Supramolecular Interactions of Group VI Metal Carbonyl Complexes:The Facilitating Role of 1,3-Bis(p‑isocyanophenyl)ureaShaun Millard,† Jenny W. Fothergill,‡ Zoe Anderson,† Eric. C. Brown,† Matthew D. King,†,‡

and Adam C. Colson*,†

†Department of Chemistry and Biochemistry, Boise State University, Boise, Idaho 83725, United States‡Micron School of Materials Science and Engineering, Boise State University, Boise, Idaho 83725, United States

*S Supporting Information

ABSTRACT: An investigation of supramolecular phenomenainvolving zerovalent transition metal complexes was facilitatedby the production of the ditopic isocyanide ligand 1,3-bis(p-isocyanophenyl)urea, which was synthesized via substoichio-metric phosgenation of 4-isocyanophenylamine and used tocoordinate group VI metal carbonyl fragments. The resultingbinuclear organometallic complexes were observed to packinto ladder-like anisotropic arrays in the solid state. Crystallo-graphic and computational evidence suggests that this packingmotif can be attributed to a combination of intermolecularπ−π and urea−π interactions. Similar to other N,N′-diarylureas bearing electron-withdrawing groups, 1,3-bis(p-isocyanophenyl)urea and the organometallic complexesprepared therefrom also exhibit an affinity toward anion binding in nonaqueous solution. Equilibrium constants (K) for theformation of host−guest complexes between the organometallic derivatives of 1,3-bis(p-isocyanophenyl)urea and chloride,nitrate, and acetate anions exceed 103, 104, and 105 M−1, respectively.

■ INTRODUCTION

Noncovalent interactions are foundational to the field ofsupramolecular chemistry and have enabled emergingapplications in chemical sensing, molecular recognition, andthe self-assembly of hierarchical materials.1 Such interactionsare also exploited in the field of crystal engineering to directthe organization of molecular solids in which constituentmolecules are arrayed according to desired structural motifs.2

Although research efforts in these fields often emphasize theassembly of purely organic molecules, the integration oforganometallic and coordination complexes into supramolec-ular structures and engineered molecular solids is also well-documented.3 Less common, however, are accounts ofsupramolecular phenomena or crystal engineering involvinglow- or zerovalent transition metal carbonyl complexes, despitethe fact that such complexes have long attracted interest due totheir unique structural and electronic properties as well as theconvenient spectroscopic monitoring afforded by the metal-bound carbon monoxide ligands.4

One challenge associated with the study of supramolecularbehavior in low-valent transition metal complexes is theincorporation of organic ligands capable of binding to theelectron-rich metal centers while simultaneously participatingin noncovalent intermolecular interactions. To address thischallenge, we have designed a ditopic organic ligand (1)bearing isocyanide functional groups appended to the well-known N,N′-diarylurea moiety (Figure 1). Organic isocyanides

are isolobal analogues of carbon monoxide, and their ability tostabilize neutral and anionic transition metal complexes hasbeen broadly demonstrated.5 Unlike carbon monoxide, addi-tional functionality can be conferred upon isocyanides bymodification of the appended organic component, a usefulfeature that has been exploited in the preparation of chelatingligands, coordination polymers, and functional components inadvanced materials.6

Our selection of the N,N′-diarylurea moiety as a facilitatingagent for supramolecular chemistry and crystal engineering wasmotivated by the versatility of the urea group in promotingmultiple intermolecular interactions.1b,7 Examples of threeprominent supramolecular motifs associated with the N,N′-diarylurea moiety are depicted in Figure 2. The most intuitivenoncovalent interactions are those between the N−Hhydrogen bond donors and CO acceptors of the urea

Received: March 29, 2019Published: May 24, 2019

Figure 1. 1,3-Bis(p-isocyanophenyl)urea (1).

Article

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© 2019 American Chemical Society 8130 DOI: 10.1021/acs.inorgchem.9b00917Inorg. Chem. 2019, 58, 8130−8139

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group. In the absence of competing donor or acceptor species,these interactions can drive the formation of ribbon- or tape-like supramolecular structures (Figure 2a).7a In the presence ofalternative hydrogen bond acceptors such as halides oroxoanions, strong interactions are often observed betweenthe urea host and the anionic guest (Figure 2b), presentingopportunities for applications in anion detection or anion-templated supramolecular assembly.7a,8 Perhaps less intuitively,the urea group is also known to facilitate π−π interactionsbetween neighboring N,N′-diarylurea subunits (Figure 2c).The relative coplanarity of the aryl groups in N,N′-diarylureacompounds is largely determined by the extent of intra-molecular hydrogen bonding between the oxygen atom of theurea moiety and the ortho proton located on the adjacent N-aryl ring; experimental and computational evidence indicatesthat this planarity is most pronounced when the aryl rings bearelectron-withdrawing functional groups.8h,9

The objective of the present study was to identify whichifanyof the supramolecular motifs depicted in Figure 2 mightbe observed in N,N′-diarylureas appended with zerovalentmetal carbonyl subunits. Accordingly, we describe thesynthesis of 1,3-bis(p-isocyanophenyl)urea and its subsequentcoordination to M(CO)5 (M = Cr, Mo, and W) metalcarbonyl fragments. A notable structural feature of thesecomplexes is the nearly planar conformation of the N,N′-diarylurea moiety, which facilitates molecular packing intoordered stacks through a combination of π−π and urea−πinteractions. Like their purely organic counterparts, the metal-bound derivatives of 1,3-bis(p-isocyanophenyl)urea alsoexhibit an affinity toward anion binding in acetonitrile solution.

■ RESULTS AND DISCUSSION

The ditopic 1,3-bis(p-isocyanophenyl)urea ligand (1) wasprepared by treating 4-isocyanophenylamine with a sub-stoichiometric quantity of triphosgene. The formation of 1presumably proceeds through the intermediacy of anisocyanate species, although no attempts were made to isolateor characterize the latter.10 The PdO-catalyzed ligandsubstitution method described by Coville was used to appendCr(CO)5 and W(CO)5 fragments to the isocyanide groups of1, producing homobimetallic complexes 2 and 4, respec-tively.11 The Mo-containing variant (3) was prepared viachemical decarbonylation of Mo(CO)6 using trimethylamineN-oxide in the presence of 1. Complexes 2−4 are readilyidentifiable by their diagnostic infrared spectra, as summarizedin Table 1. It is worth noting that the CN stretchingfrequency of 1 increases upon metal coordination, suggesting

that the isocyanide acts primarily as a σ-donor with very littleπ-accepting character. This observation is consistent withprevious reports of aromatic isocyanide coordination inheteroleptic metal carbonyls in which pronounced shifting ofthe CN stretch to lower frequencies was not observed untiltwo or more isocyanide ligands were attached to a metalcore.11,12

Single crystals of complexes 2−4 suitable for X-raydiffraction studies were grown from acetonitrile solutions at−20 °C. As a representative example of the series, themolecular structure of 3 is shown in Figure 3. The mostprominent structural feature is the nearly planar configurationof the N,N′-diarylurea moiety. The Cortho−Cipso−Nurea−Cureatorsion angles of 0.46° and 7.5° are similar to those reportedby Nangia for other N,N′-diarylureas bearing para-substitutedelectron-withdrawing groups.9a This planarity may beattributed to intermolecular C−H···O hydrogen bondingbetween ortho aryl protons and the urea oxygen atom, asproposed by Etter; indeed, the Cortho···Ourea distances andCortho−H···Ourea angles in complexes 2−4 range from 2.84 to2.87 Å and 121 to 122°, respectively, and are consistent withthe analogous distances (2.83−2.94 Å) and angles (119−122°)observed in other N,N′-diarylureas of comparable planarity.9a,b

Salient structural features for complexes 2 and 4 are presentedin Table 2 and Figures S1 and S2 in the SupportingInformation.The packing of 3 within the molecular solid is depicted in

Figure 4a. Despite the relative steric bulk of the Mo(CO)5subunits, the planarity of the N,N′-diarylurea moiety facilitates

Figure 2. Major supramolecular motifs associated with N,N′-diarylurea compounds.

Table 1. Diagnostic Infrared CN and CO StretchingFrequencies for Compounds 1−4a

νCN (cm−1) νCO (cm−1)

1 2127 N/A2 2143 2058, 19553 2143 2063, 19564 2144 2059, 1950

aRecorded in CH2Cl2.

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packing into ordered stacks, resulting in staggered, ladder-likearrays of metal atoms (Figure 4b). It is tempting to view theorientation and proximity of these metal sites and speculate onthe potential development of charge transfer materials,especially considering that adjacent metal atoms are separatedby only 6.0−7.6 Å. This separation is well within the 4−14 Årange over which the majority of electron transfer processesoccur in benchmark biological systems.13 In their present form,however, complexes 2−4 undergo electrochemically irrever-sible oxidation and are therefore unlikely candidates for suchapplications (Figure S3, Supporting Information).The molecular packing of 3 is reminiscent of the π-stacking

motif introduced in Figure 2c in which planar N,N′-diarylurea

moieties are arranged parallel to one another in an offset or“slipped” orientation; however, close inspection of theinteractions between molecules of 3 reveals several subtledeviations from the archetypal packing motif. As shown inFigure 5a, the separation between the adjacent N,N′-diarylurea

planes of complex 3 is ∼3.32 Å, while the distance between thecentroids of interacting aromatic rings alternates between 3.76and 4.79 Å. By comparison, previous structural studies of N,N′-

Figure 3. Molecular structure of complex 3. Thermal ellipsoids are rendered at the 50% probability level. Only urea N−H atoms and aromatichydrogen atoms participating in hydrogen bonding are shown.

Table 2. Selected Bond Distances, Close Contacts, andTorsion Angles for Complexes 2−4

2 (M = Cr) 3 (M = Mo) 4 (M = W)

distances (Å)M−Cisocyanide 1.981(2),

1.979(2)2.127(2),2.130(2)

2.119(3),2.113(3)

Cortho−H···Ourea 2.24, 2.22 2.25, 2.23 2.25, 2.23Cortho···Ourea 2.86, 2.84 2.86, 2.84 2.87, 2.84π−π contactsa 3.31 3.33 3.32urea−π contactsa 3.29 3.32 3.31

angles (deg)Cortho−H···Ourea 121.8, 121.3 121.8. 121.0 121.0, 122.0Cortho−Cipso−Nurea−Curea 1.1, 6.1 0.46, 7.5 0.45, 7.8

aDistance between adjacent mean N,N′-diarylurea molecular planes.

Figure 4. (a) Molecular packing of 3 showing the orientation of the molecules relative to the unit cell axes. (b) Alternative representation of themolecular packing of 3 viewed through the N,N′-diarylurea planes with the CO ligands omitted and Mo atoms depicted at full van der Waals radius.

Figure 5. (a) Distances between adjacent N,N′-diarylurea planes andthe aromatic ring centroids of complex 3. (b) Alternating stackingmotifs viewed normal to the N,N′-diarylurea planes. Hydrogen atomsand all ring substituents are omitted for clarity.

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diarylureas bearing electron-withdrawing functional groupshave reported interplanar distances of 3.4−3.5 Å, with thearomatic centroids separated by no more than 4.2 Å.9a,14

Although the interplanar distances associated with complex 3are consistent with other π−π interactions reported in theliterature, the significant separation between alternating pairsof ring centroids suggests that π−π interactions are not theonly noncovalent interactions contributing to the close contactbetween N,N′-diarylurea planes.15 Additional insight into thenature of a secondary interaction can be gained by viewingadjacent pairs of N,N′-diarylurea moieties from a directionnormal to the molecular planes (Figure 5b). From thisperspective, it becomes apparent that the interactions betweenan arbitrary N,N′-diarylurea moiety of complex 3 and themolecules positioned directly above and below are notequivalentone face of the aromatic ring experiencessubstantial overlap with the aromatic ring of a neighboringmolecule, while the opposite face interacts with the nitrogenatom of a different neighbor. In the latter case, the distancebetween the aromatic ring centroid and the urea nitrogen atomis only 3.34 Å, suggesting the existence of a noncovalenturea−π interaction. Similar close contacts are also observed inthe Cr- and W-containing derivatives (2 and 4), as tabulated inTable 2.Interactions between aromatic π-systems and urea or amide

moieties have been predicted computationally and observedexperimentally, although reports of such interactions outside ofa biological context are scarce.16 Because urea−π interactionsare not commonly observed in synthetic systems, computa-tional methods were employed to determine whether urea−πinteractions truly contribute to the molecular packing observedfor complexes 2−4, or if the packing motif is simply an artifactof the steric encumbrance imposed by the Mo(CO)5 groups.Initial solid-state DFT calculations were performed on the

crystal structure of complex 3 to assess lattice energy in termsof cohesive and conformational strain energies associated withcrystal packing. The basis set superposition error-correctedlattice energy was determined to be −123.20 kcal mol−1, whichaccounts for the balance of −172.86 kcal mol−1 of cohesiveenergy offset with 49.65 kcal mol−1 of adverse molecular strainenergy due to deformations emerging from crystal packingforces (Table 3). This significant contribution of molecularstrain to the overall lattice energy, primarily in the N,N′-diarylurea moieties, likely results from steric effects ofgeometric packing of the bulky Mo(CO)5 end groups.

To allocate the relative contributions of the substituent π−πand urea−π stacking motifs described in Figure 5 to thecomplex 3 lattice energy, appropriate dimer configurationswere isolated from the DFT-optimized solid-state structure.Single-point energy calculations were performed to obtaindimer and single-molecule energies for the models representa-tive of their strained solid-state molecular conformations. Itwas found that the two motifs are comparable in interactionenergy, with total binding energies of −118.43 and −111.64kcal mol−1 for the π−π and urea−π stacking motifs,respectively, corresponding to a ΔEbind of only 6.80 kcalmol−1 favoring the π−π intermolecular interactions. Thesestacked dimer models, however, still contained interactingMo(CO)5 end groups, thereby not adequately representing thetrue nature or relative consequence of the two participatingN,N′-diarylurea stacking motifs.Subsequent calculations isolated the interaction energy

contributions to the N,N′-diarylurea π−π and urea−π stackingmotifs, for which the Mo(CO)5 groups were removed from themolecular models. The p-substituent isocyanides groups wereterminated with H atoms to provide a better representation ofthe bonding configuration of the N,N′-diarylurea moieties asthey exist in the complex 3 crystal structure. The resultingcalculations produced absolute N,N′-diarylurea dimer inter-action energies of −57.92 and −55.62 kcal mol−1, respectively.Remarkably, this amounts to an approximate difference of only4% (i.e., 2.30 kcal mol−1) between the π−π and urea−πstacking interactions of the N,N′-diarylurea groups, despite thereduction of surface contact area from 94.8 to 76.4 Å2 for theurea−π stacked dimer (based on MSMS surface area calculatedusing 1.5 Å probe sphere). These results demonstrate that theurea−π stacking is indeed a robust intermolecular interactionand plays an important role in molecular ordering duringcrystal growth, and that the association is not an ancillaryproduct of crystal packing.The underlying nature of the prominent urea−π stacking

interactions observed in the synthetic complexes can beexplained through observation of electrostatic potentials andmolecular orbital interactions. An electrostatic potential map ofthe urea−π stacked N,N′-diarylurea dimer shows that thepositive electrostatic potential arising from the urea nitrogenatom overlaps with the negative potential central to anaromatic ring of the adjacent molecule (Figure 6). Theproximity of the molecules in the urea−π dimer pair, separatedby 3.32 Å, provides a favorable distance for strong electrostaticinteraction between these substituent groups. In addition,inspection of molecular orbitals reveals that the N,N′-diarylurea HOMO and LUMO share significant constructivespatial overlap in their relative dimer positions (Figure 7). Theindividual molecule orbital energies were calculated at −6.344and −1.601 eV for the HOMO and LUMO, respectively. Theresulting HOMO of the dimer complex, representative of thesingle-molecule orbital combination formed in the crystallineorbital, was calculated at −6.439 eV. The combination ofadvantageous electrostatic interactions and molecular orbitalinteractions provides the physical description for the stronginteraction evident in the urea−π stacking of the N,N′-diarylurea dimer in the crystal structure of complex 3.Having investigated the intermolecular interactions between

complexes 2−4 in the solid state, a short study of the anion-binding behavior of 1−4 in nonaqueous solution was initiated.Urea-containing compounds are known to act as effectivereceptors or hosts for anionic guests.7a,8 The host−guest

Table 3. Calculated Energies per Molecule (kcal mol−1) ofComplex 3 in the Crystal and Dimer Conformations

complex 3

total lattice energy −123.20cohesive energy −172.86conformational energy 49.65

complex 3 dimers

π−π urea−πtotal binding energy −118.43 −111.64relative binding energy 0.000 6.797

N,N′-diarylurea dimers

π−π urea−πtotal binding energy −57.92 −55.62relative binding energy 0.000 2.295

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interaction of interest can be represented by the followingequilibrium relationship: host + guest− ⇌ [host·guest]−. Thecorresponding equilibrium constant (K) may be estimated byperforming spectroscopic titration experiments, followed byfitting of the resultant data to established binding models.17 Inthe present study, solutions of 1−4 were titrated with thetetrabutylammonium salts of chloride, nitrate, and acetate asrepresentative examples of the halides, inorganic oxoanions,and organic oxoanions, respectively.

1H NMR spectroscopy was selected as the primary methodfor estimating equilibrium binding constants, as incrementaltitration of urea hosts 1−4 with anionic guests consistentlyproduced a detectable response in the spectral features. Underthe dilute conditions of the titration experiments (≈ 0.1 mM),broadening of the urea N−H proton NMR signals prevented

unambiguous chemical shift assignment. Consequently, the 1HNMR signals corresponding to the aromatic ring protons wereused to probe anion binding behavior. Two unique aromaticproton resonances were observed for compounds 1−4, adownfield signal corresponding to the pair of protons nearestthe urea moiety (Hα) and an upfield signal representing thepair of protons nearest the isocyanide functional group (Hβ).The latter signal shifts upfield upon titration and was used todetermine K for all host−guest complexes, as the former signal(Hα) was prone to considerable broadening, especially duringtitrations with acetate. As a representative example, Figure 8aillustrates the effects of chloride titration on the aromatic 1HNMR signals of 1, while Figure 8b compares the magnitude ofthe 1H NMR chemical shifts of Hβ observed upon titration of 1with nitrate, chloride, and acetate anions.The respective upfield and downfield shifts observed for the

Hβ and Hα1H NMR signals are consistent with previous

observations of signal shifting during titration of N,N′-diarylureas with anions.8d,18 It should be noted thatdeprotonation of N,N′-diarylureas has been reported duringtitrations with basic anions such as fluoride or acetate,especially when the urea (or thiourea) hosts are particularlyacidic.8g,19 As a means of confirming that the observed 1HNMR chemical shifts may be attributed to host−guest complexformation rather than deprotonation of the N,N′-diarylureahost, UV−vis spectroscopy was used to probe the magnitudeof the bathochromic spectral shift accompanying addition ofexcess acetate anion. Formation of hydrogen bonding host−guest complexes is typically characterized by a modest spectralshift (<50 nm), while formal deprotonation produces anabsorption band that is generally red-shifted by more than 100nm.8d,f,20 Addition of excess acetate to 1 produced abathochromic spectral shift of only 15 nm with no additionalfeatures appearing at higher wavelengths, indicating that theurea moiety remains protonated under the titration conditions(Figure S5, Supporting Information).Equilibrium constants for the formation of host−guest

complexes of 1−4 with chloride, nitrate, and acetate werederived from 1H NMR titration profiles and are reported inTable 4. All values of K assume a 1:1 binding ratio between theurea host and anion guest, an assumption substantiated bydirect observation of the 1:1 host−guest complexes usingelectrospray ionization mass spectrometry as well as theconsistency of the 1:1 binding model in accounting for theexperimental data (Figures S6−S10, Supporting Informatio-n).17a Compounds 1−4 exhibit similar affinities for each of theanions studied, the values of K becoming progressively largerwith increasing anion basicity. The equilibrium constants forthe formation of host−guest complexes of 1−4 with chlorideand nitrate anions are comparable to those reported byFabbrizzi for the 1,3-bis(p-nitrophenyl)urea receptor, althougha higher affinity toward acetate was measured for the latter (logK = 6.61).8d The results presented in Table 4 suggest that theisocyanide-functionalized N,N′-diarylurea (1) is capable ofstrong anion binding and that attachment of organometallicsubunits does not substantially impact the ability of the ureamoiety to act as an anion receptor.

■ CONCLUSIONSThe synthesis of 1,3-bis(p-isocyanophenyl)urea, its coordina-tion to group VI metal carbonyl fragments, and the structuralcharacterization of the binuclear organometallic products havebeen reported. The nearly planar configuration of the N,N′-

Figure 6. Electrostatic potentials mapped onto the electron densityisosurface (isovalue: 1.0 × 10−6 e bohr−3) of the N,N′-diarylureaurea−π stacked dimer. (a) Top view of urea−π dimer indicating ureanitrogen atoms (blue circle) and the aromatic ring (red circle) of theupper molecule involved in electrostatic binding interactions. (b)Locations of the two nitrogen−ring interactions (green arrows) in theN,N′-diarylurea urea−π dimer.

Figure 7. Molecular orbital surface representations of N,N′-diarylurea(a) HOMO and LUMO orbitals for noninteracting molecules in theurea−π stacked dimer orientation, and (b) the resulting HOMO ofthe N,N′-diarylurea urea−π dimer.

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diarylurea moiety enables the packing of the organometalliccomplexes into ladder-like anisotropic arrays in which thezerovalent metal atoms are separated by 6.0−7.6 Å. Crystallo-graphic and computational evidence suggests that theformation of these arrays can be attributed to a combinationof intermolecular π−π and urea−π interactions. Although themetal carbonyl fragments employed in this exploratory studyundergo irreversible electrochemical oxidation, the methodsand observations reported herein might be extended toproduce similar molecular solids containing more electro-chemically robust organometallic fragments; specimens of thelatter may in turn find application as charge transfer materials.Similar to other N,N′-diarylureas bearing electron-withdrawinggroups, 1,3-bis(p-isocyanophenyl)urea and its organometallicderivatives were also found to behave as anion receptors innonaqueous solvent. Equilibrium constants (K) for theformation of host−guest complexes of 1,3-bis(p-isocyanophenyl)urea with chloride, nitrate, and acetate exceed103, 104, and 105 M−1, respectively. Complexes of 1,3-bis(p-isocyanophenyl)urea with group VI metal carbonyl complexesexhibit similar anion binding behaviors, presenting oppor-tunities for additional research into anion detection or anion-templated supramolecular assembly using low-valent organo-metallic species.

■ EXPERIMENTAL SECTIONGeneral Considerations. All synthetic operations were carried

out under a nitrogen atmosphere using standard Schlenk techniquesto exclude moisture and oxygen. Nitrogen was prepurified by passagethrough columns of activated copper catalyst (BASF PuriStar R3-11G) and molecular sieves (RCI-DRI 13X). Glassware was dried inan oven at 130 °C, assembled while hot, and allowed to cool underreduced pressure. All solvents were dried according to published

procedures and degassed with nitrogen prior to use.21 Cr(CO)6(Beantown Chemical, 99%), Mo(CO)6 (Acros, 98%), W(CO)6(Beantown Chemical, 97%), PdO (Acros), trimethylamine N-oxidedihydrate (Beantown Chemical, 98%), and triphosgene (Chem-Impex, 99%) were used as received without further purification. 4-Isocyanophenylamine was prepared according to literature proceduresand sublimed prior to use.22

Infrared spectra were obtained using a Thermo Scientific NicoletiS5 FTIR spectrometer equipped with a 0.2 mm BaF2 liquid cell. 1Hand 13C NMR data were recorded on a 600 MHz Bruker AVANCEIII spectrometer. Electrospray ionization mass spectrometry (ESI-MS) was carried out using a Bruker HCTultra CTD II spectrometerin negative ion mode. Samples of 1−4 were dissolved in CH3CN andtreated with the tetrabutylammonium salts of chloride, nitrate, andacetate prior to injection into the mass spectrometer. Elementalanalyses were performed by Atlantic Microlab, Inc., in Norcross, GA,USA.

Synthesis of 1,3-Bis(p-isocyanophenyl)urea (1). 4-Isocyano-phenylamine (3.00 g, 25.4 mmol) was dissolved in 80 mL ofanhydrous dichloromethane, followed by addition of 7.8 mL oftriethylamine. The solution was cooled to 0 °C, and triphosgene (1.20g, 4.04 mmol) was slowly introduced into the reaction vessel.(Caution! Triphosgene is toxic and its reaction with 4-isocyanophenyl-amine generates considerable heat and an abundance of hydrogen chloride.Triphosgene should be added very slowly and in several portions to allowfor suf f icient heat exchange with the cooling media.) The light yellowreaction mixture was magnetically stirred for 3 h at 0 °C, then stirredfor an additional 45 h at 25 °C. Methanol (10 mL) was added to thereaction mixture, and stirring was continued for an additional hour.Organic solvents were removed under reduced pressure, and theresidues were dissolved in 60 mL of dimethylformamide (DMF).Deionized water (60 mL) was slowly added, and the reaction vesselwas gently heated to ensure that the solution remained clear. Afteraddition of deionized water, the solution was allowed to cool slowly toroom temperature, whereupon an off-white precipitate formed. Theprecipitate was filtered and washed with three 20 mL portions ofwater, followed by 20 mL of diethyl ether and 20 mL of hexanes,respectively. After drying under reduced pressure for 1 day, 1 wasobtained with sufficient purity for further experimentation. Yield: 2.82g (84.6%). IR (CH2Cl2, cm

−1): νCN 2027 (vs). 1H NMR (CD3CN, 20°C): δ 7.38 (d, 4H, Ar−H, J = 8.86), 7.53 (d, 4H, Ar−H, J = 8.86),7.60 (br s, 2H, N−H). 13C NMR (CD3CN, 20 °C): δ 119.3, 127.1,140.3, 151.9, 163.5, ipso C not observed. MS(ESI): m/z 297 [M +Cl−]−, 324 [M + NO3

−]−, 321 [M + CH3COO−]−. Anal. Calcd for

C15H10N4O: C, 68.69; H, 3.84; N, 21.36. Found: C, 68.47; H, 4.05;N, 21.17.

Synthesis of Complex 2. Cr(CO)6 (317 mg, 1.44 mmol) and 1(182 mg, 0.694 mmol) were combined with 25 mL of DMF andheated to 90 °C, whereupon PdO (14 mg, 0.12 mmol) was added to

Figure 8. (a) Overlay of 1H NMR spectra obtained during the titration of 1 (0.90 × 10−4 M in CD3CN) with [Bu4N]Cl. (b) Comparison of 1HNMR chemical shifting observed during titration of 1 with nitrate, chloride, and acetate anions. The dotted lines represent the results of nonlinearfitting to a 1:1 host−guest binding model.

Table 4. Equilibrium Constants (log K) for Formation ofHost−Guest Complexes of 1−4 with Selected Anions

log Ka

urea host NO3− Cl− CH3COO

−

1 3.62(5) 4.42(3) 5.30(8)2 3.52(3) 4.35(3) 5.41(7)3 3.60(3) 4.35(8) 5.50(3)4 3.70(3) 4.58(3) 5.66(4)

aIn CD3CN solution at 25 °C. Values in parentheses indicateuncertainty in the last figure

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the reaction vessel. The reaction mixture was magnetically stirred at90 °C for 15 min, then allowed to cool to room temperature. DMFwas removed by vacuum distillation, leaving behind an oily residue.The oily residue was extracted with dichloromethane and filtered toremove insoluble impurities. Hexanes were added slowly to thedichloromethane filtrate until the solution became cloudy; then thesolution was centrifuged at 5000 rpm for 2 min, after which the clearsupernatant was decanted and dried under reduced pressure. Theresidual pale yellow solid was dissolved in warm acetonitrile, thencooled slowly to −20 °C to yield crystals of 2·CH3CN. Yield: 242 mg(50.7%). IR (CH2Cl2, cm

−1): νCN 2143 (m), νCO 2058 (s), 1955 (vs).1H NMR (CD3CN, 20 °C): δ 7.44 (d, 4H, Ar−H, J = 8.75), 7.58 (d,4H, Ar−H, J = 8.89), 7.71 (br s, 2H, N−H). 13C NMR (CD3CN, 20°C): δ 120.2, 127.9, 141.1, 153.0, isocyanide and adjacent ipsocarbons not observed. MS(ESI): m/z 681 [M + Cl−]−, 708 [M +NO3

−]−, 705 [M + CH3COO−]−. Anal. Calcd for C27H13Cr2N5O11:

C, 47.18; H, 1.91; N, 10.19. Found: C, 47.23; H, 1.83; N, 10.16.Synthesis of Complex 3. Mo(CO)6 (811 mg, 3.07 mmol) and 1

(403 mg, 1.54 mmol) were dissolved in 25 mL of tetrahydrofuran(THF). A dropping funnel charged with trimethylamine N-oxidedihydrate (342 mg, 3.07 mmol), THF (10 mL), and methanol (10mL) was attached to the reaction flask, the contents of which wereadded dropwise to the reaction mixture over the course of 1 h. Thereaction mixture was magnetically stirred for 6 h at room temperature,after which the solvents were removed under reduced pressure. Theresidues were extracted with dichloromethane and filtered to removeinsoluble impurities. Hexanes were added slowly to the dichloro-methane filtrate until the solution became cloudy; then the solutionwas centrifuged at 5000 rpm for 2 min, after which the clearsupernatant was decanted and dried under reduced pressure. Theresidual off-white solid was dissolved in warm acetonitrile, thencooled slowly to −20 °C to yield crystals of 3. Yield: 871 mg (77.0%).IR (CH2Cl2, cm

−1): νCN 2143 (m), νCO 2063 (s), 1956 (vs). 1H NMR(CD3CN, 20 °C): δ 7.43 (d, 4H, Ar−H, J = 8.86), 7.57 (d, 4H, Ar−H, J = 8.82), 7.69 (br s, 2H, N−H). 13C NMR (CD3CN, 20 °C): δ119.3, 127.2, 140.4, 151.8, isocyanide and adjacent ipso carbons notobserved. MS(ESI): m/z 769 [M + Cl−]−, 796 [M + NO3

−]−, 793 [M+ CH3COO

−]−. Anal. Calcd for C25H10Mo2N4O11: C, 40.89; H, 1.37;N, 7.63. Found: C, 41.03; H, 1.38; N, 8.15.Synthesis of Complex 4. W(CO)6 (369 mg, 1.05 mmol) and 1

(132 mg, 0.503 mmol) were combined with 25 mL of DMF andheated to 90 °C, whereupon PdO (10 mg, 0.082 mmol) was added tothe reaction vessel. The reaction mixture was magnetically stirred at90 °C for 5 min, then cooled to room temperature. DMF wasremoved by vacuum distillation, leaving behind an oily residue. Theoily residue was extracted with dichloromethane and filtered toremove insoluble impurities. Hexanes were added slowly to thedichloromethane filtrate until the solution became cloudy; then thesolution was centrifuged at 5000 rpm for 2 min, after which the clearsupernatant was decanted and dried under reduced pressure. Theresidual yellow solid was dissolved in warm acetonitrile, then cooledslowly to −20 °C to yield crystals of 4·CH3CN. Yield: 211 mg(44.1%). IR (CH2Cl2, cm

−1): νCN 2144 (m), νCO 2059 (s), 1950 (vs).1H NMR (CD3CN, 20 °C): δ 7.43 (d, 4H, Ar−H, J = 8.93), 7.57 (d,4H, Ar−H, J = 9.00), 7.69 (br s, 2H, N−H). 13C NMR (CD3CN, 20°C): δ 119.3, 127.3, 140.4, 151.8, isocyanide and adjacent ipsocarbons not observed. MS(ESI): m/z 945 [M + Cl−]−, 972 [M +NO3

−]−, 969 [M + CH3COO−]−. Anal. Calcd for C27H13N5O11W2:

C, 34.10; H, 1.38; N, 7.36. Found: C, 34.29; H, 1.31; N, 7.41.Determination of Equilibrium Formation Constants (K) by

1H NMR. In typical titration experiments, CD3CN solutions of ureahosts 1−4 (0.75 mL, 0.10 mM) were loaded into standard NMRtubes and initial 1H NMR spectra were collected. Aliquots of ananion-containing solution were then delivered to the NMR tubesusing a microsyringe, the mass of each aliquot being recorded on amicrobalance. The first 10 aliquots of titrant were taken from a stocksolution of the anion guest (2.0 mM) prepared by dissolving a knownquantity of the appropriate tetrabutylammonium salt in a 0.10 mMsolution of the urea host, thereby minimizing host dilution effects.Subsequent aliquots of titrant were taken from a stock solution of

anion guest (4.0 mM) prepared in the same manner. Sufficient anionwas delivered during each titration step to enable collection of 1HNMR spectra at the following approximate [anion]/[urea] ratios: 0.1,0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.2, 2.0, 3.0, and 4.0. Theupfield shifting of the aromatic proton signals centered between 7.38and 7.44 ppm was recorded, and values of K were calculated bynonlinear fitting to a 1:1 binding model using the WinEQNMR2software package.23

Crystal Structure Determination of Complexes 2−4. X-raydiffraction data were collected at 100 K on a Bruker D8 Venture usingMoKα radiation (λ = 0.71073 Å). Data were corrected for absorptioneffects using the SADABS area detector absorption correctionprogram.24 The structures were solved by direct methods usingOlex2 with the SHELXT structure solution program and refined withthe SHELXL refinement package using least-squares minimization.25

All non-hydrogen atoms were refined with anisotropic thermalparameters. Hydrogen atoms attached to heteroatoms were identifiedfrom the residual density maps and refined with isotropic thermalparameters. All other hydrogen atoms in the investigated structurewere located from difference Fourier maps, but their positions wereultimately placed in geometrically calculated positions and refinedusing a riding model. Additional calculations and refinement ofstructures were carried out using APEX3 and SHELXTL software.26

Graphical representations of crystallographic data were generatedusing the Mercury software package.27 X-ray data collection andrefinement parameters are tabulated in Table S1 (SupportingInformation).

Computational Methods. All DFT calculations were performedusing the Crystal14 software package.28 Calculations utilized apairwise dispersion-corrected B3LYP-D2 density functional withatom-centered Dunning cc-pVDZ basis sets on all nonmetalatoms.29 Mo atoms were treated with LANL2DZ effective corepotentials with standard Dunning D95V valence orbitals.30 Atomiccoordinates from the X-ray structural determination of complex 3were taken as the initial geometry for the crystal structuraloptimization. The energy minimization allowed full relaxation ofatom positions and lattice parameters within the constraints of spacegroup symmetry. A shrinking factor of 4 was used in defining the kpoint sampling of the density matrix and commensurate grid inreciprocal space to achieve precision on energy convergence of 10−6

hartree for all geometry optimizations and single-point energycalculations.31 A pruned (75,974) integration grid was used to definethe radial and angular grid-point distribution. Corrections for basis setsuperposition error were performed using a counterpoise method.32

Interaction energies for stacked dimer structures were calculated usingdimers extracted from the expanded DFT-optimized complex 3crystal structure. Single-point energies were used to determineinteraction energies from Eint

AB = E(AB) − [E(A) + E(B)], wheresingle-molecule energies for A and B were calculated in the full ABbasis.

Calculated and experimental molecular bond lengths, bond angles,dihedral angles, and RMSDs for complex 3 are provided in TablesS2−S4 (Supporting Information).

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.inorg-chem.9b00917.

Molecular structures of complexes 2 and 4, electro-chemical methods and data, ESI-MS data, tables of X-raydata collection and refinement parameters, and calcu-lated and experimental molecular bond lengths, bondangles, dihedral angles, and RMSDs for complex 3(PDF)

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Accession CodesCCDC 1905535−1905537 contain the supplementary crys-tallographic data for this paper. These data can be obtainedfree of charge via www.ccdc.cam.ac.uk/data_request/cif, or byemailing data_request@ccdc.cam.ac.uk, or by contacting TheCambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: adamcolson@boisestate.edu.ORCIDMatthew D. King: 0000-0003-1011-3108Adam C. Colson: 0000-0002-1114-7791NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors acknowledge Boise State University for financialsupport and Prof. Orion Berryman and the Center forBiomolecular Structure and Dynamics CoBRE (NationalInstitutes of Health, CoBRE NIGMS P20GM103546) at theUniversity of Montana for facilitating X-ray data collection.Single crystal X-ray diffraction data were collected usinginstrumentation supported by NSF MRI CHE-1337908.

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Inorganic Chemistry Article

DOI: 10.1021/acs.inorgchem.9b00917Inorg. Chem. 2019, 58, 8130−8139

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