ligands offers promise for accessing new metallodrug candidates capable of bifunctional [18F]-positron emission tomography (PET) imaging, but this requires water soluble and hydrolytically stable ligands that can be fluorinated under mild conditions. Toward this goal, here we report the synthesis and characterization of water-soluble Pt(II) complexes containing a triaminoborane-bridged diphosphoramidite ligand called MeOTBDPhos that can be fluorinated using simple fluoride salts. NMR and XRD studies show that (MeOTBDPhos)PtCl2 (1) dissolves in water with cooperative H-OH addition across the bridgehead N-B bond to form 1-H2O. The B-OH bond in 1-H2O undergoes rapid displacement with fluoride (<10 min) when treated with CsF in MeCN to form 1-HF. 1-HF can also be prepared in <10 min by addition of KF to 1 in the presence Kryptofix® 222 and (HNEt3)Cl in MeCN. In addition to using fluoride salts, we show how mononuclear 1 can be fluorinated with HBF4·Et2O to form dinuclear [(MeOTBDPhos-HF)Pt(μ-Cl)]2(BF4)2 (4-HF). Comparative studies show that the B-F bond in 1-HF undergoes hydrolysis as soon as it is dissolved in water or saline, but the B-F bond persists for hours when the pH of the solution is lowered to pH ≤ 2. In contrast to 1-HF, the B-F bond in dinuclear 4-HF persists for days when dissolved in water, which may be attributed to slow, sacrificial release of fluoride from the BF4- anion. The results show how cooperative N-B reactivity on the ligand can be leveraged to rapidly fluorinate water-soluble MeOTBDPhos complexes under mild conditions and afford suggestions for how to enhance hydrolytic B-F stability, as required for use in biomedical applications.

Here we show that the praseodymium N,N-dimethylaminodiboranate complex Pr(H3BNMe2BH3)3 and the 2,2,6,6-tetramethylheptane-3,5-dionate complex Pr(thd)3 can serve as volatile carriers for 225Ac. The actinium coordination complexes Ac(H3BNMe2BH3)3 and Ac(thd)3 are the likely species subliming with the carrier material. A sample of 225Ac-doped Pr(H3BNMe2BH3)3 was used to deposit amorphous 225Ac-doped praseodymium boride films on glass and Si(100) at 300 °C. The α emission spectra of the refractory films are well-resolved, suggesting that they could be used as radioactive implants for brachytherapy and related treatments.

In this Forum Article, we review the development of chelating borohydride ligands called aminodiboranates (H3BNR2BH3-) and phosphinodiboranates (H3BPR2BH3-) for the synthesis of trivalent f-element complexes. The advantages and history of using mechanochemistry to prepare molecular borohydride complexes are described along with new results demonstrating the mechanochemical synthesis of M2(H3BPtBu2BH3)6, where M = U, Nd, Tb, Er, and Lu (1-5). Multinuclear NMR, IR, and single-crystal X-ray diffraction data are reported for 1-5 alongside complementary density functional theory calculations to reveal differences in their structure and reactivity with and without tetrahydrofuran. The results demonstrate how mechanochemistry can be used to access f-element complexes with chelating borohydrides in improved and reproducible yields, which is an important step toward investigating the properties of lanthanide and actinide phosphinodiboranate complexes with different phosphorus substituents. The relevance of these results is contextualized by a discussion of structural factors known to influence the volatility of f-element borohydrides and applications that require the development of volatile f-element complexes.

Here we report a convenient synthesis of new diphosphoramidite ligands derived from 1,8,10,9-triazaboradeca-lin (TBD) and describe their complexes with group 10 metals. Treating the chlorinated and structurally char-acterized ligand precursor (Cl)TBDPhos (L1) with four equivalents of HOR (R = C3H7 or C3HF6) in the presence of NEt3 yielded the diphosphoramidite ligands iPrOTBDPhos (L2) and F-iPrOTBDPhos (L3) in good yields. L2 and L3 were used to prepare a variety of Ni, Pd, and Pt complexes with chloride and 1,2-benzenedithiolate ligands so their structures and spectroscopic properties could be compared to similar complexes with methoxy-substituted( MeO)TBDPhos such as (MeOTBDPhos)PdCl2, which is reported here for the first time. Single-crystal XRD studies on the ((ROT)BDPhos)PtCl2 complexes revealed that increasing the size of alkoxy substituents from MeO to iPrO to F-iPrO decreases the P-M-P bite angle from 97.47(3) in ((MeO)TBDPhos)PtCl2 to 93.98(4) in (F-iPrOTBDPhos)PtCl2. Similar changes were observed in the dithiolate complexes (iPrOTBDPhos)Pt(S2C6H4) and (F-iPrOTBDPhos)Pt (S2C6H4), and the structural studies revealed longer Pt-P bond distances compared to the dichloride complexes that correlated to ca. 1000 Hz decrease in their 195Pt-31P coupling constants. No significant changes were observed in the ligand bond distances in complexes containing the methoxy and isoproxy-substituted ligands, but complexes with fluorinated F-iPrOTBDPhos revealed subtle, but significant differences in their P-N, P-O, and B-N distances that reflect substituent-induced electronic changes in the ligand. Overall, this work establishes a more convenient synthetic entry into the chemistry of alkoxy-substituted TBDPhos ligands for ongoing studies with these and related transition metal complexes.

We describe the syntheses of a series of sodium aminodiboranate salts, Na(H3B-NR2-BH3), with different substituents on nitrogen, including sodium salts of the unsubstituted aminodiboranate, H3B-NH2-BH3 -, and of the N-substituted anions H3B-NRR '-BH3 -, where NRR ' = NHMe, NHEt, NH(SiMe3), NEt2, N(i-Pr)2, N(SiMe3)2, NMe(i-Pr), NMe(t-Bu), NMe(SiMe3), and the pyrrolidide and piperidide derivatives NC4H8, NC5H10, and NC5H8-cis-2,6-Me2. The compounds have been characterized by 1H and 11B NMR spectroscopy and IR spectroscopy; crystallographic studies have been carried out for the unsolvated N,N-dimethylaminodiboranate salt Na(H3B-NMe2-BH3) and several sodium aminodiboranate salts in which the sodium ions are solvated with ethers (dioxane, diglyme, tetrahydrofuran, and 12-crown-4) or amines (N,N,N ',N '- tetramethylethylenediamine). One of the structures contains a rare example of an ether ligand in which one oxygen atom bridges between two metal ions. General structural and spectroscopic trends as a function of the substituents on nitrogen are discussed.

Tandem catalysts that perform two different organic transformations in a single pot are highly desirable because they enable rapid and efficient assembly of simple organic building blocks into more complex molecules. Many examples of tandem catalysis rely on metal-catalyzed reactions involving one or more metal complexes. Remarkably, despite surging interest in the development of chemically reactive (i. e., non-innocent) ligands, there are few examples of metal complexes that leverage ligand-centered reactivity to perform catalytic reactions in tandem with separate catalytic reactions at the metal. Here we report how multifunctional Pd complexes with triaminoborane-derived diphosphorus ligands, called TBDPhos, appear to facilitate borenium-catalyzed cycloaddition reactions at the ligand, and Pd-catalyzed Stille and Suzuki cross-coupling reactions at the metal. Both transformations can be accessed in one pot to afford rare examples of tandem catalysis using separate metal and ligand catalysis sites in a single complex.