Synthesis and characterization of a mercury-containing trimetalloboride

Article abstract of DOI:10.1039/c4cc01246a

The reaction of phenylmercuric chloride with an anionic dimanganese borylene (Li⁺[Cp₂(CO)₄Mn₂B] ^-) led to the formation of a trimetalloboride featuring the first reported bond between mercury and a non-cluster boron atom. Examination by ¹⁹⁹Hg NMR indicated ¹¹B-¹⁹⁹Hg scalar coupling. Theoretical calculations indicated the nature of bonding to be σ-donation from a B-Mn π-orbital to Hg, in conjunction with weak Hg_d → π* back-donation. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01246a

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Synthesis and characterization of a
mercury-containing trimetalloboride†
Cite this: Chem. Commun., 2014,
50, 5729
Rudiger Bertermann,a Holger Braunschweig,*a William C. Ewing,a Thomas Kramer,a
¨
Ashwini K. Phukan,b Alfredo Vargasc and Christine Wernera
Received 17th February 2014,
Accepted 8th April 2014
DOI: 10.1039/c4cc01246a
www.rsc.org/chemcomm
The reaction of phenylmercuric chloride with an anionic dimanganese
borylene (Li+[Cp2(CO)4Mn2B]ꢀ) led to the formation of a trimetalloboride
featuring the first reported bond between mercury and a non-cluster
boron atom. Examination by 199Hg NMR indicated 11B–199Hg scalar
coupling. Theoretical calculations indicated the nature of bonding
to be r-donation from a B–Mn p-orbital to Hg, in conjunction with
weak Hgd - p* back-donation.
Save for one very recent report,1 covalent interactions between
mercury and boron have been limited to systems containing
borane clusters. Early instances of Hg–B bonding featured the
insertion of Hg into the open face of decaborane (nido-B10H14),
bound either via tetrahapto-coordination to 4-boron atoms in
the 1 : 1 complex [B10H12HgMe]ꢀ or sandwiched between the
open faces of two decaborane molecules in the 2 : 1 species
Scheme 1 Syntheses of trimetalloborides 2 and 3. 2a, M = Au; 2b, M = Ag;
2c, M = Cu. ITol = N,N0-bis(4-methylphenyl)imidazol-2-ylidene.
[(B10H12)2Hg]2ꢀ 2,3
Structural assignments of these two com-
.
pounds were proposed solely on the basis of NMR or IR data, one of the two BQMn bonds, elongating the proximate borylene
without the aid of X-ray crystallography. A handful of crystal bond with respect to the unaffected bond on the opposite side of
structures were subsequently reported describing the binding the molecule. In 2c, Cu occupied a symmetric position equidistant
of Hg to boron vertices of borane or carborane clusters as exo- from each Mn, forming Mn–B–Cu angles near 901. Computational
polyhedral substituents on individual boron atoms,4 or as investigation indicated diffuse delocalized bonding between Au,
bridging moieties between two or more boron vertices.5
For some time, we have been interested in the addition of low- (ITol)Cu+ and the borylene anion was primarily electrostatic.
valent members of the late d-block to metal–borylene bonds. To
The isoelectronic nature of the closed-shell d10-Au+ and -Hg2+
B and the proximate Mn in 2a, while in 2c the interaction of
date, work in this area has yielded a number of new compounds cations, as well as their shared relativistic behavior and propen-
with interesting bonding motifs.6 The addition of base-stabilized sity toward the formation of linear 14-electron species,9 suggested
group 11 metals across the central MnQBQMn unit of anionic the reactivity of Hg2+ complexes with 1 similar to the addition of
dimanganese borylene 17 has recently been shown to give addi- [(ITol)Au]+ in the formation of 2a. When 1 was treated with
tion products 2a–c with the elimination of LiCl (Scheme 1).8 In PhHgCl in toluene, the emergence of a bright red color was
compounds 2a and 2b, Au and Ag inserted asymmetrically across accompanied by the precipitation of a colorless solid. Examination
of the reaction mixture by 11B NMR indicated the growth of a low-
a
field resonance at 208 ppm, coincident with the disappearance of a
peak at 199 ppm characteristic of 1. After filtration and concen-
tration of the filtrate, the product (3) was crystallized at ꢀ35 1C.
Though stable at ꢀ35 1C, slow decomposition of 3 was evident at
room temperature.
¨
¨
¨
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat Wurzburg,
¨
Am Hubland, 97074 Wurzburg, Germany. E-mail: h.braunschweig@uni-wuerzburg.de
b Tezpur University, Napaam 784028, Assam, India
c University of Sussex, Brighton BN1 9QJ, Sussex, UK
† Electronic supplementary information (ESI) available: Full experimental and
spectroscopic description of the work described herein, as well as relevant
computational details and Fig. S1–S5. CCDC 975393. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c4cc01246a
The solid-state structure of 3, determined by single crystal
X-ray crystallography (Fig. 1), bears a striking resemblance to 2a.
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Fig. 2 (a) 199Hg{1H} NMR spectrum of 3; (b) 199Hg{1H, 11B} NMR spectrum
of 3. Both spectra were recorded on a 300 MHz Bruker Avance III HD NMR
spectrometer.
Fig. 1 The crystallographically determined structure of one of the two
independent molecules of 3. Thermal ellipsoids represent 50% probability.
Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and
bond angles (1) averaged for the 2 independent molecules: B–Hg, 2.2915;
B–Mn1, 1.860; B–Mn2, 1.9555; Hg–Mn2, 2.655; Hg–Cphenyl, 2.089; Mn2–B–Hg,
76.9; Mn2–Hg–Cphenyl, 165.3; Mn2–B–Mn1, 173.5; Mn1–B–Hg, 109.2.
arises from a combination of scalar coupling due to the neighbor-
ing boron (10B and 11B) and motional narrowing due to fluxion-
ality in solution. As scalar coupling ( J) is indicative of covalent
interactions between nuclei,10 the narrowing upon decoupling
confirms the existence of bonding interactions between B and Hg.
The Hg atom resides between B and Mn2 (Hg–B, 2.2915 Å; A 11B-coupled 199Hg NMR spectrum obtained at higher sensitivity
Hg–Mn2, 2.655 Å; (avg.)). Whereas the Mn–B–Mn unit in 1 is (500 MHz) gave better resolution, showing a rough quartet of
symmetric, with B–Mn bonds measuring 1.8812(14) Å and sufficient detail to allow the determination of JHg–B (103 Hz)
1.8809(14) Å,7 the addition of mercury substantially elongates through fitting of a simulated spectrum (Fig. S2, ESI†).
the B–Mn2 bond (1.9555 Å (avg.)), while inducing a slight
To examine the role mercury plays in the bonding of 3, we
contraction of the B–Mn1 bond (1.860 Å (avg.)). The Mn–B–Mn performed theoretical calculations at the OLYP/TZP, QZ4P level
angle is nearly linear both before and after addition (1, 176.11(9)1; of density functional theory using the ADF program.11–13 The
3, 173.51 (avg.)), while the Mn2–Hg–Cphenyl angle is slightly bent optimized geometry is in good agreement with the observed
toward boron (165.271 (avg.)), reminiscent of the Mn–Au–C angle structure (Fig. S3, ESI†). The asymmetric interaction of [PhHg]+
in 2a (160.29(7)1), indicating a more complicated bonding with the [{CpMn(CO)2}2B]ꢀ fragment replicates the observed
environment than a pure s-interaction between Mn and Hg.
difference in Mn–B bond lengths (Mn2–B, 1.930 Å; Mn1–B,
The 1H and 13C{1H} NMR spectra of 3 in solution each show 1.824 Å). The Mayer Bond Order (MBO) of the shorter of the two
only one set of signals for the Cp-moiety at 4.17 ppm (1H) and Mn–B bonds is more than double that of the longer (Mn2–B,
82.6 ppm (13C). Furthermore, the 13C{1H} NMR spectrum shows 0.566; Mn1–B, 1.168), further indicating diminished bonding
only one signal for the four CO groups (226 ppm), suggesting between Mn2–B relative to Mn1–B. The MBO values obtained
fluxional behavior in solution. Low-temperature 1H NMR failed for the Hg–Mn2 (0.373) and Hg–B (0.471) bonds suggest both
to induce separation of the Cp-resonances at temperatures as low interactions to be weaker than true single bonds. A very weak
as ꢀ90 1C. Spectroscopic inquiry at a faster time-scale (FT-IR) interaction was observed between the distant Mn1 and Hg, with
demonstrated the asymmetry of the ground state structure through a MBO value of 0.134. Though this interaction is insignificant
excellent agreement between the room temperature solution-state in the solid state, it may enable facile conversion between the
FT-IR spectrum of 3 and a DFT-simulated IR spectrum based on an two enantiomeric forms of 3 via transfer of the [HgPh+] between
optimized structure of C1-symmetry, closely resembling the solid- the two B–Mn bonds in solution, explaining the fluxionality
1
state structure of 3 (Fig. S1, ESI†).
observed by H NMR spectroscopy.
Covalent bonding between Hg and B was confirmed by 199Hg
Extended Transition State-Natural Orbitals for Chemical
NMR spectroscopy. Fig. 2 compares the 11B-coupled (Fig. 2a) Valence (ETS-NOCV)14 calculations are useful tools for the identifi-
and 11B-decoupled (Fig. 2b) 199Hg{1H} spectra of 3. The broad, cation and quantification of orbital interactions involved in the
slightly structured peak located at 83.9 ppm (FWHH = 460 Hz, bonding between molecular fragments.15 The two pairs of com-
Fig. 2a) was observed to substantially narrow with application plementary NOCV determined to be relevant for bonding between
of 11B decoupling (FWHH = 33 Hz, Fig. 2b). The existence of the [PhHg]+ and [Mn–B–Mn]ꢀ fragments are a s-interaction of a
well resolved 199Hg satellites in the 500 MHz 1H NMR spectrum p-orbital between Mn2 and B with [PhHg]+ (Fig. 3a) and back-
of the ortho- and meta-protons (FWHH E 7 Hz) of the PhHg donation from a mercury d-orbital to an orbital of p*-symmetry
moiety indicated that relaxation via chemical shift anisotropy between B and Mn2 (Fig. 3b). The deformation densities (Dr) depict
(CSA) was negligible for the 199Hg nucleus in solution. A line- the stabilization attributed to each interaction, quantified by their
width comparison of the 199Hg{1H} NMR signal at 53.8 MHz respective DE values (s, ꢀ73.7 kcal molꢀ1; p, ꢀ5.7 kcal molꢀ1),
(300 MHz NMR spectrometer) and the signal at 89.6 MHz indicating that the primary contributor to bonding is the
(500 MHz NMR spectrometer) showed no difference, indicating s-interaction. The transferred charge in this interaction, indicated
that the observed line-width of the 199Hg{1H} resonance (460 Hz) by Dr1, is evenly spread across both Mn and B, explaining both
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lower oxidation state and is thought to more readily employ its
d-orbitals in bonding.25 The classification of Au+ as a transition
metal is not in question,9 and recent analysis has attributed
significant strength to Aud - p* interactions with alkenes.26
ETS-NOCV analysis of 2aIMe (Fig. S5b, ESI†) indicates very
similar d - p* back-donation to that seen in the analysis of 3.
As expected, the back-donation in the model compound 2aIMe
(DEp = 15.1 kcal molꢀ1) is stronger than in 3, comprising B23%
of the combined bonding interactions, whereas the Hgd - p*
back-donation is only B7% of the calculated total.
The similarities between the back-donation interactions of an
accepted transition metal (Au) and a ‘‘controversial’’ transition
metal (Hg) placed in the same system suggest the capability of
Hg to function akin to its neighbors to its left, though the small
magnitude of the interaction indicates a reluctance to do so in
this system. Ongoing work on engineering related bonding
environments with this capability in mind will no doubt provide
further experimental evidence for, or against, the transition-
metal-nature of Hg.
Financial support in the form of an advanced grant from the
European Research Council is gratefully acknowledged.
Fig. 3 Plots of relevant bonding NOCV pairs (Cn/Cꢀn) with accompanying
eigenvalues, the associated deformation densities (Drn) and the orbital
stabilization energies (DE) corresponding to (a) s-donation and (b) p-back
donation. Positive and negative eigenvalues correspond to bonding and
antibonding interactions, respectively. The direction of charge flow in the
deformation densities is from red - blue.
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