MAGNETIC RESONANCE IN CHEMISTRYMagn. Reson. Chem. 2007; 45: 484–487Published online 19 April 2007 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/mrc.1995

Solvent effects on one-bond B–Li coupling constants inboryllithium compounds

Janet E. Del Bene1∗ and Jose Elguero2

1 Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555, USA2 Instituto de Quımica Medica, CSIC, Juan de la Cierva, 3, E-28006 Madrid, Spain

Received 1 February 2007; Revised 18 February 2007; Accepted 22 February 2007

EOM-CCSD 11B–7Li coupling constants and B chemical shifts have been computed for Li-diazaboroleand its complexes with one H2O or FLi molecule. B–Li coupling constants for a model compoundH2BLi and its complexes with up to 4 H2O or FLi molecules have also been obtained in an attempt toresolve discrepancies between the computed values of these properties for isolated Li-diazaborole andexperimentally determined values for boryllithium in a THF solution. The presence of solvent moleculesincreases the ion-pair character of the B–Li bond, with the result that 1J(B–Li) decreases systematicallyas the basicity and the number of solvent molecules increases. In the presence of even a single solventmolecule, the boron chemical shift for Li-diazaborole increases, and approaches the experimental value.The computed results emphasize the role of the solvent in determining these NMR properties. Copyright 2007 John Wiley & Sons, Ltd.

KEYWORDS: NMR; 11B; 7Li; spin-spin coupling constants; chemical shifts; solvent effects; boryllithium compounds

INTRODUCTION

In a recent paper, Segawa, Yamashita, and Nozaki reportedthe syntheses of boryllithium moieties in which the boronatom acts as a nucleophile.1 These investigators employedNMR spin–spin coupling constants and chemical shifts tocharacterize the anion (C2H2Ar2B1N2

�, where Ar is a 2,6-diisopropylphenyl group) and two related neutral moleculeswith B–H (C2H3Ar2B1N2) and B–Li (C2H2Ar2B1Li1N2)bonds. In a recent paper, we reported ab initio, equation-of-motion coupled cluster singles and doubles method (EOM-CCSD) spin–spin coupling constants and MP2 chemicalshifts for models of these species in which the diisopropy-lphenyl groups were replaced by H atoms, giving diazabo-role anion, diazaborole, and Li-diazaborole, respectively.2

Although the computed coupling constant 1J(B–H) and theboron chemical shift for diazaborole were in good agreementwith the corresponding experimental values, there were dis-crepancies between the experimental B–Li bond distance andthe B chemical shift for the boryllithium compound and thecomputed values for Li-diazaborole. In that paper, we notedthat the computed chemical shift for the diazaborole anionclosely resembled the experimental value for the neutralboryllithium compound, which suggested that the discrep-ancies might arise from the presence of solvent molecules(DME in the crystal and THF in solution), since interactionwith the solvent would increase the ion-pair character of theB–Li bond.

ŁCorrespondence to: Janet E. Del Bene, Department of Chemistry,Youngstown State University, Youngstown, Ohio 44555, USA.E-mail: jedelbene@ysu.edu

In the present paper, we present computed 11B–7Licoupling constants and B chemical shifts for complexes of Li-diazaborole with solvent molecules. For this study, we havechosen two solvents: H2O, with computed and experimentalproton affinities of 165.8 and 165 kcal/mol,3 respectively; andFLi, which has a computed proton affinity of 212.3 kcal/mol.The proton affinities of these two molecules bracket thatof THF (196.5 kcal/mol),3 the solvent used for the NMRexperiments. Because the EOM-CCSD calculations for Li-diazaborole with multiple solvent molecules are not possible,we have also used H2BLi as a model, and have examinedchanges in 1J(B–Li) as a function of the number of solventmolecules bonded to Li.

COMPUTATIONAL METHODS

Structure optimizations were carried out at second-orderMøller–Plesset perturbation theory (MP2)4 – 7 as imple-mented in Gaussian 03,8 with the 6-311CCG(d,p) basisset.9 – 11 The fully optimized structures of Li-diazaborole,H2BLi, Li-diazaborole : FLi, and H2BLi : FLi have C2v symme-try, and are equilibrium structures with no imaginary fre-quencies. However, the C2v structures of Li-diazaborole:OH2

and H2BLi : OH2 are not equilibrium structures, but haveone low-frequency imaginary mode corresponding to anH–O–H bending motion, which facilitates interaction ofthe acidic hydrogens with the boron-containing molecules.Therefore, these two complexes and the complexes contain-ing more than 1 solvent molecule were optimized undersymmetry constraints to eliminate such secondary interac-tions, and to make the calculations of coupling constantsfeasible. In particular, the B–Li bond is a local symmetry

Copyright 2007 John Wiley & Sons, Ltd.

One-bond B–Li coupling in boryllithium compounds 485

axis (C2 for complexes with 1, 2, or 4 solvent molecules andC3 for complexes with 3 solvent molecules). The complexeswith H2O have the solvent molecule positioned so that Lisits at the negative end of the H2O dipole moment vector. Asimilar positioning for complexes with FLi results in a linearLi. . .F–Li arrangement. Our purpose in this study is not toobtain fully optimized structures, but to mimic the NMRsolvent molecule THF, which has a rigid structure with noacidic H atoms, thereby precluding secondary interactions.

Coupling constants were evaluated using the EOM-CCSD method in the configuration interaction (CI)-likeapproximation,12 – 15 with all electrons correlated, as imple-mented in ACES II.16 For these calculations, the Ahlrichs qzpbasis set was placed on C, N, O, and F atoms,17 the Dun-ning cc-pVDZ basis on H atoms,18,19 and the hybrid basis setdeveloped previously on B and Li.20 All terms that contributeto the total coupling constant (the paramagnetic spin-orbit(PSO), diamagnetic spin-orbit (DSO), Fermi-contact (FC), andspin-dipole (SD)) were evaluated for Li-diazaborole, H2BLi,H2BLi : OH2, and H2BLi : FLi. Since these data show that theFC term is an excellent approximation to 1J(B–Li), only FCterms have been evaluated for the remaining complexes, andare used to approximate 1J(B–Li).

Chemical shieldings have been evaluated at MP2 employ-ing the GIAO method21 and the same basis sets used for thecoupling constant calculations. The 11B chemical shift (υ) canbe related to the computed chemical shielding (�) by theequation

υ�11B� D 106.5 � 0.90 � �11B�

which was developed in our previous paper.2 All calculationshave been performed at the Ohio Supercomputer Center onthe Cray X1 or the Itanium cluster.

RESULTS AND DISCUSSION

Li-diazaboroleTable 1 presents B–Li distances, FC terms, and total couplingconstants 1J(B–Li) for isolated Li-diazaborole and thismolecule with either one H2O or one FLi molecule bonded toLi. The complex Li-diazaborole : OH2 is shown in Scheme 1.1J(B–Li), as approximated by the FC term, decreases from136.8 Hz in Li-diazaborole to 92.5 Hz in the complex withone H2O molecule bonded to Li, and to 81.6 Hz when oneFLi molecule is bonded. This change is not simply due to achange in the F–Li distance, since 1J(B–Li) increases slightlyto 139.7 Hz when the B–Li distance is stretched to 2.291 A, theexperimental value in the crystal.1 Thus, the presence of evena single solvent molecule increases the ion-pair character ofthe B–Li bond and decreases the value of 1J(B–Li), with thestronger base having a larger effect.

The effect of the solvent is also manifested by changesin boron chemical shifts (υ). Li-diazaborole has a computedυ�11B� of 31.0 ppm, much smaller than the experimental valueof 45.4 ppm for the boryllithium molecule.1 Moreover, asnoted previously, the computed value of 43.7 ppm for theB chemical shift for diazaborole anion is much closer to theexperimental value for the neutral boryllithium molecule,again suggesting that the B–Li bond in isolated, gas-phase

Table 1. B–Li distances (A), Fermi-contact terms (FC) andtotal coupling constants 1J(B–Li) (Hz), and B chemical shifts (υ,ppm) for Li-diazaborole and its complexes with H2O and FLia

Species R(B–Li) FC 1J(B–Li) υ

Li-diazaborole 2.184 136.8 136.9 31.0 (45.4)b

(stretched) 2.291 139.7 – 31.4Li-diazaborole : OH2 2.204 92.5 – 32.3Li-diazaborole : FLi 2.235 81.6 – 34.0Diazaborole anion – – – 43.7

a Li-diazaborole and its complexes have C2v symmetry.b The experimental value for the boryllithium molecule fromRef. 1.

Scheme 1. Diazborole : H2O.

Li-diazaborole lacks sufficient ion-pair character. In thepresence of one solvent molecule, υ increases to 32.3 ppmwhen H2O is the solvent, and to 34.0 ppm when the solventis the more basic FLi. These results are consistent with anincreasing ion-pair character of the B–Li bond in the presenceof solvent molecules of increasing base strength.

H2BLiTo further investigate solvent effects, coupling constants1J(B–Li) have been evaluated for H2BLi in the presence ofup to four solvent molecules. The complexes of H2BLi withFLi are illustrated in Scheme 2. The structure of H2BLi : 4FLiis not an optimized structure, but is one found along thedissociation path to the ion-pair H2B� with LiC:4FLi ata B–Li distance of 5.5 A. The tetrahedral coordination ofLiC is evident. Table 2 presents the symmetries of thesecomplexes, B–Li distances, and B–Li coupling constantsapproximated by the FC terms. 1J(B–Li) for H2BLi is 113.7 Hzat the optimized B–Li distance, and 113.8 Hz when the B–Lidistance is stretched to 2.220 A. Thus, simply changing theB–Li distance has little effect on 1J(B–Li). However, the B–Lidistance also increases as the number of solvent moleculesbonded to Li increases, but in the presence of solvent 1J(B–Li)decreases dramatically, as shown in Fig. 1. The second-ordercurves have intercepts within 1 Hz of the value for isolatedH2BLi. Figure 1 also illustrates that the change in 1J(B–Li) isgreater when FLi, the stronger base, is the solvent molecule.Given that the proton affinities of H2O and FLi bracket thatof THF, it is expected that the corresponding curve for THFwould lie somewhere between these two curves.

Copyright 2007 John Wiley & Sons, Ltd. Magn. Reson. Chem. 2007; 45: 484–487DOI: 10.1002/mrc

486 J. E. Del Bene and J. Elguero

(a) (b)

(d)(c)

Scheme 2. H2BLi with 1, 2, 3, and 4 FLi molecules.

Table 2. Symmetries, B–Li distances (A), and Fermi-contact(FC) terms and total coupling constants 1J(B–Li) (Hz) forH2B–Li and its complexes with H2O and FLi

Species Symmetrya R(B–Li) FC 1J(B–Li)

H2B–Li C2v 2.208 113.7 114.4H2B–Li (stretched) C2v 2.220 113.8 –H2B–Li : 1 OH2 C2v 2.227 82.1 82.8H2B–Li : 2 OH2 C2v 2.264 63.4 –H2B–Li : 3 OH2 Cs 2.299 52.1 –H2B–Li : 4 OH2 C2v 2.355 43.9 –H2B–Li : 1FLi C2v 2.253 74.3 75.1H2B–Li : 2FLi C2v 2.353 50.0 –H2B–Li : 3FLi Cs 2.553 28.6 –H2B–Li : 4FLi C2v 5.5 <5b –

a The B–Li bond is a local C2 or C3 (for complexes with 3 solventmolecules) rotational axis for the solvent molecules. Complexeswith H2O have Li positioned at the negative end of the dipolemoment vector of H2O. This same arrangement for complexeswith FLi makes Li. . .F–Li linear.b A point along the dissociation path.

Missing from Table 2 and Fig. 1 is the value of 1J(B–Li)for H2BLi:4FLi, since as noted above, optimization ofthis complex leads to dissociation to the ion pair H2B�

with LiC: 4FLi. As H2BLi : 4FLi dissociates, 1J(B–Li) shouldapproach 0 Hz when B and Li are completely isolated. Atone point along the dissociation path at a B–Li distanceof 5.5 A, the computed value of 1J(B–Li) is less than5 Hz.

0

20

40

60

80

100

120

0 1 2 3 4

No. of solvent molecules, n

Hz

Figure 1. 1J(B–Li) for complexes H2BLi:(n solvent) for solventsH2O (♦) and FLi (�).

Segawa and coworkers reported a broad singlet peakwith a half-width of 36 Hz in the 7Li NMR spectrum of theboryllithium compound. They attributed this broadening tothe interaction of Li with the quadruple boron nucleus, butthey did not report a value for 1J(B–Li). We have estimatedthe value of 1J(B–Li) employing two different approaches.Using an experimental NMR band for B–H coupling ina trispyrazolylborate22 along with the splittings and half-width observed for this band, we have constructed a similarband for B–Li coupling in the boryllithium species. This gavean estimate of about 15 Hz for 1J(B–Li). A second estimatehas been obtained from the curves in Fig. 1 and the computedB–Li coupling constants for Li-diazaborole and its complexeswith one solvent molecule, H2O or FLi. From these data weobtained a value for 1J(B–Li) for Li-diazaborole:3THF of

Copyright 2007 John Wiley & Sons, Ltd. Magn. Reson. Chem. 2007; 45: 484–487DOI: 10.1002/mrc

One-bond B–Li coupling in boryllithium compounds 487

about 25 Hz. If a complex of the boryllithium molecule withfour THF molecules does not dissociate, then 1J(B–Li) wouldbe further reduced. It should be noted, however, that thisestimate does not take into account the influence of the twodiisopropylphenyl groups on the B–Li coupling constant.

No chemical shifts are reported for complexes of H2BLiwith solvent molecules, since BH2

� has a huge computedchemical shift, the result of a large negative charge on thissmall ion and the inability to delocalize this charge onto otherelectronegative elements. This observation emphasizes theuniqueness of the newly synthesized boryllithium moietiesas stable, nucleophilic species.

SUMMARY

EOM-CCSD coupling constants and MP2 chemical shifts forcomplexes of Li-diazaborole with one H2O or FLi molecule,and coupling constants for a model compound H2BLi and itscomplexes with up to four H2O or FLi molecules, have beenobtained in an attempt to resolve discrepancies betweenthe computed values of these properties for isolated Li-diazaborole and the experimentally determined values fora boryllithium molecule in a THF solution. The presence ofsolvent molecules increases the ion-pair character of the B–Libond, with the result that 1J(B–Li) decreases systematicallyas the basicity and the number of solvent molecules increases.In the presence of even a single solvent molecule, the boronchemical shift for Li-diazaborole increases, and approachesthe experimental value. The computed results emphasize theimportance of the solvent in determining the NMR propertiesof boryllithium molecules.

AcknowledgementsJEDB thanks the Ohio Supercomputer Center for continuing supportof this research. JE also thanks the Spanish Ministry of Science andEducation for economic support (Project no. CT2006-14487-C02-01/BQU).

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Copyright 2007 John Wiley & Sons, Ltd. Magn. Reson. Chem. 2007; 45: 484–487DOI: 10.1002/mrc